Ion-Molecule Reactions Involving Fluorine-Containing Organic

6 Ion-Molecule Reactions Involving Fluorine-Containing Organic Compounds S H A R O N G. LIAS

Downloaded by UNIV OF ROCHESTER on June 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0066.ch006

Physical Chemistry Division, National Bureau of Standards, Washington, DC 20234

Since the mid-1950's the elementary chemical reactions of charged species with molecules - so-called ion-molecule reactions - have been extensively studied. Most of these investigations have been carried out in the gas phase, in the ion sources of various types of mass spectrometers where the ions are generated by electron bombardment or by absorption of high energy photons. Through the use of such mass spectrometric techniques as well as other approaches, a large body of information about the chemistry of ions has been generated during the last quarter of a century, so that it is now possible to understand the factors which in fluence the chemistry of ions (1,2,3). In classical organic chemistry, a common approach to inter preting patterns of chemical reactivity has always been to examine the changes brought about in reaction rates and mechanisms when an atom or group of atoms in a reactant species is replaced by some other substituent. Within the past five years, a number of in vestigators have followed this approach in order to examine the factors influencing the mechanisms of the reactions of ions. Of particular interest have been the results obtained in fluorine -substituted organic compounds. Such substitution changes the electron density distribution within a reactant molecule, and therefore, as organic chemistry would predict, can often direct the site of attack of an approaching ion. Furthermore, it is seen that in an ion-molecule complex, fluorine atoms are much more reluctant to indulge in the rearrangements which are char acteristic of hydrogen atoms in analogous situations. For these reasons, fluorine "labelling" can lead to the observation of reactions whose mechanisms are very specific, and therefore, are particularly interesting for ascertaining the effects of molecular structure, reaction enthalpy, or other factors on the course of a reaction. In addition, it has been shown that fluorinated molecu lar ions have a tendency to be formed with excess internal energy and to dissociate giving vibrationally excited fragment ions; the elucidation of the reactions and collisional deactivation of such species is also of interest. © 0-8412-0399-7/78/47-066-152$10.00/0

Root; Fluorine-Containing Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

6.

LIAS

Ion-Molecule Reactions

153

T h i s chapter presents a b r i e f r e v i e w of the r e a c t i o n s of p o s i t i v e l y - c h a r g e d polyatomic ions i n f l u o r i n a t e d systems. The r e s u l t s o f many s u c h s t u d i e s h a v e a l s o p r o v i d e d i n f o r m a t i o n a b o u t the thermochemistry of f l u o r i n a t e d i o n s which i s a l s o reviewed here.


Reactions i n P e r f l u o r i n a t e d Alkanes The i o n - m o l e c u l e r e a c t i o n s w h i c h o c c u r i n p e r f l u o r i n a t e d a l k a n e s have been i n v e s t i g a t e d u s i n g i o n c y c l o t r o n resonance s p e c t r o m e t r y ( 4 , 5 ) , tandem mass s p e c t r o m e t r y ( 6 , 7 ) , h i g h - p r e s s u r e mass s p e c t r o m e t r y ( 8 , 9 ) , t i m e - o f - f l i g h t mass s p e c t r o m e t r y ( j 6 , 7 ) , and r a d i a t i o n c h e m i s t r y t e c h n i q u e s ( 1 0 , 1 1 , 1 2 ) . T a b l e I l i s t s r e a c t i o n s o b s e r v e d b e t w e e n f r a g m e n t i o n s and parent molecules i n CF (9), (b 5 6 8,), C ^ ( 5 , 7 ) , and c - C ^ F g ( 5 ) , and t h e r a t e c o n s t a n t s m e a s u r e d f o r t h e s e r e a c t i o n s . The r e s u l t s show t h a t e x c e p t f o r C F i o n s , w h i c h u n d e r g o c o n densation r e a c t i o n s , F or F t r a n s f e r r e a c t i o n s predominate i n these systems. 9

4

9

9

I n T a b l e I , we a l s o e s t i m a t e t h e p r o b a b i l i t i e s t h a t t h e c o l l i s i o n s between t h e s e r e j e t a n t p a i r s l e a d t o r e a c t i o n . Except f o r t h e r e a c t i o n s o f CF^ , we s e e t h a t t h e F o r F t r a n s f e r r e a c t i o n s observed i n p e r f l u o r i n a t e d alkanes are extremely i n e f ficient. T h i s low r e a c t i o n p r o b a b i l i t y i s e s p e c i a l l y s t r i k i n g when compared w i t h t h e e f f i c i e n c i e s o f t h e c o r r e s p o n d i n g r e a c t i o n s i n hydrocarbon systems ( 3 ) . The m e t h y l i o n r e a c t i o n w i t h e t h a n e c o r r e s p o n d i n g to r e a c t i o n 4 o c c u r s a t every c o l l i s i o n between t h e r e a c t a n t p a i r ; the h y d r i d e t r a n s f e r r e a c t i o n s from propane t o v i n y l i o n s o r e t h y l i o n s , c o r r e s p o n d i n g t o r e a c t i o n s 6 and 7 , occur w i t h e f f i c i e n c i e s of about 0 . 5 . These d i f f e r e n c e s between t h e e f f i c i e n c i e s o f i o n - m o l e c u l e r e a c t i o n s i n f l u o r o a l k a n e s and a l k a n e s c a n be e x p l a i n e d i n t e r m s o f t h e t h e r m o c h e m i s t r y o f t h e s e s y s t e m s , remembering t h a t endothermic o r t h e r m o n e u t r a l i o n m o l e c u l e r e a c t i o n s a r e q u i t e i n e f f i c i e n t , and o f t e n c a n n o t e v e n be s e e n on t h e t i m e s c a l e o f i o n c o l l e c t i o n i n a mass s p e c t r o meter. The h i g h l y e f f i c i e n t h y d r i d e t r a n s f e r r e a c t i o n s o b s e r v e d i n hydrocarbon systems a r e a l l e x o t h e r m i c . In the fluorocarbon s y s t e m s , oiji t h e o t h e r h a n d , t h e F t r a n s f e r r e a c t i o n s l i s t e d f o r 3 * 2 3 * 2 5 a c t u a l l y s l i g h t l y endothermic, as d e m o n s t r a t e d b y t h e i o n c y c l o t r o n r e s o n a n c e t e c h n i q u e known a s double resonance (5). T h i s i s a s u r p r i s i n g r e s u l t f o r those used t o t h i n k i n g i n terms of h y d r i d e t r a n s f e r r e a c t i o n s i n hydrocarbon s y s t e m s , w h e r e f r a g m e n t c a r b o n i u m i o n s c a n u s u a l l y be assumed t o undergo e f f i c i e n t exothermic r e a c t i o n s w i t h the parent m o l e c u l e . The t h e r m o c h e m i s t r y o f t h e f l u o r o c a r b o n i o n s w i l l be d i s c u s s e d i n more d e t a i l b e l o w . +

C F

C

F

9

a n c

C

F

a

r

e


FLUORINE-CONTAINING FREE RADICALS

154

Table I .

Ion-Molecule Reactions i n Perfluoroalkanes. k χ

F

-

and F T r a n s f e r R e a c t i o n s

(1) (2)


(3)

CF

+

0

+CF.-K:F. +CF +

2 2

C F

+ + C

2

C F

4

+ + C

3

2 6"* 2 5 F

C

3 8" 3 7 F

C

F

^ 3 F

(4) (5) (6) (7) (8)

3

+ + C

2 6" 2 5

C F

3

+ + C

3 8^3 7

C F 2

C

C

F

3

+ + C F

3

F

4

+ 4 < : F

4

+C Fg^C F

3

3

2 5 F

+ + C

3

3 8F

> C

(1.9)

0.27

3 7 F

4

F

(9) (10)

F

+

b. c. d. e.

F

4

> C

2 5

C

F

+ 4 < :

d

0.04

(0.8)

d

0.10

F

e

3.3

+

0.01

d

0.40

b

CF +C F ->[C F ]*-> +

3

2

+ + C

6

3

7

+

3

(0.13) ,

F

3

+ + C

+

2 4

d

CF +C F +[C F C F

a.

C

CF +CF.->C„F

C F

(11)

+ + C

(0.06)

(0.3)

(0.086)

2 6

3 5 - 4 8- 4 7 3 6 Condensation Reactions C

0.34°

d

2

+ + C

d

(0.4) ,

4C F

7

1.7

d

(0.86)

F

+ + C F

F

k k a Rn/ C o l l i s i o n 0.36

b

14.5+3°

3 7

+ + C

C F

F

+ + C F

F

1 0

cm / m o l e c u l e - s e c o n d 2.7

0

3

10

3

g

3 6 F

4

9

+

1.9°

(0.01),

0.20

]*+ 0.32

d

0.03

k ( c o l l i s i o n ) estimated w i t h the Langevin-Gioumousis-Stevenson e q u a t i o n , reference 13. H i g h p r e s s u r e mass s p e c t r o m e t e r , r e f e r e n c e 9 . Rate constant for thermal i o n s . H i g h p r e s s u r e p h o t o i o n i z a t i o n mass s p e c t r o m e t e r , r e f e r e n c e 8 . Rate constant for thermal i o n s . Tandem mass s p e c t r o m e t e r , r e f e r e n c e 7. These r a t e c o n s t a n t s c o r r e s p o n d t o t h o s e f o r i o n s h a v i n g 0 . 3 eV k i n e t i c e n e r g y . Observed i n an i o n c y c l o t r o n resonance s p e c t r o m e t e r , r e f e r e n c e 5.


6.

OAs

155



In s e v e r a l of the s t u d i e s of f l u o r i n a t e d alkanes 5^,6^,_7) endothermic c o l l i s i o n - i n d u c e d d i s s o c i a t i o n s of the r e a c t a n t ions were a l s o o b s e r v e d . I t was s u g g e s t e d (_4,5^6^2.,8) t h a t when a f l u o r i n a t e d a l k a n e i s i o n i z e d by e l e c t r o n i m p a c t , i t undergoes d i s s o c i a t i o n t o g i v e fragment i o n s c o n t a i n i n g i n t e r n a l e x c i t a t i o n energy. The o b s e r v a t i o n o f t h e s e e n d o t h e r m i c r e a c t i o n s u n d e r c o n d i t i o n s such t h a t the r e a c t i n g i o n s have n o m i n a l l y t h e r m a l e n e r g i e s , a l s o i n d i c a t e s t h a t t h e f r a g m e n t i o n s , i f formed w i t h e x c e s s e n e r g y , a r e c a p a b l e o f r e t a i n i n g t h i s e n e r g y f o r t i m e s as l o n g a s milliseconds. I f indeed the f l u o r i n a t e d carbonium ions observed i n these i n v e s t i g a t i o n s a r e r e a c t i n g f r o m a n e x c i t e d s t a t e , i t w o u l d be e x p e c t e d t h a t t h e a v e r a g e r a t e c o n s t a n t o f r e a c t i o n w o u l d change w i t h p r e s s u r e as the r e a c t a n t i o n i s c o l l i s i o n a l l y d e a c t i v a t e d . The e f f e c t o f p r e s s u r e on t h e r a t e c o n s t a n t f o r r e a c t i o n 5 was examined q u a l i t a t i v e l y i n an i o n c y c l o t r o n resonance s t u d y ( 5 ) , and i t was f o u n d t h a t t h e a v e r a g e r a t e c o n s t a n t d e c r e a s e s a s t h e p r e s s u r e i s i n c r e a s e d , a s one w o u l d e x p e c t f o r a n e n d o t h e r m i c reaction. I n a h i g h p r e s s u r e mass s p e c t r o m e t r i c s t u d y ( 8 ) o f r e a c t i o n 4 , i t was shown t h a t a t p r e s s u r e s a b o v e a b o u t 0 . 5 t o r r , e s s e n t i a l l y a l l o f t h e C F ^ i o n s h a d r e a c t e d t o f o r m C^F,- . On t h e o t h e r h a n d , t h e r a t e c o n s t a n t s w h i c h have been r e p o r t e d (7,8) for t h i s r e a c t i o n may be t o o h i g h f o r c o l l i s i o n a l l y d e a c t i v a t e d i o n s , s i n c e no r e a c t i o n was s e e n b e t w e e n C F ^ and ^2^6 * ξ con d i t i o n s o f a n i o n c y c l o t r o n r e s o n a n c e e x p e r i m e n t (10 t o r r , 0.5 sec. observation time) (14). From t h i s l a t t e r r e s u l t , i t c a n be estimated t h a t f o r thermal r e a c t a n t i o n s , |tp r ^ t e constant f o r r e a c t i o n 4 c a n n o t be g r e a t e r t h a n 5 χ 10 cm / m o l e c u l e - s e c o n d . This corresponds to a c o l l i s i o n e f f i c i e n c y of 0.0066. I f the i n e f f i c i e n c y o f t h i s r e a c t i o n c a n be a t t r i b u t e d t o a n e n e r g y b a r r i e r , t h e n t h e maximum e n d o t h e r m i c i t y o f r e a c t i o n 4 i s 3 . 0 k c a l / mole (0.13 e V ) . +

υ η (

β

t

n

e

I n agreement w i t h t h e h i g h p r e s s u r e mass s p e c t r o m e t r i c r e s u l t (8), there i s a l s o evidence t h a t r e a c t i o n 4 occurs i n the r a d i o l y s i s o f ^2^6 P r e s s u r e s a s h i g h a s two a t m o s p h e r e s ( 9 , 1 0 ) , o r i n t h e l i q u i a p h a s e ( 1 1 ) , s i n c e C F ^ i s o b s e r v e d as a p r o d u c t u n d e r t h e s e c o n d i t i o n s , even i n the presence o f a f r e e r a d i c a l s c a v e n ger. The f o r m a t i o n o f t h i s p r o d u c t f r o m t h e d i s s o c i a t i o n o f n e u t r a l e x c i t e d C F i n the r a d i o l y s i s : a

t

9

A

C F * -> C F + C F 2

6

4

2

(12)


156


c a n be e x c l u d e d s i n c e C F ^ i s n o t f o r m e d when b r o m i n e i s added t o t h e s y s t e m , a n d t h e r e i s no r e a s o n t o e x p e c t t h a t s m a l l c o n c e n t r a t i o n s o f b r o m i n e w o u l d h a v e a n y e f f e c t on t h e p r i m a r y d e c o m p o s i t i o n of an e l e c t r o n i c a l l y e x c i t e d molecule. We c a n t e n t a t i v e l y c o n c l u d e (8) t h e r e f o r e , t h a t i o n i c f r a g m e n t a t i o n o f C^F^ t o f o r m C F ^ does o c c u r a t p r e s s u r e s a s h i g h a s two a t m o s p h e r e s , a s w e l l as i n the l i q u i d p h a s e , and t h a t t h e fragment i o n s undergo r e a c t i o n 4 under these c o n d i t i o n s .


Reactions i n F e r f l u o r i n a t e d Olefins T a b l e I I shows t h e i o n - m o l e c u l e r e a c t i o n s w h i c h h a v e o b s e r v e d i n C F . ( 1 5 , 1 6 ) and C F . ( 5 ) . 0

been

0

Table I I .

Ion-Molecule Reactions i n Perfluorinated O l e f i n s . Reaction

C

2 4 F

+

+

C

2 4 " F

C

3 5 F

+

+

C F

3

(

1

3

)

C

2 3

+

+

C

2 4

C

3 5

+

+

C F

2

(

1

4

)

C

F

(

1

5

)

F

2

CF

+

+

+

C

F

2 4 + F

+ C F 2

C

F

2 4 F

-> C F

4

3

+

+

2

Q

+

+ C F 3

-> C F

6

3

R e s u l t s from r e f e r e n c e s

5,

2

+ C F

+

C F * 4- C L F , -> C F _ 3 3 6 3 5 CF

C F

5

+ CF. 4

+

+ CF

+

(16)

2

(17) (18)

2

1 5 , and 16.

I n C F ^ , i t h a s b e e n shown (16) t h a t when i o n i z a t i o n i s e f f e c t e d b y bombardment w i t h 13 eV e l e c t r o n s , o n l y t h e _ g a r e n t ^ F ^ i o n s a r e p r o d u c e d i n i t i a l l y a t p r e s s u r e s o f a b o u t 10 torr. I n tljie i o n c y c l o t r o n r e s o n a n c e s p e c t r o m e t e r (16) t h e p a r e n t C F, i o n undergoes r e a c t i o n 13. The r a t e c o n s t a n t . ^ o r | h i s r e a c t i o n i n c r e a s e s f r o m a l i m i t i n g ^ a l u e o f 3 . 0 χ 10 cm / m o l e c u l e - s e c o n d a t l o w ^ p r e n s u r e s (10 t o r r ) t o a v a l u e of 1.0 (16) o r 1^5 (17) χ 10 cm / m o l e c u l e - s e c o n d a t h i g h e r p r e s s u r e s (10 torr). T h i s was i n t e r p r e t e d i n t e r m s o f t h e r e a c t i o n scheme: 2

2

C F 2

(C F 2

+ e~

4

4

+

( C

)* + C F 2

4

F

2

-

4

V + 2 (C F 4

8

+

(19)

e" ) * -> C F 3

5

+

+ CF

3

(20)


157


LIAS

6.

(21)


+ CF, 3

(22)

The m o l e c u l a r i o n s f o r m e d i n i t i a l l y by e l e c t r o n i m p a c t a r e c o n sidered to c o n t a i n i n t e r n a l e x c i t a t i o n energy. Some o f t h e s e i o n s r e a c t t o f o r m p r o d u c t C^F^ i o n s , w h i l e t h e o t h e r s u n d e r g o n o n - r e a c t i v e c o l l i s i o n s o r r e s o n a n t c h a r g e exchange r e a c t i o n s w i t h t h e n e u t r a l C^F^ m o l e c u l e s i n t h e s y s t e m , p r o d u c i n g m o l e c u l a r i o n s of lower i n t e r n a l energy. These moderated i o n s t h e n r e a c t t o form products. I t w a s f o u n d (16) t h a t t h e r a t e c o n s t a n t o b s e r v e d f o r f o r m a t i o n o f C^F^ r e a c h e s a c o n s t a n t v a l u e a f t e r t h e p a r e n t i o n has undergone an average o f s i x c o l l i s i o n s . +

The b i m o l e c u l a r r a t e c o n s t a n t f o r t h e c o n d e n s a t i o n r e a c t i o n 13 is_ l o w r e l a t i v e t o t h e r a t e c o n s t a n t f o r c o l l i s i o n b e t w e e n ? 4 and C ^ F ^ . A p p r o x i m a t e l y one c o l l i s i o n i n f i f t y l e a d s t o the f o r m a t i o n of p r o d u c t s d i f f e r e n t from the r e a c t a n t s . In the d i s c u s s i o n below of condensation r e a c t i o n s i n p a r t i a l l y f l u o r i n a t e d e t h y l e n e s , i t w i l l be shown t h a t t h e r e a r r a n g e m e n t w h i c h i s necessary f o r m e t h y l r a d i c a l e l i m i n a t i o n from a C^(H,F)g complex i s s u b s t a n t i a l l y i n h i b i t e d by t h e p r e s e n c e o f F - a t o m s i n t h e complex. T h i s would e x p l a i n the f a c t t h a t the r a t e constants of t h e s e r e a c t i o n a r e c o n s i d e r a b l y l o w e r , and a l s o more d e p e n d e n t on t h e | . n t e r n a l e n e r g y c o n t e n t o f t h e r e a c t a n t s a s compared t o t h e 2 4 " 2 4 Pair (18,19). C

F

+

C

H

C

H

r

e

a

c

t

i

o

n

The d e c r e a s e i n r e a c t i o n r a t e w i t h i n c r e a s e i n i n t e r n a l e n e r g y c a n be r a t i o n a l i z e d i n t e r m s o f t h e l i f e t i m e o f t h e c o l l i s i o n complex. R e a r r a n g e m e n t s s u c h as t h o s e i n v o l v e d i n r e a c t i o n 13 may be e x p e c t e d t o i n c r e a s e i n p r o b a b i l i t y when t h e c o l l i s i o n p a r t n e r s remain together f o r a longer p e r i o d of t i m e . B e c a u s e t h e l i f e t i m e o f t h e c o m p l e x w i l l be s h o r t e n e d by i n c r e a s e d i n t e r n a l or t r a n s l a t i o n a l energy the r a t e c o e f f i c i e n t of such an e x o t h e r m i c r e a c t i o n w i l l a l s o be r e d u c e d . I n t h e n e x t s e c t i o n we wilj.1 p r e s e n t e v i d e n c e t h a t t h e i n t e r mediate condensation i o n ( C , F i n t h i s case) formed i n f l u o r i n a t e d e t h y l e n e s has the c y c l o o u t a n e s t r u c t u r e . I t h a s b e e n shown (20) t h a t c y c l o b u t a n e i o n s f o r m e d w i t h a l a r g e amount o f i n t e r n a l energy have a h i g h p r o b a b i l i t y of d i s s o c i a t i n g to g i v e an e t h y l e n e i o n and e t h y l e n e m o l e c u l e , w h i l e t h o s e f o r m e d w i t h o n l y a s m a l l amount o f e x c e s s e n e r g y d i s s o c i a t e by l o s i n g a m e t h y l r a d i c a l , i . e . t h r o u g h a p r o c e s s a n a l o g o u s t o r e a c t i o n s 20 and 2 2 . R


158


The r e s u l t s g i v e n i n T a b l e I I show t h a t i n p e r f l u o r i n a t e d p r o p y l e n e , f r a g m e n t CF and C F ^ i o n s u n d e r g o e x o t h e r m i c F t r a n s f e r reactions w i t h the parent molecule. The o n l y o t h e r r e a c t i o n s o b s e r v e d i n C^F^ a s w e l l a s i n 2 - C . F g (5) a r e e n d o thermic c o l l i s i o n - i n d u c e d d i s s o c i a t i o n r e a c t i o n s . +

+


Reactions i n P a r t i a l l y Fluorinated Ethylenes A number o f s t u d i e s h a v e b e e n c a r r i e d o u t e x p l o r i n g t h e e f f e c t s of f l u o r i n e s u b s t i t u t i o n on t h e i o n - m o l e c u l e c h e m i s t r y o f ethylenes (21,22,23,24,25,26,27). From r e s u l t s o b t a i n e d i n C H , C H F , C H C F , c i s - C H F C H F , t r a n s - C H F C H F , C ^ H , and C F . , and m i x t u r e s t h e r e o f , a c o n s i s t e n t p i c t u r e o f t h e r e a c t i o n s o r f l u o r i n a t e d ethylene parent ions with f l u o r i n a t e d ethylene emerges. 2

4

2

3

2

2

2

I n g e n e r a l , when c h a r g e t r a n s f e r i s e x o t h e r m i c f o r a g i v e n C ( H , F ) reactant p a i r , i t predominates. The o n l y o t h e r r e a c t i o n c h a n n e l w h i c h i s observed i n n e a r l y a l l systems i s c o n d e n s a t i o n : 2

4

C (H,F) 2

4

+

+ C (H,F) + C (H,F) 2

4

4

g

(23)

+

(where t h e two r e a c t a n t s p e c i e s may o r may n o t h a v e t h e same e m p i r i c a l formula and/or s t r u c t u r e ) . The l i f e t i m e o f t h e C ( H , F ) g c o n d e n s a t i o n i o n i s dependent on i t s i n t e r n a l energy c o n t e n t . When t h e p r e s s u r e i s i n c r e a s e d s u f f i c i e n t l y , t h e C , ( H , F ) ~ adducts a r e c o l l ^ s i o n a l l y s t a b i l i z e d . The c o l l i s i o n a l s t a b i l i z a t i o n o f the C Hg i o n formed i n e t h y l e n e , a t p r e s s u r e s above a^out 0 . 1 t o r r , h a s b e e n w e l l documented ( 2 8 , 2 9 , 3 0 ) . The C H F ion formed i n v i n y l f l u o r i d e : +

4

4

4

C H F 2

3

+ C H F

+

2

C H F

3

4

6

2

6

2

(24)

+

h a s b e e n o b s e r v e d i n a mass s p e c t r o m e t e r a t p r e s s u r e s i n t h e 0 . 0 1 - 0 . 3 t o r r ( 2 6 ) , and t h e C.ÎLF. i o n formed i n C F C H : — 4 4 4 2 2 0

CH CF 2

2

+

+ CH CF 2

2

+ C H F 4

4

4

(25)

+

i s s t a b i l i z e d at^the pressures t y p i c a l of e x p e r i m e n t s (10 t o 10 torr).

range

0

ion cyclotron

resonance

Two modes o f d i s s o c i a t i o n a r e o b s e r v e d f o r t h e C ( H , F ) g condensation ions. One o f t h e s e l e a d s t o t h e f o r m a t i o n o f a s u b s t i t u t e d e t h y l e n e i o n and e t h y l e n e m o l e c u l e h a v i n g i d e n t i t i e s d i f f e r e n t from those of the o r i g i n a l r e a c t a n t s p e c i e s . Because i n t h i s r e a c t i o n t h e r e i s n e v e r any s c r a m b l i n g o f H ( o r i n d e u t e r i u m l a b e l l i n g e x p e r i m e n t s , D) a n d / o r F s p e c i e s , i t i s c a l l e d a " m e t h l y e n e s w i t c h i n g r e a c t i o n " a n d i s assumed t o o c c u r f r o m a 4


+

LIAS

6.

159


condensation i o n having the structure of a cyclobutane i o n . E x amples o f methylene s w i t c h i n g r e a c t i o n s w h i c h have been o b s e r v e d (17) i n c l u d e :

C H F 9

C H F


9

CH - C D F + -> CHF CDF CHF-UDF

+ CH CDF

CH - C D F * + -* CHFCF CHF-CF

+ CH CDF

+ CDFCDF +

+ CDFCF

5

1

H

C

2 3 H

F +

+

C F

2

C F

1

2

2 * j

2

F

+

|

(26)

l

(27)

l

*

2

C H F C F

2

+

+

C H

2

C F

2

(

2

8

)

A n e x a m i n a t i o n o f t h e s e r e a c t i o n s a s w r i t t e n h e r e shows t h a t a s y m m e t r i c a l c l e a v a g e a l o n g t h e v e r t i c a l a x i s r e t u r n s t h e com p l e x t o r e a c t a n t s o r r e s u l t s i n a charge t r a n s f e r , w h i l e a cleavage along the h o r i z o n t a l a x i s produces the "methylene s w i t c h ing" products. An i s o t o p i c a l l y mixed product i s never produced, even i n s m a l l y i e l d . The o t h e r mode o f d i s s o c i a t i o n o f t h e C ^ ( H , F ) c o m p l e x e s i s through the e l i m i n a t i o n of a methyl r a d i c a l . Some e x a m p l e s o f m e t h y l r a d i c a l e l i m i n a t i o n r e a c t i o n s w h i c h have been observed i n f l u o r i n a t e d e t h y l e n e systems (17) a r e : g

+ CH CF 2

2

+

CH -CF„ + CDFCF

£

+ CDFCF

2

J

+

|

C

3

H

2

D F

2

+

C

F

(29)

3

I-CHF *

CHFCHF

-> Π J +1 -> C ^ D F ^ CDF-C' 2 M

+ CF

ÇHF-ÇHF * C Η DF + CDF-CDF 0F+ C C D HF„ HF

+

cis-CHFCHF

(30)

3

F

+ t r - C D F C D F ->

30

02

2

+ CDF.(76%) (31) + CHF (24%) A 2

As i n t h e m e t h y l e n e s w i t c h i n g r e a c t i o n s , t h e r e seems t o b e l i t t l e s c r a m b l i n g o f t h e H(D) a n d F atoms w i t h i n t h e i o n - m o l e c u l e c o m p l e x , s i n c e i n e v e r y c a s e t h e o b s e r v e d p r o d u c t s c a n be e x p l a i n e d by t h e s h i f t o f a s i n g l e H(D) o r F atom t o a n o t h e r m e t h y l e n e group f o l l o w e d by l o s s o f t h e r e s u l t i n g m e t h y l group from t h e


160


i o n i c complex. I t h a s b e e n s u g g e s t e d t h a t t h e mechanism o f t h i s r e a c t i o n i s a 1,4-F s h i f t i n a l i n e a r tetramethylene i o n i c i n t e r m e d i a t e (17) o r t h a t t h e mechanism p r e d o m i n a n t l y i n v o l v e s a 1,2-H(D) s h i f t i n a c y c l i c i o n i n t e r m e d i a t e (22). Assuming e i t h e r one o f t h e s e m e c h a n i s m s , i t i s p o s s i b l e t o c a l c u l a t e t h e s t a t i s t i c a l p r o b a b i l i t y f o r t r a n s f e r o f H o r F t o and f r o m C ( H , F ) groups i n the ( C ^ - C ^ C H j F ) , ) c o m p l e x e s s t u d i e d by F e r r e r - C o r r e i a and J e n n i n g s ( z 2 ) . C o m p a r i n g t h e o b s e r v e d and s t a t i s t i c a l p r o b a b i l i t i e s o f H(D) o r F m i g r a t i o n t o C H F , CI> , o r C F g r o u p s i n t h e s e c o m p l e x e s , i t c a n be s e e n t h a t t h e p r o b a b i l i t y f o r F - m i g r a t i o n t o a CH«(CD ) g r o u p i s a l w a y s v e r y l o w , n o t more t h a n 20-30% o f t h e s t a t i s t i c a l p r o b a b i l i t y . The p r o b a b i l i t y o f F - m i g r a t i o n f r o m one CHF g r o u p t o a n a d j a c e n t CHF g r o u p i s s l i g h t l y g r e a t e r t h a n t h e s t a t i s t i c a l p r e d i c t i o n , and t h e p r o b a b i l i t y o f F - m i g r a t i o n t o a CF« g r o u p i s t h r e e t i m e s g r e a t e r t h a n one w o u l d p r e d i c t f r o m s t a t i s t i c a l c o n s i d e r a t i o n s ( 2 6 ) . The p r o d u c t s f o r m e d i n r e a c t i o n s 2 9 , 3 0 , a n d 31 a r e i n agreement w i t h these p r e d i c t e d trends. I t w o u l d seem f r o m t h e s e r e s u l t s t h a t t h e m i g r a t i o n o f a n F - a t o m i n a n i o n i c c o m p l e x i s a c t i v a t e d by t h e p r e s e n c e o f t h e p o l a r i z e d C - F bonds on a n a d j a c e n t c a r b o n . 2

2

2


?

I t i s somewhat more d i f f i c u l t t o g e n e r a l i z e a b o u t t h e m e c h a n i s m o f H m i g r a t i o n i n s u c h c o m p l e x e s , b u t c e r t a i n p a t t e r n s do emerge f r o m tljie d a t a r e p o r t e d by F e r r e r - C o r r e i a and J e n n i n g s ( 2 2 ) . In a C^(H,F)g c o m p l e x i n w h i c h two o r more C H g r o u p s a r e a d j a c e n t , the p r o b a b i l i t y of C H e l i m i n a t i o n i s g r e a t e r than would be s t a t i s t i c a l l y p r e d i c t e d . On t h e o t h e r h a n d , t h e H - s p e c i e s h a v e a v e r y l o w p r o b a b i l i t y o f m i g r a t i n g t o a CHF o r C F g r o u p . 2

q

2

I t i s i n t e r e s t i n g t o examine t h e c h e m i s t r y seen i n t h e C H C F s y s t e m i n t h e l i g h t o f t h e s e p r e d i c t e d trends_ f o r p r o b a b i l i t i e s o f m i g r a t i o n o f H an C F H . + HF + H (35) 2 4 The p r o t o n a t e d p a r e n t i o n f o r m e d i n r e a c t i o n 34 r e a c t s f u r t h e r w i t h methyl f l u o r i d e to generate a d i m e t h y l f l u o r o n i u m i o n which does n o t r e a c t f u r t h e r : +

0

CH FH 3

+

+ C H F -> C H F C H 3

3

3

+

+ HF

(36)

There i s evidence t h a t the s t r u c t u r e of the p r o t o n a t e d parent i o n i s a s w r i t t e n , t h a t i s , t h a t t h e p r o t o n a t i o n i s on t h e f l u o r i n e atom. When d e u t e r i u m l a b e l l i n g i s u t i l i z e d , i t i s s e e n t h a t a d e u t e r o n a t e d p a r e n t i o n u n d e r g o e s r e a c t i o n 36 w i t h no i n c o r p o r a t i o n of deuterium i n the d i m e t h y l fluoronium i o n product (31): CH FD 3

+

+ CH F + C H F C H 3

3

3

+

+ DF

(37)

The f r a g m e n t i o n s f o r m e d i n m e t h y l f l u o r i d e a r e C H ^ F * a n d C H . D e u t e r i u m l a b e l l i n g e x p e r i m e n t s (32) h a v e d e m o n s t r a t e d t h a t t h e m e t h y l i o n i n i t i a l l y f o r m e d by f r a g m e n t a t i o n r e a c t s w i t h t h e p a r e n t CH^F m o l e c u l e b y F ~ t r a n s f e r : +

3

CD

3

+

+ C H F ->· ( C D F C H ) * -> C D F + 3

3

3

+

3

CH

3

+

(38)


162


The l e s s e n e r g e t i c m e t h y l i o n s f o r m e d a s p r o d u c t s i n r e a c t i o n 38 r e a c t w i t h t h e p a r e n t m o l e c u l e t o f o r m CH^F . T h e CD^ i o n r e a c t s t o f o r m n e a r l y e q u a l amounts o f CH^F a n d CD^F : +

CD

+ CH F + ( C D . F C H * ) * -> C H F

+

9

J

+

+ CD.H

(53%)

+ CH D

(47%)

(39)

J

-> C D F 2

3

agaiç suggesting a dimethyl fluoronium ion intermediate. The CH^F i o n i s n o t s e e n t o r e a c t w i t h CH^F u n d e r t h e l o w p r e s s u r e c o n d i t i o n s of an i o n c y c l o t r o n r e s o n a n c e e x p e r i m e n t , but i n a h i g h p r e s s u r e mass s p e c t r o m e t e r (9) CH«F d i s a p p e a r s s l o w l y . The s u g g e s t e d (9) r e a c t i o n m e c h a n i s m f o r t n i s i o n i s :


+

CH F

+

2

+ S / + C H F -> ( C H F C H F ) * (CH^F H

3

3

2

+ CH F 2

2

(40)

The i o n - m o l e c u l e c h e m i s t r y o b s e r v e d ( 9 , 3 3 , 3 4 , 3 7 ) i n C F X and CF«H f o l l o w s t h e same p a t t e r n a s d e s c r i b e d a b o v e f o r m e t h y l fluoride. The C F H p a r e n t i o n r e a c t s w i t h t h e p a r e n t m o l e c u l e t o transfer a proton: 2

CF H 2

2

2

+ CF H

+

2

-> C H F H

2

2

2

+ CF H

+

(41)

2

and t h e p r o t o n a t e d p a r e n t m o l e c u l e i n t u r n r e a c t s w i t h t h e p a r e n t m o l e c u l e t o form a d i f l u o r o m e t h y l f l u o r o n i u m p r o d u c t i o n : CH F H 2

2

+ CH F

+

2

2

•> ( C H F ) F 2

+ HF

+

2

(42)

The a b u n d a n c e o f t h e p a r e n t i o n f o r m e d i n C F H by e l e c t r o n i m p a c t i s so s m a l l t h a t i t i s n o t o b s e r v e d i n t h e mass s p e c t r u m o f t h i s compound. H o w e v e r , when a p r o t o n i | t r a n s f e r r e d t o C F H f r o m some ο t h e p r o t o n a t i n g a g e n t ( s u c h a s CH,. g e n e r a t e d i n e x c e s s methane, f o r i n s t a n c e ) , the p r o t o n a t e d p a r e n t i o n i s seen to u n d e r g o a c o n d e n s a t i o n r e a c t i o n w i t h CF Η a n a l o g o u s t o r e a c t i o n s 36 and 42 o b s e r v e d i n C H F a n d C H ^ : 3

3

3

CHF H 3

+ CHF

+

-> ( C H F ) F

3

2

2

+ HF

+

(43)

I n a m i x t u r e o f C H F w i t h C F H , i t was s e e n (3|) t h a t i n i t i a l l y t h e t h r e e h a I o n i u m i o n s ( C H « ) F , (CH F ) F (reactions 36 and 4 2 ) , and C H F C H F are formed: 3

2

2

9

3

CH FH 3

CH F H 2

2

+ CF H

+

-> C H F C H F

+

+ HF

(44)

+ C H F -> C H F C H F

+

+ HF

(45)

2

+

9

2

3

2

3

3

2

2


6.

LIAS

163


At s u f f i c i e n t l y h i g h p r e s s u r e s , however, the h a l o n i u m i o n s (CH«F) F a n d CH^FCH^^F d i s a p p e a r a s a r e s u l t o f r e a c t i o n s w i t h C H ^ F , s u c h a s t h e s e c o n d s t e p o f r e a c t i o n 40 a n d : 2

(CH F) F 2

2

+

+ C H F -> C H F C H F 3

3

That i s , r e a c t i o n occurs to reduce the fluoronium i o n products.

2

+

+ C F ^

t h e number o f

(46) f l u o r i n e atoms

in

I n attempts to e s t a b l i s h i n f o r m a t i o n about the r e l a t i v e h e a t s formation of various f l u o r i n a t e d methyl i o n s , studies ( 3 3 , 3 4 , 3 8 , 3 9 ) have been c a r r i e d out e x a m i n i n g the d i r e c t i o n s of fluoride and h y d r i d e t r a n s f e r r e a c t i o n s b e t w e e n f l u o r o m e t h y l i o n s and v a r i o u s f l u o r i n a t e d methanes. Some s t u d i e s ( 3 8 , 3 9 ) h a v e a l s o i n c l u d e d i o n s and m o l e c u l e s c o n t a i n i n g c h l o r i n e a t o m s . Although the more q u a n t i t a t i v e a s p e c t s o f t h i s w o r k w i l l be c o v e r e d i n d e t a i l l a t e r i n our d i s c u s s i o n of the thermochemistry o f f l u o r i n a t e d i o n s , i t i s i n t e r e s t i n g t o examine h e r e t h e r e l a t i v e i m p o r t a n c e s o f c o m p e t i n g r e a c t i o n c h a n n e l s , shown i n T a b l e I I I . I t s h o u l d be p o i n t e d out t h a t a t s u f f i c i e n t l y h i g h p r e s s u r e s , s t a b i l i z a t i o n of the d i m e t h y l fluoronium i o n i n t e r m e d i a t e s of the r e a c t i o n s i s observed ( 9 ) .


of

I n r e a c t i o n s o f v a r i o u s i o n s w i t h C H F , C F ^ , and C F H , i t i s found t h a t exothermic f l u o r i d e t r a n s f e r r e a c t i o n s occur to the v i r t u a l e x c l u s i o n o f h y d r i d e t r a n s f e r , e v e n when t h e l a t t e r r e a c t i o n channel i s s i g n i f i c a n t l y exothermic. In reactions with C F C 1 , CF C l , C H F C 1 , o r C l U F C l i t i s s e e n t h a t w h e r e c h l o r i d e t r a n s f e r i s e x o t h e r m i c , i t s t r o n g l y p r e d o m i n a t e s e v e n when a l t e r n a t e c h a n n e l s a r e more e x o t h e r m i c . F l u o r i d e t r a n s f e r does c o m pete w i t h c h l o r i d e t r a n s f e r , but h y d r i d e t r a n s f e r i s never o b s e r o b s e r v e d when t h e r e a c t a n t m o l e c u l e c o n t a i n s c h l o r i n e . Apparentl y i t i s n o t t h e s t r e n g t h o f t h e bond b r o k e n i n t h e r e a c t i o n w h i c h d e t e r m i n e s t h e p r e f e r r e d c h a n n e l s i n c e t h e C - F bonds a r e s t r o n g e r t h a n t h e C - H bonds i n some o f t h e s e compounds. A likely interp r e t a t i o n i s that since these r e a c t i o n s a l l proceed v i a a d i alkylhaIonium i o n intermediate the e l e c t r o p h i l i c reactant ions p r e f e r e n t i a l l y a t t a c k h i g h e l e c t r o n d e n s i t y c h l o r i n e atoms i n t h e reactant molecules. A t t a c k a t f l u o r i n e atoms o c c u r s w i t h a s i g n i f i c a n t l y l o w e r p r o b a b i l i t y , and a t t a c k a t h y d r o g e n atoms o c c u r s with a negligible probability. This i s consistent w i t h the p i c t u r e given below of the i o n chemistry o c c u r r i n g i n a l k y l f l u o r i d e s , where r e a c t i o n n e a r l y always i n v o l v e s a t t a c k o f t h e i o n a t the f l u o r i n e atom. 3

3

2

2


3

164


Table I I I .

R e l a t i v e Importances of Competing C l , F , or H A b s t r a c t i o n from the Halomethanes. Mode o f R e a c t i o n and E n t h a l p y

Reactant

Pair

C l " Transfer

F~ T r a n s f e r

(eV,kcal/mole)

H~ Transfer

CFC1, CF +CFC1 +


3

3

CF H +CFC1 2

3

CF C1 +CFC1 2

C F

2

CF +CF C1 +

2

2

CF H +CF C1 2

2

CF H 2

10% (•- 1 . 9 3 , -- 4 4 . 5 )

90% (-1.37, -37.5)

10% (- - 1 . 5 5 , -- 3 5 . 8 )

99% (-0.93, -21.5)

1% (•- 1 . 1 7 , -- 2 6 . 9 )

80% (-0.67, -15.5)

20% (•- 1 . 8 3 , -- 4 2 . 3 )

86% (-0.43, -10.0)

14% (- - 1 . 4 6 , -- 3 3 . 6 )

95% (-0.03, -0.8)

(- - 0 . 8 8 ,

5% -- 2 0 . 3 )

0% (-0.88,

100% (- - 0 . 6 6 , -- 1 5 . 2 )

0% (-0.73,

(- - 0 . 8 1 ,

5% -- 1 8 . 7 )

0% (-0.78,

Major channel (-0.45, -10.4)

Minor channel (-0.67, -15.5)

100% (-0.01, -0.3)

0% (-2.61,

Major channel (-0.36, -8.3)

Minor channel (-1.2, -27.3)

2

C 1

3

3

90% (-1.60, -37)

2

2

CF C1 +CHFC1 +

2

2

CHFC1 + C H F C l

n

0% (0,0)

CH FC1 2

CF +CH FC1 +

3

2

95% (-0.004,

CH F +CH FC1 +

2

2

(0,0) C H

3

F

CF +CH F 3

C F

-0.1)

+

3

-60.6)

2 2 H

CH +CF H 3

+

2

2


LIAS

6.

165

Ion-Molecule Reactions Table I I I ( c o n t ' d . ) Mode o f R e a c t i o n a n d E n t h a l p y

Reactant

C F

Cl"" Transfer

F~ Transfer

H~ Transfer

2 2 H

CF H +CF H "2" ' ^ 2 " 2

Occurs (+0.02, +0.4)

(0,0)

C F +CCFF H H

100% 100% (-0.37, -8.6)

0% (-0.40,

-9.2)

CH +CF H

100% (-0.38, -8.7)

0% (-0.78,

-18.1)

CH i FF + C F H .

100% 100% (-0.02, -0.40)

0% (+0.12,

+2.7)

+

0

? 3 0


Pair

(eV,kcal/mole)

++

0

20

+

0

20

CF H 3

3

20

+

3

++

Results

03

f r o m r e f e r e n c e s 38 a n d 39.



166

Alkyl Fluorides. The i o n - m o l e c u l e r e a c t i o n m e c h a n i s m o b s e r v e d i n e t h y l f l u o r i d e (31) i s s i m i l a r t o t h a t d e s c r i b e d a b o v e f o r t h e CH^F s y s t e m . The p a r e n t i o n t r a n s f e r s a p r o t o n t o t h e parent molecule: C H F 2

5

+ C H F -> C H F H

+

2

5

2

5

+ C ^ F

+

(47)

and t h e p r o t o n a t e d p a r e n t i o n r e a c t s w i t h t h e e t h y l molecule to d i s p l a c e HF: C H FH


2

5

+ C H F •> C H F C H

+

2

5

2

5

2

and f o r m t h e d i e t h y l f l u o r o n i u m i o n . undergoes the r e a c t i o n s : C H F 2

4

+

+ C H F -> C H 2

5

2

5

+

+ C F H 2

2 5 2

C

H

-> C H 4

F

?

+

+

+

C

2

H

C^H^F*

(F~ t r a n s f e r ) (

+ 2 HF

(48)

The f r a g m e n t

4

2 4

+ HF

+

5

fluoride

F

+

ion

(49)

transfer)

(50)

(Condensation)

(51)

Of t h e i o n s g e n e r a t e d i n r e a c t i o n s 49, 5(j), and |1, only C ^ reacts further with C H_F, giving C H FH , C H , and C . H as j . . ZD Ζ j J .> H y product i o n s . 2

0

0

C

0

C

+

Q

A somewhat d i f f e r e n t p a t t e r n o f r e a c t i v i t y i s o b s e r v e d i n CH CHFCH (40). The p a r e n t CH CHFCH i o n fragments to y i e l d CH CFCH +, CH CHF , and CH CHF+ a s t h e a b u n d a n t p r i m a r y i o n s ^ a l l of whicn r e a c t w i t h the parent 2-fluoropropane molecule by F t r a n s f e r to y i e l d s e c - C ^ H - as a p r o d u c t . The s e c - C ^ ion, in t u r n , r e a c t s w i t h the 2-fluoropropane molecule to regenerate CH^CFCHg , p r o b a b l y t h r o u g h a d i p r o p y l f l u o r o n i u m i o n i n t e r mediate: 3

2

+

sec-C H 3

7

+

+ CH CHFCH 3

3

+ (CH ) CHFCH(CH ) 3

2

3

2

+

->• ( C H ^ C F *

+

C^g

(52) -

C H 6

1 3

+

+ HF HF (53)

Since the CH^FCH^ i o n p r o d u c e d i n r e a c t i o n 52 w i l l r e a c t t o regenerate s e c - C ^ ^ , a chain r e a c t i o n occurs i n t h i s system. The c o n c e n t r a t i o n o f t h e c h a i n c a r r i e r i s a t t e n u a t e d , h o w e v e r , the occurrence of the a l t e r n a t e r e a c t i o n channel 53.


by

6.

LIAS

167


Reactions

o f F l u o r i n a t e d Ions w i t h Other Organic

Compounds

Reactions with Alkanes. F i g u r e 1 shows the_ p r o b a b i l i t y tljiat a h y d r i d e _ t r a n s f e r r e a c t i o n w i l l o c c u r when C F ^ , C F ^ H , C F ^ C l , or CC1 F undergoes a c o l l i s i o n w i t h t h e C - C l i n e a r alkanes (41). 9

9

1

1

CX 3


Q

+

+ C R_ η 2n+2

CX.H + C H 3 η

0

Hydride transfer reactants.

+

0

Q

9

,2n+l

(54)

i s the only a v a i l a b l e r e a c t i o n channel f o r these

I

01 0 1

I

I 1 I ι I 2 3 4 5 6 7 n-ALKANE CHAIN LENGTH

1

1 8

9

International Journal of Mass Spectrometry and Ion Physics

Figure 1. Probabilities of a reactive collision be tween CF H+(X), C F / f # j , CF Cl(S7), CCl F (0), and the linear alkanes. Probabilities are estimated by dividing measured rate coefficients by the rate coefficient for collision of the particular ion-mole cule pair. 2

2

2

+

I t f o l l o w s from F i g u r e 1 t h a t f o r each o f t h e s e i o n s , t h e p r o b a b i l i t y of a reactive c o l l i s i o n increases with increasing chain l e n g t h o f the n - a l k a n e . Table IV l i s t s t h e enthalpies of r e a c t i o n f o r e a c h o f t h e s e i o n s a t a s e c o n d a r y s i t e and a p r i m a r y s i t e i n η-butane; these v a l u e s w i l l give a n i n d i c a t i o n o f t h e ordering of reaction enthalpies f o r hydride transfer reactions of these i o n s .


168


Table IV.


Reaction

E n t h a l p i e s of Hydride Transfer Reactions Halomethyl Ions w i t h η-Butane.

Enthalpy (e,V Secondary S i t e

kcal/mole) Primary S i t e

-2.0,

•46.4

•1.2,

28.4

-1.6,

36.8

0.8,

•18.8

-1.4,

•31.7

•0.6,

•13.7

•0.4,

+8.7

-0.4,

- ( C F H ) * -> C ^ H ^ + HF 2

2

4

3

4

+

(55) (56) (57)


Root; Fluorine-Containing Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978. 45% 60%

9% 36% 3%

6.7

7.6

8.2

6

8

2-C H

g

5

Results from reference 42.

4

c-C H

3

c-C H CH

4

3

c-C H

3

CH CHCH

2

84%

15%

85%

4.7

3

18%

82%

6.7

2

21%

79%

7.5

CH CCH

2

22%

3

4

78%

2

C H

7.5

Hydride Transfer

CH CCH

100%

CondensationDissociation

with Alkenes,

15%

0

3

85%

1.2

1

3 V x l O cm /molecule-second

Rate Constants and Modes of Reaction of C F Cycloalkanes, and Alkynes.

7.2

2 2

Reactant Molecule

Table V I .


LIAS

6.

171


A t p r e s s u r e s a r o u n d 10 t o 10~ t o r r , t h e a b u n d a n c e s o f t h e i n t e r m e d i a t e a d d u c t i o n s were n e g l i g i b l y s m a l l ( C F H 3

2

3

2

(58)

+

whic_h a c c o u n t s f o r a b o u t 95% o f t h e r e a c t i v e e n c o u n t e r s b e t w e e n CF and C H a t 1 0 " t o r r . H o w e v e r , o n l y 10% o f t h e C F - a c e t y lene c o l l i s i o n s lead to r e a c t i o n . T h u s , when C F a n d a c e t y l e n e c o l l i d e , the adduct has a h i g h p r o b a b i l i t y of s i m p l y f a l l i n g a p a r t to regenerate the o r i g i n a l r e a c t a n t s . Depending on the s t r u c t u r e one assumes t h e C F H c o n d e n s a t i o n i o n t o h a v e , i t c a n be e s t i m a t e d t h a t i t s i n t e r n a l e n e r g y c o n t e n t w i l l be 5 0 - 6 5 k c a l / m o l e . I n t h e r e a c t i o n s o f C F w i t h e t h y l e n e , p r o p y n e , and p r o p a d i e n e , condensation i o n s w i t h c o n s i d e r a b l e excess energy a r e a l s o formed, b u t t h i s e n e r g y i s l a r g e l y d i s p o s e d o f b y e l i m i n a t i o n o f HF (reactions 55-57). I t c a n be e s t i m a t e d t h a t t h e o v e r a l l r e a c t i o n : 3

2

2

+

3

3

3

3

2

+

3

CF

3

+

+ C H 2

2

+ (C F H 3

3

2

+

)*

HF + C ^ H *

(59)

i s a s much a s 50 k c a l / m o l e e x o t h e r m i c , a n d i n d e e d , a b o u t 5% o f t h e p r o d u c t i o n s formed i n t h e r e a c t i o n between C F and a c e t y l e n e a r e C«F H . H o w e v e r , t h e l o w p r o b a b i l i t y o f t h i s c h a n n e l i n d i c a t e s t h a t t h e d i s s o c i a t i o n l e a d i n g t o HF e l i m i n a t i o n i s n o t r e a d i l y a v a i l a b l e to the C J ? ^ 2 ^ _ * I t i s generally ac c e p t e d t h a t t h e s t r u c t u r e o f a C^H^ i o n i s c y c l i c , a n d i t i s r e a s o n a b l e t o assume t h a t t h e C^F^H a n a l o g u e w o u l d a l s o h a v e a s i t s most s t a b l e s t r u c t u r e , a c y c l i c s t r u c t u r e . The £^3^2 " d e n s a t i o n i o n w o u l d most l i k e l y i n i t i a l l y b e f o r m e d w i t η e i t h e r o f t h e two s t r u c t u r e s : 3

2

c

o

n

e

n

s

a

t

i

o

n

i o n

C

CF CHCH 3

O

N

+

III E i t h e r o f t h e s e i n t e r m e d i a t e s w o u l d h a v e t o u n d e r g o some r e a r r a n g e m e n t i n o r d e r t o e l i m i n a t e HF a n d f o r m a p r o d u c t i o n o f t h e structure:


172


I n the c a s e o f r e a c t i o n s 5 5 , 5 6 , and 5 7 , p r o d u c t i o n s of a s t a b l e s t r u c t u r e c a n be f o r m e d by e l i m i n a t i o n o f HF f r o m c o n d e n s a t i o n i o n s w i t h o u t t h e n e c e s s i t y f o r much r e a r r a n g e m e n t . When C F ^ r e a c t s w i t h p r o p y l e n e o r c y c l o p r o p a n e , 82-85% o f the r e a c t i v e encounters r e s u l t i n a d i s s o c i a t i o n of the i n t e r mediate i o n to e l i m i n a t e C H C F : +

2


CF

+ C H

+

3

3

(C F H

6

4

The r e m a i n d e r o f transfer: C

3

F

+

3

) * -> C F H

+

6

2

2

4

+ C H ^

+

(60)

the r e a c t i o n between these p a i r s i s h y d r i d e

+

C

3 6 H

C

F

3

H

+

C

3 5 H

(61)

+

E s s e n t i a l l y , t h e same p a t t e r n o f r e a c t i v i t y i s s e e n f o r h i g h e r homologue o f p r o p y l e n e , 1 - b u t e n e .

the next

I n s i g h t i n t o t h e mechanism of t h e C H C F e l i m i n a t i o n , has b e e n o b t a i n e d b y u s i n g c y c l o p r o p a n e - 1 , 1-cL and C H C H C D . If the H s and D s a r e s t a t i s t i c a l l y d i s t r i b u t e d t h r o u g h o u t £he C ^ D ^ i n t e r m e d i a t e s , one w o u l d e x p e c t t o s e e o 4 > 2 3 ' C FH D product i o n s i n the r a t i o 0 . 0 7 : 0 . 5 3 : 0 . 4 0 , w h i l e a mechanism i n v o l v i n g t r a n s f e r of an u n r e a r r a n g e d C H ( C D ) group would result i n a 0.33:0:0.67 d i s t r i b u t i o n . I n the propylene experiment, the r a t i o C F H : C FH D : C J H D was 0 . 0 8 : 0 . 4 9 : 0 . 4 3 , w h i c h corresponds to a s t a t i s t i c a l d i s t r i b u t i o n , w i t h i n experimental error. I n c y c l o p r o p a n e , t h e r a t i o o f t h e s e ion|.c p r o d u c t s was 15:35:49. T h i s means t h a t a b o u t 30% o f t h e C F - c y c l o p r o p a n e c o n d e n s a t i o n i o n s u n d e r g o d i s s o c i a t i o n p r o c e s s 60 t h r o u g h a c l e a n t r a n s f e r o f a m e t h y l e n e g r o u p , w i t h o u t any r e a r r a n g e m e n t . 2

2

3

f

c

2

2

+

!

2

F H

C

F H

D

a

2

2

2

2

4

3

When C F r e a c t s w i t h 2 - b u t e n e , 90% o f t h e r e a c t i v e e n c o u n t e r s a r e a c c o u n t e d f o r by t h e e x o t h e r m i c h y d r i d e - and charge transfer reactions: 3

+

C F * + C , H -> C . H _ + C F H 3 4 8 4 7 3

(62)

CF

(63)

+

Q

3

+

+ C H 4

-> C H

g

4

g

+

0

+ CF

3

which a r e of a p p r o x i m a t e l y e q u a l importance (Table ¥ 1 ) . The o n l y o t h e r p r o d u c t i o n o b s e r v e d i n a 2 - b u t e n e - C F m i x t u r e i s C^F^.^ . T h i s i s t h e i o n w h i c h w o u l d be f o r m e d i f t h e p r o d u c t o f a n H F e l i m i n a t i o n r e a c t i o n h a d enough e x c e s s e n e r g y t o d i s s o c i a t e f u r t h e r by l o s i n g C H : 4

2

CF

3

+

+ C H 4

g

4

(C F H 5

3

8

+

) * -> C F H 3

2

3

+

+ HF + C ^

(64)


LIAS

6.

173


The isomeric c y c l i c alkanes, cyclobutane and methyleyclopropane a l s o undergo r e a c t i o n s 62, 63, and 64, although f o r these compounds hydride t r a n s f e r r e a c t i o n 62 i s of major importance. The o b s e r v a t i o n of C^Hg* ions i n mixtures c o n t a i n i n g c y c l o butane or methylevclopropane i s somewhat s u r p r i s i n g , s i n c e charge t r a n s f e r from CF^ i s r e s p e c t i v e l y , 38 and 22 kcal/mole (1.6 and 0.95 eV) endothermic. To e x p l a i n the formation of C^Hg i o n s , we must p o s t u l a t e that r i n g opening and rearrangement to the 2-butene or isobutene s t r u c t u r e occurs i n the ion-molecule complex:


+

CF

+

+

+ (c-C Hg or c-C H CH )->(C HgF )*->(2-C Hg

3

4

+ CF

3

5

3

5

3

+

4

+

or i - C H ) 4

g

(65)

3

which would make t h i s r e a c t i o n

exothermic.

Table V I I gives the r e a c t i o n s o c c u r r i n g between O^F^ and or C alkenes and alkynes, as w e l l a^ the observed r a t e s of r e a c t i o n . Again, as i n the case of C F , hydride t r a n s f e r i s observed when i t i s exothermic. The C^F^ i o n a l s o undergoes a CF^ e l i m i n a t i o n r e a c t i o n with these compounds: 3

3 +

C F 2

+ 5

+ M

CF M

+

+ CF

3

(66)

2

which leads to the formation of a produc£ i o n of the same indent i t y as the condensation ions of the C F r e a c t i o n s . However, unl i k e the CF M ions formed i n condensation r e a c t i o n s of C F , the ions formed i n C F e l i m i n a t i o n r e a c t i o n s of C F^ do not undergo f u r t h e r d i s s o c i a t i o n , because excess energy i s c a r r i e d away by the departing C F s p e c i e s . 3

3

3

2

2

2

T h i s i s s u b s t a n t i a t e d by the r e a c t i o n between C Ç ^ + and C H , where the r a t e constant f o r formation of C^F^B.^ i s nearly an order of magnitude greater when the r e a c t a n t i o n i s C.F,, (Table VII) than when i t i s C F . In the r e a c t i o n of ^ 5 ' 2

2

2

2

3

C F 2

+

+ C H

5

2

2

-> C F C H 3

2

+

+ CF

2

(67)

2

the o 3 2 formed w i t h a maximum of about 13 kcal/mole excess energy, while i n r e a c t i o n 58 the i o n has about 50-65 kcal/mole excess energy, and d i s s o c i a t e s with a high proba b i l i t y to g i v e back the o r i g i n a l r e a c t a n t s : c

F

H

+

i

o

n

i

s

C F H 3

3

+ 2

·*· C F

+ 3

+ C H

T h i s d i s s o c i a t i o n i s endothermic r e a c t i o n 67.

2

(68)

2

for 3 3 2 C

F

H

+

i

o

n

s

f

o r m e < i

i

n


FLUORINE-CONTAINING F R E E RADICALS

174

Table V I I .


Reactant

R a t e C o n s t a n t s a n d Modes o f R e a c t i o n o f w i t h A l k e n e s and A l k y n e s .

Molecule

C

2 2

C

2 4

cm

10 χ 10 *Rn /molecule-second 5.0

H

3.8

H

C

3 3 F

C H 2

6.5

CH CCH 2

7.0

2

H

2

F

5

+

+

3

2

3

+

4 3

C

5 3 4

F

C H 3

4

H

5

2

+ C F

+

4

H

2

C F.

+

F

C

H

+ C F

+ C F H

+

3

C H

C

R e s u l t s from r e f e r e n c e

3

H

2 3 2

W 3

2

Products

C

CH CCH

C

2

+ C F H 2

+

5

+ C F

2

+ HF

F

3

+

4 3 F

+ C F H 2

+

5

+ C F

42.


2

LIAS

6.

175


Reactions with Aromatic Compounds. In the reaction of fluorinated methyl ions with benzene and toluene, the most important process i s the HF elimination reaction (42,43). For instance, CFg reacts with these compounds as follows: C F

3

+

+

C

6 6 ~*

( C F

H

C

H

3 6 6

+ )

C

*

C

H

C F

" 6 5 2

+ +

H F

( 6 9 )

C F * + c-C.H.CH. -»• (CF CJK,CH )* -> CH C,H.CF + HF 3 653 3 6 5 3 3 6 4 2 (70) +


0

+

0

0

0

These reactions apparently proceed without any rearrangements occuring i n the ion-molecule complex, since i n the reaction of CH F with C D only DF loss i s observed (43): 2

6

6

CFH

+ 2

+ C D 6

6

C D CH 6

5

+ 2

+ DF

(71) +

An examination of the reaction between CF and c-C^H^CD^ confirmed that the HF elimination process 2 involved only ring hydrogens rather than hydrogens from the methyl group (42). For these aromatic compounds, the major competing reaction channel i s charge transfer, whicji accounts for 8 and 15% of the reactive encounters between CF^ and benzene or toluene, respectively. In toluene, the highly exothermic hydride transfer reaction: C F * + c-C.H.CH -> CF H + c-C,H CH Q

0

c

+

(72)

0

accounts for only about 3% of the reactive encounters. +

The C F ^ ion also reacts with benzene to form a C^H^CF product ion. This reaction can be written: 2

C F 2

+ 5

+ C H -> C H C F 6

6

6

5

+ 2

+ CF H

+ 2

(73)

3

When chlorofluoromethyl ions react with benzene or toluene, i t i s seen that HC1 elimination strongly predominates over HF elimination in reactions analogous to 69-71. Reactions with Oxygenated Compounds. A recent s_tudy (14,4^) has reported on the reactions of CF^*, CF C1 , CC1 F , and C^ with a series of organic molecules containing a carbonyl functional group. These ions undergo four-center reactions with such compounds, which i n the case of aldehydes and ketones lead to a product i n which the carbony|. oxygen of the reactant compound has been replaced by a F (or CI ). For instance, CF^ reacts with acetone as follows: +

2

2

+



176

O...CFj CF ++R'COR

-CF 0+R'CFR

R-C . . . F

3

2

+

(

7

4

)

I R'


The C ^ F ^ ' i o n u n d e r g o e s a n a n a l o g o u s C F. 25 0

+ CH C0CH 3 3

+

o

reaction:

+ CH CFCH 3 3

o

0

0

+ C F.0 24

+

(75)

o

+

+

but i n the c h l o r i n e - c o n t a i n i n g i o n s , CF^Cl and C F C 1 , t h e p r o d u c t i o n c o n t a i n s C l r a t h e r t h a n F_j_ When l a r g e r a l d e h y d e s o r k e t o n e s are allowed to r e a c t w i t h CF^ , the monofluorinated carbonium ions h o m o l o g o u s t o t h a t f o r m e d i n r e a c t i o n 74 a r e o b s e r v e d o n l y i n v e r y s m a l l abundances. I n s t e a d , the major product ions a r e u s u a l l y the i o n s c o r r e s p o n d i n g t o t h e l o s s o f HF f r o m t h e m o n o f l u o r i n a t e d i o n s : 2

CF CF j

3

0

+

+

+ C H COCH 2

5

3

-> C F 0 + ( C H F ) * + C F 0 + HF + C H 2

4

g

+

2

4

+ c - C . H 0 •> C F 0 + ( C . H F ) * -> C F 0 + HF + C - E L Do Ζ Do Ζ D / Q

o

+

0

o

?

+

(76) (77)

+

The e x t e n t o f t h i s d i s s o c i a t i o n i s , n o t u n e x p e c t e d l y , g o v e r n e d b y t h e e n e r g e t i c s o f t h e r e a c t i o n i n w h i c h tljie f l u o r i n a t e d i o n i s formed. I t i s i n t e r e s t i n g t h a t t h e C^HgF i o n f o r m e d i n t h e r e a c t i o n between C F ^ and c y c l o p e n t a n o n e : 2

C

2 5 F

+

+

C

" 5 8° C

H

C

2 4° F

+

C

5 8 H

F +

(

7

8

)

does n o t undergo d i s s o c i a t i o n . I t c a n be e s t i m a t e d t h a t t h e f o u r c e n t e r r e a c t i o n 78 i s a s much a s 4 eV l e s s e x o t h e r m i c t h a n t h e f i r s t step of r e a c t i o n 77. The r e a c t i o n s o f t h e s e i o n s w i t h a c i d s a n d e s t e r s w e r e a l s o examined. R e a c t i o n 79, a d i s p l a c e m e n t i n v o l v i n g c l e a v a g e o f t h e a c y l - o x y g e n bond a n d t h e f o r m a t i o n o f a c y l i u m i o n p r o d u c t s :

Γ

ο

II

RCOOR'+CX +

-RCO+ + ROCX

R-C

3

1

i- cx

3

(79)

3


6.

LIAS

Ion-Molecule

177

Reactions

(where R a n d R a r e H o r a l k y l g r o u p s ) i s i m p o r t a n t f o r many s u c h reaction pairs. I n f a c t , when R i s CH ( i . e . a c e t i c a c i d , a c e t i c a n h y d r i d e , m e t h y l a c e t a t e ) , r e a c t i o n 79 i s t h e o n l y c h a n n e l o b served. f


H o w e v e r , f o r m i c a c i d a n d a l l o f t h e f o r m a t e s ( a l l compounds where R=H) w h i c h w e r e i n v e s t i g a t e d u n d e r g o a l t e r n a t e r e a c t i o n s t o a greater or lesser extent. An a l t e r n a t e r e a c t i o n which i s gener a l l y i m p o r t a n t i s a n o t h e r f o u r - c e n t e r r e a c t i o n , t h i s one i n v o l v i n g breakage of the a c y l - o x y g e n bond: Ο

II OC

3

+

+HCOOR.

H-C... X

- » C X O R + HCXO 2

CX

+

(80)

2

(where R i s Η o r a n a l k y l g r o u p ) . The p r e v a l e n c e o f one c h a n n e l o r a n o t h e r c a n be e x p l a i n e d i n t e r m s o f r e l a t i v e e x o t h e r m i c i t i e s . In t h e c a s e o f t h e R=CH compounds, r e a c t i o n 79 i s a b o u t 2 . 7 eV more exothermic than the f o u r - c e n t e r r e a c t i o n analogous t o r e a c t i o n 80, b u t when R=H, t h e f o u r - c e n t e r r e a c t i o n i s more e x o t h e r m i c . The o c c u r r e n c e o f r e a c t i o n 79 a n d r e a c t i o n 80 i n d i c a t e t h a t t h e h a l o m e t h y l i o n s a t t a c k a c i d s and e s t e r s a t t h e a c y l oxygen r a t h e r t h a n the c a r b o n y l oxygen. R e a c t i o n o f f l u o r o m e t h y l c a t i o n s w i t h ROH (R=H, C H ^ , C ^ ) h a v e b e e n e x a m i n e d ( 4 2 , 4 3 ) , a n d c a n be r e p r e s e n t e d i n a g e n e r a l fashion as: C(H,F)

3

+

+ ROH ·> C ( H , F ) = 0 R 2

+

+ HF

-> C ( H , F ) 0 H + R 3

(82)

+

When R = C H ^ , o n l y t h e l a t t e r c h a n n e l i s 2

(81)

observed.

Thermochemical I n f o r m a t i o n about F l u o r i n a t e d Species Ion-Molecule Reactions

Derived

from

Under t h e u s u a l c o n d i t i o n s used t o i n v e s t i g a t e i o n - m o l e c u l e r e a c t i o n s , r e a c t i o n s w h i c h a r e s t r o n g l y e n d o t h e r m i c c a n n o t be o b s e r v e d , a n d r e a c t i o n s w h i c h a r e c l o s e t o t h e r m o n u e t r a l , (ΔΗ= 0 + ^ 2-3 k c a l / m o l e ) occur w i t h e f f i c i e n c i e s such t h a t l e s s than one c o l l i s i o n i n 10 i s r e a c t i v e . On t h e o t h e r h a n d , i t c a n u s u a l l y be s a f e l y assumed t h a t a n y r e a c t i o n w h i c h i s o b s e r v e d t o o c c u r w i t h a high e f f i c i e n c y i s exothermic. W i t h i n t h e framework o f t h e s e


178

FLUORINE-CONTAINING

F R E E

RADICALS

g e n e r a l o b s e r v a t i o n s , i t i s p o s s i b l e to d e r i v e upper a n d / o r lower l i m i t s for the heats of formation of p a r t i c u l a r ions or molecules from the o b s e r v a t i o n or n o n - o b s e r v a t i o n of p a r t i c u l a r i o n - m o l e c u l e reactions. More q u a n t i t a t i v e i n f o r m a t i o n a b o u t i o n i c h e a t s o f f o r m a t i o n c a n be o b t a i n e d f o r s y s t e m s i n w h i c h a n i o n - m o l e c u l e e q u i l i brium: A


can be o b s e r v e d . v a l u e f o r AG°:

+

+ Β t

C

+

+ D

(83)

The e q u i l i b r i u m c o n s t a n t o b s e r v e d

-RTlnK

eq

= AG° = ΔΗ -

leads to a

TAS

(84)

I f the temperature v a r i a t i o n of the e q u i l i b r i u m constant i s d e t e r m i n e d , o r i f some o t h e r means o f e s t i m a t i n g AS c a n b e e m p l o y e d , a v a l u e f o r the enthalpy of r e a c t i o n i s o b t a i n e d . I n s e v e r a l r e c e n t s t u d i e s , new o r i m p r o v e d e s t i m a t e s o f t h e h e a t s of f o r m a t i o n of s e v e r a l f l u o r i n a t e d i o n s or r a d i c a l s have been o b t a i n e d from such experiments i n v o l v i n g i o n - m o l e c u l e r e a c tions. These r e s u l t s a r e summarized i n T a b l e V I I I . In this t a b l e and T a b l e I X , f o r t h e s a k e o f c o n s i s t e n c y w i t h o t h e r d i s c u s s i o n s o f t h e s u b j e c t i n t h e l i t e r a t u r e , we s h a l l u s e a s t h e u n i t of energy k c a l / m o l e ; 1 k c a l / m o l e corresponds to 4.18 k j o u l e o r 0.04 eV. I n t h e c a s e o f u n c h a r g e d r a d i c a l s and m o l e c u l e s , i t i s w e l l known t h a t t h e s u b s t i t u t i o n o f a f l u o r i n e atom f o r a h y d r o g e n atom r e s u l t s i n a " s t a b i l i z a t i o n " of the species, i . e . a lowering of the heat of formation of the r a d i c a l or molecule. This i s i l l u s t r a t e d i n T a b l e I X , where t h e h e a t s o f f o r m a t i o n o f CH , C H F , CHF , and CF« a r e l i s t e d ( 3 3 , 3 4 , 5 0 , 5 1 ) . In the case of tne corresponding c a t i o n s , i t i s i n t e r e s t i n g t o examine whether the d e c r e a s e d e l e c t r o n e g a t i v i t y d i f f e r e n c e b e t w e e n C and F ( a s compared t o C a n d F ) , and t h e i n c r e a s e d d o u b l e bond c h a r a c t e r o f t h e C - F bond w i l l c a u s e any d i f f e r e n c e s i n t h e " s t a b i l i z a t i o n " e f f e c t e d by f l u o r i n e s u b s t i t u t i o n i n t h e i o n a s compared t o t h e n e u t r a l s p e c i e s . A de t a i l e d LCAO-SCF c a l c u l a t i o n i n t h e INDO a p p r o x i m a t i o n (52) p r e d i c t s that i n these f l u o r i n e - s u b s t i t u t e d i o n s , there i s s u b s t a n t i a l p o l a r i z a t i o n o f t h e c a r b o n - f l u o r i n e π-bond t o w a r d t h e p o s i t i v e l y c h a r g e d c a r b o n - a t o m , b a l a n c e d by a p o l a r i z a t i o n i n t h e o p p o s i t e d i r e c t i o n of the C-F a bond. Thus, although the magnitude of both t h e d e s t a b i l i z i n g σ - p o l a r i z a t i o i j t and t h e s t a b i l i z | . n g π b a c k - d o n a t i o n i n c r e a s e i n g o i n g f r o m CF« t o C F H t o C F H ^ , t h e n e t t o t a l c h a r g e d e n s i t y on f l u o r i n e v a r i e s by o n l y 0 . 0 8 9 m t h i s s e r i e s . The d a t a g i v e n i n T a b l e I X ( 3 9 , 4 7 , 5 3 , 5 4 , 5 5 ) i n d e e d c o n f i r m t h a t t h e energy r e q u i r e d t o e f f e c t i o n i z a t i o n of the C F H , C F H , and CF^ r a d i c a l s i s w i t h i n t h e l i m i t s o f e r r o r , t h e same f o r t h e t h r e e fluorine-substituted species. That i s , the degree of 2

+

2

2

2


6.

LIAS

179


s t a b i l i z a t i o n e f f e c t e d by s u b s t i t u t i o n of one f l u o r i n e atom i s , w i t h i n the l i m i t s of e r r o r of p r e s e n t l y a v a i l a b l e data, not s t r o n g l y i n f l u e n c e d by the presence of the p o s i t i v e charge on the carbon except f o r the f i r s t s u b s t i t u t i o n . jj>ignificantly g r e a t e r s t a b i l i z a t i o n i s e f f e c t e d i n going from CEL to CH^F than from CH^ to CH~F. Some authors (55,56) have defined the " s t a b i l i z a t i o n energy or f l u o r i n e - s u b s t i t u t i o n i n these methyl ions a s t h e d i f f e r e n c e i n the CHg -H bond energy and the corresponding CX^ -H bond energy (where X i s H or F^. _These bond energies are l i s t e d i n Table IX, along w i t h the CX« -F and CX« - C l bond e n e r g i e s . Here i t can be seen that although c o n s i d e r a b l e " s t a b i l i z a t i o n " i s e f f e c t e d by the presence of F atoms i n the methyl group, the number of F-atoms present i n the i o n has, w i t h i n the l i m i t s of e r r o r , l i t t l e e f f e c t upon the amount of s t a b i l i z a t i o n . (This i s j u s t another way of saying that the degree of s t a b i l i z a t i o n per F atom v a r i e s i n the same way i n the i o n as i n the corresponding RH, RF, and RC1 molecules.) E a r l i e r c o n s i d e r a t i o n s of t h i s subject (33,34,55,56) have u s u a l l y l e d to the c o n c l u s i o n that the s u b s t i t u t i o n of F f o r H i n CF H r e s u l t e d i n s i g n i f i c a n t l y l e s s s t a b i l i z a t i o n than the corresponding s u b s t i t u t i o n i n ÇFH^ , p a r t l y because u n t i l r e c e n t l y , the heat of formation of CF« was taken as 99 kcal/mole (46), r a t h e r than 94.1 kcal/mole 145) or 91.5 kcal/mole (47). The data l i s t e d i n Table IX show that the the p r e s e n t l y a v a i l a b l e l i m i t s of e r r o r on the heats of formation of these ions are such that s m a l l d i f ferences i n the degree of " s t a b i l i z a t i o n " e f f e c t e d by consecutive f l u o r i n e - s u b s t i t u t i o n can not be d i s c e r n e d with c e r t a i n t y . 11


+

2

A n i c i c h and Bowers (17) have reached a s i m i l a r c o n c l u s i o n about the s t a b i l i z a t i o n brought about by f l u o r i n e s u b s t i t u t i o n i n the f l u o r i n a t e d ethylene i o n s . They p o i n t out that «| p l o t o£ the experimenta1ly determined heats of formation of C^H^ , C^H^F , ^ 2 2 2 ' ^ 2 3 ' * C^F, f a l l on a s t r a i g h t l i n e , as snown i n F i g u r e 2. +

H

F

H F

+

a n c

Figure 2. Plot of the heat of formation (49) vs. the number of fluorine substituents in the ion series C (H,F) . Figure from Reference 17. 2

+

U



5

+

3

?

+

3

+

2

F

+

2

CC1 F

2

+

+

CF C1

2

CF H

2

CH F

sec-C F

C

+

+

+

+

+

2

3

6

8

2

2

C F -C F

2

C F ->C F

+

3

5

2

4

F

3

+

3

sec-C F

+CF

6 +

+

a 9 b

a

7

5

+

3

7

+

2

2

2

2

2

2

+

"

3

2

+

+

3

3

7

+

C H Cl

2

2

a

sec-C H 4€FCl ^CFCl +

2

+

2

a

5

b

4

C H +CF Cl ->€F Cl +C H Cl

2

+

+

a

1

Rn 1

k

c

l

/ m

o

l

e

a

l

/ m

o

l

e

> -1.2

2

k

kcal/mole

R n / c o l l i s i o n ^0.008

+

+

f

ATÎ >157 + 1.5

f

128+5

Ion-Molecule Reactions Involving Fluorine-Containing Organic

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