Radical Production from the Interaction of Closed-Shell Molecules

3 Radical Production from the Interaction of Closed-Shell Molecules Part 7. Molecule-Assisted Homolysis, One-Electron Transfer, and Non-Concerted Cycloaddition Reactions

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch003

WILLIAM A. PRYOR Department of Chemistry, Louisiana State University, Baton Rouge, L A 70803 For some time, my research group has been i n t e r e s t e d in the processes by which c l o s e d - s h e l l , s t a b l e molecules i n t e r a c t t o pro duce f r e e r a d i c a l s (1-6). We have d i v i d e d these r e a c t i o n s i n t o three mechanistic types: m o l e c u l e - a s s i s t e d homolyses (ΜΑΗ), one - e l e c t r o n t r a n s f e r s , and non-concerted p e r i c y c l i c r e a c t i o n s . These processes u s u a l l y occur a t s u r p r i s i n g l y moderate temperatures and often have an i n t r i g u i n g and c h a l l e n g i n g k i n e t i c and mechanistic complexity. A l a r g e number o f r e a c t i o n s of all three mechanistic types are now known. Many of these processes f i n d p r a c t i c a l use. For example, some processes t h a t appear t o i n v o l v e ΜΑΗ r e a c t i o n s are used i n chemical i n d u s t r y : the commercial p o l y m e r i z a t i o n of styrene o f t e n is self-initiated (7); the a u t o x i d a t i o n of acetaldehyde is ini tiated by ppm l e v e l s of ozone (8); and some halogenations are c l e a r l y ΜΑΗ processes (1,9). E l e c t r o n - t r a n s f e r r e a c t i o n s f i n d p r a c t i c a l use as low-temperature initiation systems: f o r example, benzoyl peroxide and d i m e t h y l a n i l i n e produce r a d i c a l s at tempera tures as low as 10°, perhaps by an e l e c t r o n t r a n s f e r process (3, 4 ) . R a d i c a l production from [2+2] c y c l o a d d i t i o n r e a c t i o n s a l s o has been c l e a r l y demonstrated by a number of workers, using both experimental and t h e o r e t i c a l techniques (9a). However, the i n t e r c e p t i o n of the intermediate 1 , 4 - d i r a d i c a l s by added r a d i c a l t r a p ping reagents has been reported in only a few cases (5,10-16). In the l i m i t e d space a v a i l a b l e here, I will not attempt a complete summary or a d e t a i l e d review of the three c l a s s e s of r e a c t i o n s ; r a t h e r , I will present an e c l e c t i c report of a few r e a c t i o n s on which my own group has done research in the past s e v e r a l years. First, l e t me define the three c l a s s e s of processes which we will consider. M o l e c u l e - a s s i s t e d homolyses g e n e r a l l y have a form u l a t i o n l i k e that given in eqs 1a o r 1b, where the homolysis of the A-B bond is a s s i s t e d by some type of bond-formation process

©0-8412-0421-7/78/47-069-033$10.00/0

Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

ORGANIC F R E E

34

RADICALS


with molecule(s) C * E l e c t r o n t r a n s f e r r e a c t i o n s can be formulat ed as shown i n eqs 2a or 2b; these r e a c t i o n s can be followed by A-B + C

• A* + B O

(la)

A-B + 2C

• AO

(lb)

+ BO

A-B + C

• A-B*

+ Ο

AB + C:

• AB~ + C*

(2a) (2b)

the s c i s s i o n of the A-B· or A-B~ bond. We s h a l l l i m i t our d i s cussion of p e r i c y c l i c r e a c t i o n s to [2+2] c y c l o a d d i t i o n s which lead to scavengable r a d i c a l s ; these processes can be described as i n eqs 3 and 4, where S i s a r e a c t i v e molecule or r a d i c a l that can trap the 1 , 4 - d i r a d i c a l i n competition with the r i n g c l o s u r e r e a c t i o n , eq 4a, l e a d i n g to cyclobutanes.

ii

+

ii — •

3

J~L

(·Μ ·) 2

[~|

(4a)

•SM *

(4b)

2

M o l e c u l e - A s s i s t e d Homolyses I n t r o d u c t i o n . The f i r s t systematic d i s c u s s i o n of t h i s f i e l d , to my knowledge, was by Semenov (17), i n h i s book published i n E n g l i s h t r a n s l a t i o n i n 1958. I discussed the p r i n c i p l e s of ΜΑΗ r e a c t i o n s and reviewed a number of examples i n my book i n 1966 (1), and Benson (18) has considered the a p p l i c a t i o n of h i s thermochemical techniques to such processes. In 1974, Harmony presented a r e l a t i v e l y complete survey of ΜΑΗ r e a c t i o n s , i n c l u d i n g 150 r e f erences ( 9 ) . Her review does not cover non-concerted c y c l o a d d i t i o n s , nor does she t r e a t one-electron t r a n s f e r r e a c t i o n s i n depth. Her chapter a l s o gives an u n s a t i s f a c t o r y p i c t u r e of

*In the textbook "Free R a d i c a l s " , réf. 1, I used the expression "molecule-induced homolysis" and the a b b r e v i a t i o n MIH, a phraseology r e t a i n e d by Harmony ( 9 ) . More r e c e n t l y , I have used "molecule-assisted homolysis" to avoid confusion of ΜΑΗ processes w i t h the induced decomposition of i n i t i a t o r s caused by r a d i c a l s , a propagation s t e p , rather than an i n i t i a t i o n process: AB + R»

A* + RB


3.

PRYOR

Radical Production from Closed-Shell Molecules

35


r a d i c a l production from the s e l f - i n i t i a t e d p o l y m e r i z a t i o n of v a r i ous v i n y l monomers ("thermal p o l y m e r i z a t i o n s " ) , but we have r e c e n t l y reviewed that area i n d e t a i l ( 7 ) . Nevertheless, her review does provide, f o r the f i r s t time, a c o m p i l a t i o n of most of the key references on ΜΑΗ processes i n non-polymer systems. An a c c e l e r a t e d homolysis i s d e f i n e d as one that proceeds at an a c c e l e r a t e d r a t e over that expected f o r a simple unimolecular homolysis. I f a compound A-B undergoes unimolecular homolysis, eq 5, the r a t e constant can be p r e d i c t e d from the Arrhenius equation, eq 6, where BDE(A-B) i s the bond d i s s o c i a t i o n energy of the A-B bond (18). However, i f AB undergoes an a s s i s t e d homolysis A-B

• Α· + Β·

(5)

16

k = A exp(-E /RT) = 10 exp[-BDE(A-B)/RT] s e c "

1

a

(6)

with another molecule (or molecules) C, eq 7, then the heat of r e a c t i o n f o r t h i s process can only be c a l c u l a t e d i f the s t o i c h i o metry of the r e a c t i o n i s known. For example, i f the process can be described by eq l a , then the heat of r e a c t i o n equals the BDE of the A-B bond minus that of the B-C bond; thus, the o v e r a l l AB + nC

• Radicals

(7)

endothermicity of the process i s reduced. In general the a c t i v a t i o n energy a l s o i s reduced and an a c c e l e r a t i o n i n the r a t e of the r e a c t i o n i s observed. ΜΑΗ T r a n s f e r of a Hydrogen Atom. The a s s i s t e d t r a n s f e r of a hydrogen atom, o f t e n from a C-H bond, i s perhaps the most f a s c i n a t i n g of these processes to an organic chemist. A s s i s t e d C-H bond homolyses were p o s t u l a t e d very e a r l y to r a t i o n a l i z e complex r a d i c a l c h a i n r e a c t i o n s . For example, i n 1947 Hinshelwood (19) pro posed that the p r i m o r d i a l i n i t i a t i o n process i n hydrocarbon auto x i d a t i o n s i s the spontaneous hydrogen a b s t r a c t i o n r e a c t i o n of oxygen, eq 8. T h i s process remains c o n t r o v e r s i a l to t h i s day (20, 21). Since oxygen i s not a c l o s e d - s h e l l molecule, but a ground RH + 0

2

• R* + Η 0 · 2

(8)

s t a t e t r i p l e t , eq 8 a c t u a l l y does not meet the formal d e f i n i t i o n of an ΜΑΗ process. However, i t i s u s u a l l y discussed with other examples, s i n c e i t has a s u p e r f i c i a l s i m i l a r i t y (and perhaps a l s o because i t has some of the same e x a s p e r a t i n g l y i n a c c e s s i b l e exper imental f e a t u r e s ) . A hydrogen atom t r a n s f e r from carbon i n an ΜΑΗ process was proposed by F l o r y i n 1937 to r a t i o n a l i z e the spontaneous polymeri z a t i o n of styrene ( 7 ) . He suggested that 1 , 4 - d i r a d i c a l s are formed v i a eq 9 and are converted to monoradicals by the t r a n s f e r r e a c t i o n shown i n eq 10. However, Hammond, Kopecky, and our group


36

ORGANIC F R E E RADICALS

have used k i n e t i c i s o t o p e e f f e c t s to r u l e out r e a c t i o n 10 as the predominant process that converts d i r a d i c a l s to monoradicals i n styrene ( 7 ) . 2PhCH=CH

• PhCH-CH -CH -CHPh

2

2

(9)

2

(·Μ ·) 2

·Μ · + M

• PhCH=CH-CH -CHPh + PhCH-CH

2

2

(ΗΜ ·) Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch003

2

3

(10)

(ΗΜ·)

Another mechanism f o r the spontaneous i n i t i a t i o n r e a c t i o n i n styrene was proposed by Mayo i n 1961 (7). I t i n v o l v e s the reac t i o n s shown i n eqs 11-12.

(11)

(12)

A*

(or ΗΜ·) + M

• M^

(13)

(Growing polymeric chain r a d i c a l ) Recently, the occurrence of the ΜΑΗ process, eq 12, has been probed by two methods: ( i ) Buchholz and K i r c h n e r (22) and Pryor and P a t s i g a (6a,20) used the uv absorptioh of AH t o f o l l o w i t s r a t e of appearance and measure i t s steady s t a t e c o n c e n t r a t i o n . Buchholz and K i r c h n e r o b t a i n a steady s t a t e concentration f o r AH of about 0.6 χ 10" M a t 64°. ( i i ) We had p r e v i o u s l y published a computer s i m u l a t i o n (23) of the thermal polymerization of styrene i n which we assumed that eqs 11-12 were the only i n i t i a t i o n mechanism; t h i s s i m u l a t i o n p r e d i c t s the steady s t a t e concen t r a t i o n of AH t o be 5 χ 10" M a t 60°. T h i s c e r t a i n l y i s i n acceptable agreement with the l a t e r experimental measurement by Buchholz and K i r c h n e r . In a d d i t i o n , our s i m u l a t i o n gives the chain t r a n s f e r constant f o r AH, i . e . , k i ^ / k - i s , t o be about 1. Thus, AH i s a remarkably r e a c t i v e hydrocarbon toward r a d i c a l s . α

4


3.

PRYOR


37

(For example, the t r a n s f e r constant of Ph CH i s only 3 χ 10"** at 60° (23a).) 3

AH + M£ M

.

+

• Α· + Mn-H

Μ

>

Μ

Ή

+

(14) (15)

1


A l a r g e number of ΜΑΗ t r a n s f e r s of hydrogen atoms have been i n f e r r e d , and i n the space a v a i l a b l e here we can do no more than l i s t a few to i n d i c a t e the d i v e r s i t y and wide occurrence of the process. For example, r e a c t i o n 16 was p o s t u l a t e d by Seraenov to r a t i o n a l i z e gas phase cracking r a t e s (17). C H -H + CH =CH 2

5

2

• 2C H *

2

2

(16)

5

Many of the halogens g i v e ΜΑΗ r e a c t i o n s , but the mechanism u s u a l l y i n v o l v e s a t t a c k on the halogen molecule by π-electrons from an unsaturated molecule as a halogen atom adds t o the unsat urated l i n k a g e : X R C=CR 2

+

2

X

• R !:-CR

2

2

2

+

X·

However, some halogenations i n v o l v e ΜΑΗ Η-atom t r a n s f e r s . The f i r s t example discovered, and perhaps the most c l e a r l y e s t a b l i s h e d occurs i n the f l u o r i n a t i o n of organic m a t e r i a l s (27). Here, the d r i v i n g f o r c e i s c l e a r l y the very l a r g e exothermic!ty of processes l i k e eq 17; e.g., f o r propane, eq 17 would be 3 kcal/mole exo thermic . R-H + F

2

• R*

+ HF + F*

(17)

A v a r i e t y of halogenation agents evidence ΜΑΗ processes. Many of the r e a c t i o n systems are q u i t e complex, and i t i s not always c l e a r whether C-H bond breaking i s i n v o l v e d or not. For example, W a l l i n g and h i s coworkers have s t u d i e d tert-butyl hypo c h l o r i t e i n d e t a i l . T h i s reagent r e a c t s with a v a r i e t y of organic compounds, i n c l u d i n g ethers, aldehydes and a l c o h o l s i n ΜΑΗ pro cesses (28) . We have already mentioned the r e a c t i o n of ozone with a l d e hydes, and the f a c t that an ΜΑΗ process i s used i n commerce to i n i t i a t e the a u t o x i d a t i o n of acetaldehyde to produce p e r a c e t i c a c i d . Ozone r e a c t s w i t h a wide v a r i e t y of organic m a t e r i a l s to produce r a d i c a l s ; these r e a c t i o n s are f a s c i n a t i n g because they occur at very low temperatures and with almost every c l a s s of organic compound (29-33). Even alkanes r e a c t — i n t h i s case to give a l c o h o l s , eq 18. RH +

0

3

[R-

·0 Η] 3

ROH

+

0

2


(18)

38


Although the process i s p a r t i a l l y s t e r e o s p e c i f i c , r a d i c a l s a r e postulated t o be involved (34,34a,35). We w i l l r e t u r n to t h i s subject when we d i s c u s s r e a c t i o n 19, i n which the ΜΑΗ of an 0-H bond occurs. ROOH + 0

• R00- + Η0· + 0

3

(19)

2


R a d i c a l s are produced i n the low temperature r e a c t i o n s of dimethylaminomalononitrile, but the mechanism i s unknown (36). An ΜΑΗ s c i s s i o n of a C-H bond, eq 20, can be suggested (36), although t h i s r e a c t i o n would be s t r o n g l y endothermic and probably too slow. NMe

2

NMe

NMe

2

CN—CH—CN + H-C(CN)

2

NMe

2

2

• CN—CH* + HNC + *C(CN)

2

(20)

A s s i s t e d t r a n s f e r s of a hydrogen atom from two other a t o m s — s u l f u r and oxygen—might be mentioned t o demonstrate the v a r i e t y of ΜΑΗ processes. H i a t t and B a r t l e t t (24) have shown that e t h y l t h i o g l y c o l a t e and styrene r e a c t t o produce r a d i c a l s . At high t h i o l / s t y r e n e r a t i o s , the r a t e of r a d i c a l production i s too f a s t to be accounted f o r by the thermal r e a c t i o n of styrene with i t s e l f , and we (2,7) have suggested that the r e a c t i o n i s a d i r e c t ΜΑΗ of an S-H bond, eq 21. RSH + PhCH=CH

• RS« + PhCH-CH

2

(21)

3

Styrene a l s o r e a c t s with hydroperoxides to produce r a d i c a l s (25, 26). A priori one might have thought that a hydrogen atom t r a n s f e r would be i n v o l v e d , eq 22; however, isotope e f f e c t data of 9

ROOH + PhCH=CH

2

• R00- + PhCH-CH

3

(22)

Walling and Heaton do not support t h i s formulation. This reaction system has some of the most complex and b a f f l i n g k i n e t i c and mech a n i s t i c d i f f i c u l t i e s of any ΜΑΗ system y e t s t u d i e d . Demonstration of ΜΑΗ Hydrogen Atom T r a n s f e r s from C-H Bonds. The S e l f - I n i t i a t e d Polymerization Methylenecyclohexadiene

of Styrene and the Chemistry of

(Isotoluene)

I n t r o d u c t i o n . The b r i e f review given above i n d i c a t e s that ΜΑΗ r e a c t i o n s i n general, and a s s i s t e d homolysis of C-H bonds i n p a r t i c u l a r , can no longer be regarded terra incognita. Yet, few systems are w e l l understood o r have mechanisms that are known with c e r t a i n t y . Our love a f f a i r w i t h t h i s f i e l d began some twenty years ago with s t u d i e s of the thermal polymerization of styrene.



3.

PRYOR

39


T h i s i s a p a r t i c u l a r l y f a s c i n a t i n g process. Styrene i s an un u s u a l l y s t a b l e molecule ( i t has only v i n y l i c and aromatic hydro gens) and yet i t i n i t i a t e s i t s own polymerization at a w e l l defined and r e p r o d u c i b l e r a t e . (The r a t e i s about 1%/hr at 90°.) The i n i t i a t i o n r e a c t i o n has been c o n v i n c i n g l y shown to i n v o l v e styrene i t s e l f , and not some a d v e n t i t i o u s impurity ( 7 ) . C u r r e n t l y the most popular mechanism to r a t i o n a l i z e the s e l f i n i t i a t e d polymerization of styrene, eqs 11-13, i n v o l v e s the form a t i o n of the D i e l s - A l d e r adduct, AH, from two styrene molecules, and the ΜΑΗ r e a c t i o n of t h i s dimer w i t h a t h i r d styrene molecule. T h i s novel and very elegant mechanism was o r i g i n a l l y suggested by Frank Mayo, and d e r i v e d from h i s observation of p h e n y l t e t r a l i n and phenylnaphthalene among the oligomers. I t i s c l e a r that AH i s the source of the p h e n y l t e t r a l i n - t y p e products. However, while a c r i t i c a l review (7) of the evidence e s t a b l i s h e s that AH i s present during p o l y m e r i z a t i o n , there i s no c o n c l u s i v e evidence that i t i s i n v o l v e d i n eq 12, the ΜΑΗ step. No synthesis or i s o l a t i o n of AH has been reported, although our recent work (6) suggests that AH could be prepared in situ, at l e a s t . However, we have reported the synthesis of an analogue of AH which was chosen so as to have a lower d r i v i n g f o r c e f o r Η-atom t r a n s f e r (and consequently to be more e a s i l y i s o l a b l e than AH) and s t i l l be s u f f i c i e n t l y r e a c t i v e so as to be able to i n i t i a t e the p o l y m e r i z a t i o n of o l e f i n s by an ΜΑΗ mechanism. The adduct, BH, formed from 2-vinylthiophene and 4-phenyl-l,2,4t r i a z o l i n e - 3 , 5 - d i o n e , appears to have t h i s property. Solutions (0.01 to 0.1 M) of BH appear to i n i t i a t e the p o l y m e r i z a t i o n of styrene and a l s o of methyl a c r y l a t e , a monomer which does not i n i t i a t e i t s own p o l y m e r i z a t i o n , eq 23.

(23)

(BH)

(BO

ΜΑΗ Reactions of Methylenecyclohexadiene ( I s o t o l u e n e ) . In a quest f o r a simpler and perhaps more r e a c t i v e model f o r AH, my group (6,6a), and Kopecky and Lau (37) independently and s i m u l taneously, have s t u d i e d the r e a c t i o n s of methylenecyclohexadiene (MCH) . I n i t i a l l y , both we and Kopecky u t i l i z e d the synthesis invented by B a i l e y (38), but that method can provide only d i l u t e s o l u t i o n s of MCH and r e q u i r e s r e p e t i t i v e and time-consuming


40


p r e p a r a t i v e g l p c s e p a r a t i o n s . Therefore, both groups developed new syntheses. Ours u t i l i z e s the p y r o l y s i s of compound I .

+


(I)

CO

(24)

(MCH)

S u r p r i s i n g l y , MCH i s r e l a t i v e l y s t a b l e i n a c i d f r e e , degassed heptane or benzene; however, MCH does rearrange to form toluene with a h a l f l i f e of about 50 hrs at 80°.* However, MCH decomposes i n styrene s o l u t i o n (ca. 10" M) with a f i r s t order r a t e constant of 4.3 χ 10~ s e c " at 60°, corresponding to a h a l f l i f e of 27 minutes. T h i s a c c e l e r a t e d disappearance of MCH i n styrene could be due to s e v e r a l processes: (1) an ΜΑΗ i n i t i a t i o n r e a c t i o n ; (2) a D i e l s - A l d e r r e a c t i o n of MCH w i t h styrene or, l e s s l i k e l y at the concentrations s t u d i e d , with i t s e l f ; (3) an ene r e a c t i o n of MCH with styrene or i t s e l f ; (4) c h a i n t r a n s f e r of MCH with the p o l y s t y r y l r a d i c a l produced from the s e l f - i n i t i a t e d polymerization of styrene. F u l l y expecting that MCH would i n i t i a t e the p o l y m e r i z a t i o n of styrene, we i n v e s t i g a t e d s o l u t i o n s from 0.001 to 0.01 M at 60°. To our disappointment and c o n s i d e r a b l e s u r p r i s e , MCH does not cause an increase i n the observed r a t e of thermal p o l y m e r i z a t i o n . Kopecky and Lau have reached the same c o n c l u s i o n . As I have s a i d , the h a l f l i f e of MCH i s about 27 minutes i n styrene at 60°, and only a few percent of the styrene i s converted to polymer i n the f i r s t hour of r e a c t i o n . Therefore, we were concerned that the t r a n s f e r and ene r e a c t i o n s of MCH might be d e p l e t i n g i t s c o n c e n t r a t i o n so r a p i d l y that there was none l e f t to undergo the ΜΑΗ r e a c t i o n . That i s , we were concerned that we had not used s u f f i c i e n t l y high i n i t i a l concentrations of MCH to observe i n i t i a t i o n by i t . However, as I w i l l show below, t h i s i s not the case. Before we consider the p o s s i b l e r e a c t i o n s of MCH and the f r a c t i o n of MCH undergoing each p o s s i b l e pathway, l e t ' s examine the p o s s i b l e r e a c t i o n s of AH i t s e l f . 3

4

1

*The r a t e appears to be slower i n the presence of hydroquinone, suggesting a r a d i c a l c h a i n mechanism f o r the rearrangement of MCH to toluene (6b). ( I n the presence of oxygen, the product of the r e a c t i o n i s b e n z y l hydroperoxide (6b). ) However, i t i s d i f f i c u l t to f i n d an i n h i b i t o r that produces a r a d i c a l that does not ab s t r a c t hydrogen from the s u p e r - r e a c t i v e MCH. (For example, DPPH and g a l v i n o x y l destroy MCH, and of course styrene cannot be used as an i n h i b i t o r . ) Thus, i t i s d i f f i c u l t to prove that the r e arrangement i n v o l v e s a r a d i c a l c h a i n .



3.

PRYOR


41

Figure 1 shows the r e a c t i o n s of AH, and i t can be seen that major processes using up AH are: the formation of trimer, the ΜΑΗ process, and chain t r a n s f e r . The r a t e constants f o r a l l three of these r e a c t i o n s can be estimated i n the f o l l o w i n g way (6,6a). If i t i s assumed that a l l of the t r l m e r i c product Α-Sty i s produced by an ene r e a c t i o n (eq £ i n F i g u r e 1A) r a t h e r than by r a d i c a l r e combination, r e a c t i o n Jj, then the r a t e constant f o r the ene reac t i o n of AH can be c a l c u l a t e d from the r a t e of appearance of the t r i m e r Α-Sty measured by Buchholz and Kirchner (22)* The r a t e constant f o r t r a n s f e r of AH can be c a l c u l a t e d from i t s t r a n s f e r constant, obtained from our computer s i m u l a t i o n (23), and the known value of k f o r styrene. And f i n a l l y , the r a t e constant of the ΜΑΗ r e a c t i o n of AH can be c a l c u l a t e d from the r a t e at which r a d i c a l s are formed i n styrene ( c a l c u l a t e d from the observed r a t e of thermal p o l y m e r i z a t i o n ) , assuming that a l l r a d i c a l s come from t h i s postulated ΜΑΗ r e a c t i o n . The steady s t a t e concentration of AH was measured by Kirchner, and i s 6 χ 10~ M at about 60°. Thus, knowing the r a t e constants f o r the ene, ΜΑΗ, and t r a n s f e r r e a c t i o n s of AH, and the steady-state concentration of AH, the f r a c t i o n of AH that undergoes each of these three r e a c t i o n s can be c a l c u l a t e d . Table I shows these r a t e constants and a l s o shows the percent of AH that r e a c t s by each of these paths. The data are q u i t e s u r p r i s i n g : 94% of AH undergoes an ene r e a c t i o n , and only about 4% undergoes an ΜΑΗ process. T r a n s f e r i s even l e s s import ant, d e s p i t e the l a r g e t r a n s f e r constant. Table I a l s o gives the moles of AH that pass through each pathway i n the f i r s t hour of r e a c t i o n ; as can be seen, 7 χ 10~ moles of f r e e r a d i c a l s are produced. 5

7

For MCH, a s i m i l a r c o n c l u s i o n i s reached (6,6a). Kopecky and Lau showed that almost a l l of the MCH i s converted to the ene (or r a d i c a l recombination) product. Thus, we can set the r a t e constant f o r the ene r e a c t i o n as approximately equal to the t o t a l r a t e constant (measured by uv) f o r the disappearance of MCH. Again, the t r a n s f e r constant i s known (see below), from which the r a t e constant f o r t r a n s f e r can be c a l c u l a t e d . I f we assume that the r a t e constants f o r the ΜΑΗ r e a c t i o n s of MCH and AH are equal,

* I t i s not l i k e l y that a l l Α-Sty trimer a r i s e s from an ene r e a c tion. ( I . e . , from r e a c t i o n £ i n F i g . 1A.) As I have pointed out (J) * p h e n y l t e t r a l i n probably a r i s e s from the d i s p r o p o r t i o n a t i o n r e a c t i o n of caged r a d i c a l s , shown as eq ^ i n F i g . 1A. (Any Α· that d i f f u s e s i n t o f r e e s o l u t i o n would be expected to add to styrene and not to a b s t r a c t hydrogen to give p h e n y l t e t r a l i n . ) I f cage d i s p r o p o r t i o n a t i o n occurs, then cage combination a l s o must occur. (See eq ji i n F i g . LA.) However, the c o n c l u s i o n reached here i s not dependent on whether trimer a r i s e s from the combina t i o n of r a d i c a l s w i t h i n a cage or an ene process, s i n c e n e i t h e r r e a c t i o n produces f r e e r a d i c a l s that can i n i t i a t e the polymeriza t i o n of styrene.




Β

CYCLIZATION. g . D C

Φ

CYCLIC OLIGOMERS

s

'CYCLIZATION

© TRANSFER STEPS

*CH -CAr* 2

n

MONORADICAL PROPAGATION B R A N C H E D and C R O S S L I N K ED

M # Μ

Π

© 1' POLYMER

POLYMER

Figure 1. (A) This chart outlines the Dieb-Alder dimerization of 2 styrene monomers (or similar vinyl aromatics) to form AH and the subsequent possible reactions of AH to give oligomers or to produce free radicah. The [2 + 2] cycloaddition of 2 monomer units to form the 1,4-airadical -M - also is shown. (B) The reactions of l,4~diradical that convert it to oligomers such as dicyclobutanes (DCB) or to monoradicals. ( See Refs. 5, 6, and 7.) B


3.

PRYOR


then the data i n Table I are obtained: v i r t u a l l y a l l of the MCH disappears by the ene r e a c t i o n and l e s s than 1% disappears by t r a n s f e r or by an ΜΑΗ process. However, by doing a point-by-point i n t e g r a t i o n of the amount of MCH that goes through each r e a c t i o n , the number of moles of MCH that would have undergone the ΜΑΗ r e a c t i o n i n 1 hour i s c a l c u l a t e d to be 6 χ 10~ . Thus, i f we assume that the r a t e constants f o r the ΜΑΗ r e a c t i o n s of MCH and AH are i d e n t i c a l , then 85-fold more moles of r a d i c a l s would have been pro duced i n 1 hour from a s o l u t i o n i n i t i a l l y 0.01 M i n MCH than are produced by the steady s t a t e (6 χ 10~ M) concentration of AH. I f t h i s were t r u e , the r a t e of thermal polymerization (propor t i o n a l to the square root of the r a t e of i n i t i a t i o n ) would be observed to increase by about 9 - f o l d i n the MCH s o l u t i o n . Since we could e a s i l y observe an i n c r e a s e i n R ^ of 2 - f o l d , the r a t e constant f o r the ΜΑΗ r e a c t i o n of MCH must'be at l e a s t 21-times smaller than that f o r AH. Thus, the c a l c u l a t i o n s shown i n Table I can be summarized as f o l l o w s . I f we assume that MCH and AH undergo an ΜΑΗ r e a c t i o n with the same r a t e constant, then an i n i t i a l l y 0.01 M s o l u t i o n of MCH (the most concentrated we studied) would produce 85-times more r a d i c a l s i n 1 hour than does the steady s t a t e concentration of AH, d e s p i t e the f a c t that only a very small f r a c t i o n of MCH undergoes the ΜΑΗ r e a c t i o n . Since t h i s would produce a r a t e of polymerization 9-times greater than the thermal r a t e , our i n i t i a l assumption that MCH and AH undergo the ΜΑΗ r e a c t i o n with the same r a t e constant must be i n e r r o r . In f a c t , since we could q u i t e e a s i l y detect a 2 - f o l d increase i n the r a t e of polymerization, the r a t e of r a d i c a l production from MCH must be at l e a s t 21-fold slower than f o r AH. We a l s o measured the t r a n s f e r constant of MCH, and i t i s about 10 at 60°. Thus, MCH i s e x t r a o r d i n a r i l y r e a c t i v e toward r a d i c a l s ; 10 i s not only the world's record f o r a t r a n s f e r con s t a n t f o r a hydrocarbon, but i t i s one of the l a r g e s t t r a n s f e r constants known. Only t h i o l s and t r a n s i t i o n metal compounds have t r a n s f e r constants t h i s l a r g e or l a r g e r . For comparison, the t r a n s f e r constant of triphenylmethane ( i n styrene at 60°) i s about 10 smaller than that of MCH! Despite t h i s s t r i k i n g l y l a r g e t r a n s f e r constant, MCH undergoes the ΜΑΗ r e a c t i o n an order of magnitude (or more) more slowly than does AH. Surely t h i s i s extremely s u r p r i s i n g and i s hard to r e c o n c i l e with the D i e l s A l d e r mechanism f o r the i n i t i a t i o n of styrene. There are two p o s s i b i l i t i e s at t h i s p o i n t . The most conser v a t i v e i s that MCH i s a poor model f o r AH; t h i s allows the postu l a t e d ΜΑΗ r e a c t i o n of AH to be r e t a i n e d as the i n i t i a t i o n process i n styrene. The l e a s t conservative i s that the e n t i r e D i e l s A l d e r mechanism f o r styrene's i n i t i a t i o n i s wrong! I t c e r t a i n l y does seem p o s s i b l e that MCH i s a poor model for AH, and that MCH might undergo a more n e a r l y concerted ene r e a c t i o n than does AH, thus g i v i n g a s m a l l e r y i e l d of f r e e r a d i c a l s . Figure 2 o u t l i n e s the r e a c t i o n s of an MCH-like molecule. 5

5


43

p

5



Steady s t a t e AH c o n c e n t r a t i o n (22).

7

1

(d) from r e f (22).

= 0.01-

-

5

5

5

(h) Ref (37).

(k) Not i n agreement w i t h experiment.

than f o r AH.

value,

+d[AH]/dt at the steady s t a t e ,

that -d[AH]/dt

•

(e) from r e f (23).

must be smaller f o r MCH

( j ) Assumed as i d e n t i c a l t o the AH

(22) and assuming

C l e a r l y , the v a l u e of

( i ) Ref (j> ) .

(g) from the measured r a t e of AH appearance

100.

0.7-

0.5

80-100^

(c) Moles reacted

8.2 χ 10~

6 χ HT

4 χ 10"

i s 0.5 hr ( 6 ) .

5

5.3 χ 1 0 " -

[3.7 χ 1 0 " ]

0

Moles/hr-

ÏMCH1

C a l c u l a t e d from the r a t e of r a d i c a l appearance i n d i c a t e d by the observed r a t e of thermal

polymerization,

(f)

3

10 ±

—

k

(b) The h a l f l i f e of MCH

100.

5

1.7 χ 10~

6

9.0 χ 1 0 " *

4.

7

7 χ ΙΟ"

7

3.7 χ 1 0 " -

0.1

94.

5

8

%

5

£

1.6 χ Ι Ο "

Moles/hr

2 χ 10"

g s

145*

6

8.5 χ 1 0 " ^

[AH]

= 6 χ 1(T -

of the Rate Constants f o r Reaction and Moles Reacted i n 1 Hour f o r AH and ΜΑΗ

i n Styrene at 60° (6)

A Comparison

by t h i s path i n f i r s t hour by p o i n t - t o - p o i n t i n t e g r a t i o n .

(a)

Total

ΜΑΗ

Transfer

Ene

Process

Table I .



3.

PRYOR


V E N E

45

VI PRODUCTS

Figure 2. Possible reactions of methylenecyclohexadiene (MCH) and the formation of ene products. Compound VI can be produced either bu a concerted ene reaction or by a process (Equations 2a and 2c) involving diraaicals. The possible leakage of radicals from the ene reactions extended transition state (II) to initiation polymerization is shown (6, 6SL).



46


1

Ene products V and VI can be produced by r e a c t i o n s b and J j . In a d d i t i o n , one of the ene products, VI, can be formed by the recom b i n a t i o n of f r e e r a d i c a l s , r e a c t i o n s ^ , c . (Kopecky and Lau have reported that V and VI are produced i n approximately a 1:3 r a t i o from MCH.) For a more hindered molecule such as AH, models i n d i cate that an extended t r a n s i t i o n s t a t e such as I I may be r e l a t i v e l y more p r e f e r a b l e to I I I or IV, thus e x p l a i n i n g the l a r g e r y i e l d of r a d i c a l s r e l a t i v e to ene products. Alternatively, i t i s possi b l e that a t r a n s i t i o n s t a t e l i k e IV may have a more d i s t o r t e d shape f o r AH, and that more r a d i c a l s "leak out" of the ene reac t i o n v i a eq jj. In e i t h e r case, the f a s t e r r a t e of r a d i c a l produc t i o n from AH r e l a t i v e to MCH i s r a t i o n a l i z e d as due to the greater hindrance i n the AH-styrene r e a c t i o n r e l a t i v e to the MCH-styrene ene process. Thus, the f a s t e r production of r a d i c a l s from AH r e l a t i v e to MCH could be r a t i o n a l i z e d i n terms of an ene r e a c t i o n that i n volves a t r a n s i t i o n s t a t e w i t h v a r y i n g amounts of r a d i c a l charac ter and leading to varying y i e l d s of scavengable f r e e r a d i c a l s . As the argument has been presented above, t h i s r a t i o n a l i z a t i o n does not appear to be impossible. However, F i g u r e 3 shows a tabu l a t i o n of a l l of the model compounds synthesized to date i n an a t tempt to model the behavior of AH. Although the second and t h i r d compounds i n the l i s t are reported to i n i t i a t e polymerization, MCH does not. Furthermore, as shown i n Figure 3, a compound that would appear to g i v e an even more hindered ene r e a c t i o n t r a n s i t i o n s t a t e than AH a l s o does not i n i t i a t e the polymerization of styrene ( p r i v a t e communication from D. Aue). This i s hard to r e c o n c i l e with a r a t i o n a l i z a t i o n of the l a c k of ΜΑΗ r e a c t i o n by methylene cyclohexadiene based on the hindrance of i t s ene r e a c t i o n . There are other problems w i t h the D i e l s - A l d e r mechanism. For example, i t appears from UV evidence that AH b u i l d s up to i t s steady-state concentration so slowly that an i n d u c t i o n period should be observed i n the r a t e of polymerization. However, none has been reported, although an i n d u c t i o n p e r i o d i n chain t r a n s f e r by AH i s e a s i l y observed (6a). Thus, I suggest that the D i e l s - A l d e r mechanism can continue to be accepted as the mechanism f o r i n i t i a t i o n as long as i t i s c l e a r l y recognized that i t i s not c o n c l u s i v e l y e s t a b l i s h e d and that i t may be i n c o r r e c t . I f the D i e l s - A l d e r mechanism i s wrong, i t i s d i f f i c u l t to suggest a s u p e r i o r mechanism to take i t s p l a c e . The only mechanism that has been suggested and not yet disproven i s one i n v o l v i n g 1 , 4 - d i r a d i c a l s (5,7), and i t too has i t s d i f f i c u l t i e s (5). The only conclusive experiment appears to be the s y n t h e s i s and t e s t i n g of AH i t s e l f , and we are attempting to do that. The Reaction

of Ozone w i t h t e r t - B u t y l Hydroperoxide.

An

ΜΑΗ

Hydrogen Atom T r a n s f e r from Oxygen Introduction.

As we

have remarked above, ozone r e a c t s with


3.

PRYOR

47


Compound

Initiation

Reference

Pryor, Lasswell


A d v . Free Radical Chem

5 (1975)

P r y o r , C o c o , Houk, Oaly JACS

(1974)

Sato, Abe, Otsu Makromol

Chem 1 9 7 7

P r y o r , Graham , Green 1 9 7 7 K o p e c k y , Lau 1 9 7 7

D. A u e , A . K o s 1 9 7 6 Unpublished

Figure 3. Compounds synthesized and tested as models for AH. The Dieb-Alder adduct of styrene, AH itself, is shown at the top of the figure; it is postulated to initiate the polymerization of styrene. Of the models tested, two initiate and two do not.

American Chemical Society Library 1155 16th St. N. W. Pryor; Organic Free Radicals WashmgtM, 6. 20036 ACS Symposium Series; AmericanD, Chemical Society: Washington, DC, 1978.

ORGANIC

48

F R E E RADICALS

v i r t u a l l y every type of organic molecule to form r a d i c a l s (29,30, 31). Our i n t e r e s t i n ozone was sparked by i n d i c a t i o n s that the pathology caused by ozone i n smog i s p a r t i a l l y due to the a u t o x i d a t i o n of polyunsaturated f a t t y a c i d s (PUFA) i n lung l i p i d s i n i t i ated by ozone (32,51). A priori, s e v e r a l mechanisms can be en v i s i o n e d f o r r a d i c a l production from 0 3 - s u b s t r a t e r e a c t i o n s . ( i ) The simplest r a d i c a l - p r o d u c i n g r e a c t i o n i s the unimolecu l a r homolysis of ozone, eq 25. The heat of t h i s r e a c t i o n i s 0

• 0

3

2

+ 0

(25)

25 kcal/mole (assuming O2 i s formed i n i t s ground s t a t e ) , p r e d i c t ing a h a l f l i f e f o r ozone of 10** hrs at -20 , 1 0 at 0°, and 10 min at 37°C (18,39). Thus, eq 25 i s too slow to be an important i n i t i a t i o n process at the low temperatures at which ozonolyses are u s u a l l y conducted, but i t could play an important r o l e at 37°C i n b i o l o g i c a l systems where ozone t o x i c i t y i s observed. {ii) Perhaps the next simplest r a d i c a l - p r o d u c i n g r e a c t i o n would be an e l e c t r o n - t r a n s f e r , eq 26. B a i l e y has suggested that


2

X + 0

• xt

3

+

0 "

(26)

3

an i n i t i a l e l e c t r o n t r a n s f e r i s r e s p o n s i b l e f o r the r a d i c a l pro d u c t i o n observed at -78° i n the r e a c t i o n s of ozone with t r i m e s i t y l v i n y l a l c o h o l (40). I f 0 3 were produced, i t might form reac t i v e r a d i c a l s . The r a t e constant f o r d i s s o c i a t i o n of O 3 " i n aqueous s o l u t i o n at 25°, eq 27, i s 1 0 s e c " (41). Thus, HO* T

3

0

T 3

+

H0 2

• 0

1

2

+ Η0· + HO"

(27)

could be the r a d i c a l that i n i t i a t e s a u t o x i d a t i o n (and pathology i n bio-systems) i n some of the r e a c t i o n s of ozone (42). T h i s i s reminiscent of the suggestion that the pathology caused by super oxide, 0 , i s mediated by Η0· r a d i c a l s (43,43a,43b) generated by by the r e a c t i o n of superoxide with H2O2, eq 28.* T

2

0

T 2

+ H2O2

• 0

2

+'Η0· + HO"

(28)

•Reaction 28 i s too slow to be important i n b i o l o g i c a l systems (43b). However, we have r e c e n t l y shown that the r e a c t i o n of the superoxide anion r a d i c a l w i t h organic hydroperoxides, eq i , i s f a s t (45b). Since PUFA forms hydroperoxides in vivo both by an enzymatic path and by a u t o x i d a t i o n , i t appears that r e a c t i o n i could be r e s p o n s i b l e f o r the Η0· r a d i c a l s observed i n b i o l o g i c a l systems. 0

T 2

+ ROOH

• 0

2

+ RO*

+ HO"


(i)

3.

PRYOR

49


{Hi) i n t o a C-H

Ozone could undergo a concerted 1,3-dipolar i n s e r t i o n bond or O-H bond, eq 29a (44,44a). When R-H i s

R-H

+ *0=0-0~

• R0 H —

• R00-

3

+ ΗΟ·

(29a)

benzaldehyde, PhCO-OOOH i s an intermediate and an i n s e r t i o n mech anism appears reasonable (35a). However, when R-H i s an alkane, an ΜΑΗ hydrogen a b s t r a c t i o n mechanism i s u s u a l l y w r i t t e n , eq 29b, although the r e a c t i o n i s s t e r e o s p e c i f i c (34,34a,35).


R-H

+ 0

• [R*

3

Η0 ·]

• ROH

3

+ 0

(29b)

2

{iv) Ozone r e a c t s with o l e f i n s some 10 f a s t e r than with alkanes (44). Thus, i n systems that c o n t a i n o l e f i n i c unsaturat i o n , the f a s t e s t r e a c t i o n i s a d d i t i o n of ozone to give the t r i oxide, eq 30. Even at 10~ M o l e f i n , t h i s a d d i t i o n i s so f a s t that ozone homolysis, eq 25, cannot compete at 37° C or below. The t r i o x i d e then undergoes r a p i d d i s s o c i a t i o n and recombination to form the Criegee ozonide, eqs 31-32. 6

3

0-0-0 0

3

+ R C=CR 2

2

• RC 2

CR

(30)

2

(VII) •0 (VII)

00·

• [R C— £R ] 2

2

• R C « 0 + [R C-00* « 2

• R C—00~]

2

(VIII)

2

(31)

(IX)

R C=0 + (IX) 2

• R C-0-CR 2

Criegee

2

(32)

ozonide

To the extent that the z w i t t e r i o n - d i r a d i c a l , IX, has f r e e r a d i c a l character, i t can be the source of i n i t i a t i n g r a d i c a l s : f

(IX) + R H

f

• R C00H + R · 2

However, i t appears d o u b t f u l that IX has s u f f i c i e n t l i f e t i m e and/or r a d i c a l character to be the i n i t i a t i n g species i n PUFA autoxidations induced by ozone. {v) We have shown that the Criegee ozonide does not homolyze to i n i t i a t e a u t o x i d a t i o n i n PUFA-oζone-air systems (32,45). In c o n t r a s t , the t r i o x i d e (VII) does d i s s o c i a t e at temperatures that are s u f f i c i e n t l y low so as to r a t i o n a l i z e r a d i c a l production i n o l e f i n s induced by ozone (46,47,48), but i t i s d o u b t f u l that the


50

ORGANIC F R E E

RADICALS

d i r a d i c a l produced i n t h i s d i s s o c i a t i o n , V I I I , has s u f f i c i e n t l i f e t i m e to i n i t i a t e PUFA a u t o x i d a t i o n . In the gas phase, d i r a d i c a l V I I I may undergo a " b a c k - b i t e " r e a c t i o n (51a): 0· RHC

1

00·

0

00H

• RC a J CHR-

CHR '--

(33a)


In analogy with t h i s process, we have suggested (45a) that the t r i o x i d e formed from ozone a d d i t i o n to one of the double bonds i n a polyunsaturated f a t t y a c i d can undergo the s i m i l a r r e a c t i o n , eq 33b, i n which an a l l y l i c hydrogen atom i s a b s t r a c t e d . Eq 33b •00

Ι

(VII)

0·

·_

I

• RCH=CH-CH —CH—CHR -CH—CHR

H00

Ο

I

I

> RCH-CH-CH—ÈH-CHR • RCH-CH-CH—CH-CHI

2

(33b)

generates a d i r a d i c a l that may w e l l have s u f f i c i e n t l i f e t i m e to i n i t i a t e autoxidation. (vi) F i n a l l y , r a d i c a l s can be produced by ΜΑΗ r e a c t i o n s of ozone w i t h 0-H bonds. In p a r t i c u l a r , the r e a c t i o n of ozone with hydroperoxides might i n v o l v e an ΜΑΗ process, eq 34. We began ROOH + 0

• R00* + Η 0 ·

3

(34)

3

i n v e s t i g a t i n g the r e a c t i o n s of ozone with hydroperoxides f o r s e v e r a l reasons. In the f i r s t p l a c e , r a d i c a l s are produced at very low temperatures (47,48,49,50). Secondly, l i p i d hydroperox ides are produced during the a u t o x i d a t i o n of PUFA by ozonecontaining smog, and we wished to know i f ozone r e a c t s w i t h these l i p i d hydroperoxides t o produce r a d i c a l s (32,51). A n a l y s i s of the r e a c t i o n of ozone with t e r t - b u t y l hydroperox ide i n CHC1 , C C l ^ , or C F C I 3 as s o l v e n t s at temperatures from 24 to -60° showed that the products are tert-butyl a l c o h o l , acetone, water, and d i - t e r t - b u t y l peroxide (44,44a). The r e a c t i o n scheme, shown i n eqs 35-43, r a t i o n a l i z e s these products as r e s u l t i n g from f r e e r a d i c a l r e a c t i o n s of ozone and the hydroperoxide, where Τ = 3

Τ00Η + 0

Τ00· + Η0· + 0

3

Η0· + Τ00Η

2Τ00·^=£[Τ0 Τ] α

• [2T0. +

> H0 2

0 ] 2

\

Τ0· + Τ00Η

(35)

2

+ Τ00·

(36)

TOOT + 0 (i-f)

(37)

2

2Τ0· + 0

2

Τ0Η + Τ00·


(38)

(39)

3.

PRYOR

CH3COCH3 +

TO*

CH * + 0 3

3

(41)

3

(42)

• N o n - r a d i c a l products

3

TOO* + 0

(40)

CH *

CH 00*

2

CH 00* + TOO*


51


[Τ0 ·]

3

• TO* +

5

20

(43)

2

Under c o n d i t i o n s where the production of acetone i s i n s i g n i f i c a n t (e.g., i n C F C I 3 at - 4 ° ) , k i n e t i c a n a l y s i s of eqs 35-43 gives the rate laws shown i n eqs 44 and 45.

Rate(-0 ) = ^ ( Τ 0 0 Η ) ( 0 ) + 3

k

3

Rate(-TOOH)

1

2k -^(Τ00Η)(0 ) + 3

(44)

3 / 2

(TOOH) * ( 0 ) 3

x

k

(T00H)^(0 )

(45)

3 / 2

3

x

Eq 44 can be rearranged to g i v e eq 46. Thus, a p l o t of the quantity of the l e f t s i d e of eq 46 versus {[TOOH]/[0 ]} / should give a s t r a i g h t l i n e w i t h slope equal to k^ and i n t e r c e p t k ( k / k ) / . We have performed the a n a l y s i s i n CFC1 at -4°C both f o r TOOH and f o r the deuterated hydroperoxide, TOOD. 1

2

3

1

x

i

2

t

3

I n i t i a l rate(-0 ) 3

(0 ) 3

3 / 2

=

(TOOHK

k

(TOOH)^

(0 )

1

3

1

H

+

(46)

k

X

1

Our data give k. = 5.4 M" s e c " f o r t e r t - b u t y l hydroperoxide, and k / k = 2.8 for tert-BuOOD at - 4 ° . T h i s primary isotope e f f e c t e s t a b l i s h e s the mechanism of eq 35 as an a s s i s t e d s c i s s i o n of an O-H bond, r a t h e r than a r a t e l i m i t i n g e l e c t r o n t r a n s f e r followed by a r a p i d proton t r a n s f e r , eqs 35a,b. In a d d i t i o n , the heat of 1

R

D

ROOH + 0

• ROOH*" + 0

3

ROOH*" + 0

T 3

T

(35a)

3

• R00* + Η 0 · 3


(35b)

ORGANIC F R E E


52

RADICALS

r e a c t i o n of eq 35a can be shown t o be too l a r g e f o r the e l e c t r o n t r a n s f e r process t o be f a c i l e (44). Although the k i n e t i c law f o r the r e a c t i o n o f ozone w i t h tertb u t y l hydroperoxide i s complex w i t h a f i r s t order and a chain term, under c o n d i t i o n s where the hydroperoxide i s present i n 10-30 mole excess, 50-80% of the r e a c t i o n goes by the f i r s t order pro cess. (That i s , c a l c u l a t i o n s show that most of the r e a c t i o n i s due t o the f i r s t term i n eqs 44 and 45.) Under these c o n d i t i o n s , approximate values o f k^ can be obtained by simply measuring i n i t i a l r a t e s . Data obtained i n t h i s way give an apparent a c t i v a t i o n energy f o r the r e a c t i o n of ozone and t e r t - b u t y l hydroperoxide of 7 kcal/mole and a l o g A v a l u e of 7. Reaction 35 can be c a l c u l a t e d to be 14 kcal/mole endothermic (44,44a), and the heats of r e a c t i o n and a c t i v a t i o n energies f o r eqs 36-43 have been measured or can be c a l c u l a t e d using Benson's methods. Thus, the a c t i v a t i o n energies f o r the two terms i n the k i n e t i c law, eq 46, are c a l c u l a t e d t o be about 14 and (5 + 14/2 - 8/2) - 8 kcal/mole, r e s p e c t i v e l y . Thus, depending on the k i n e t i c chain lengths, the observed a c t i v a t i o n energy f o r t h i s mechanism i s p r e d i c t e d t o be between 14 and 8 kcal/mole, i n e x c e l l e n t agreement w i t h the observed value of 7 kcal/mole. The heat o f r e a c t i o n f o r a concerted 1 , 3 - i n s e r t i o n by ozone i n t o the 0-H bond a l s o can be c a l c u l a t e d u s i n g Benson's estimate of the group term f o r an oxygen atom attached t o two other oxygens (18). T h i s c a l c u l a t i o n shows that the d i r e c t i n s e r t i o n t o form t e r t - b u t y l hydropentoxide as an intermediate i s f a r too endotherm i c t o be allowed by our approximate a c t i v a t i o n energy of 7 k c a l / mole (44,44a). The c a l c u l a t e d heats o f r e a c t i o n f o r the i n s e r t i o n process are shown over the arrows i n eq 35c. TOOH + 0

A H 3

*+

2 5

> T0 H

Δ

5

Η

*~

1

λ

> TOO- + 0

2

+ HO-

(35c)

By the same approximate technique that was used t o o b t a i n a c t i v a t i o n energies, the r a t e s f o r r e a c t i o n of ozone were d e t e r mined f o r a v a r i e t y of types of organic s p e c i e s . Table I I g i v e s data of t h i s type. I t can be seen that the r e l a t i v e r a t e s of r e a c t i o n of ozone w i t h alkenes: ROOH: alkanes i s approximately i n the r a t i o s of 1 0 : 1 0 : 1 0 . 6

1 , 4 - D i r a d i c a l s from

3

1

[2+2] C y c l o a d d i t i o n Reactions

1 , 4 - D i r a d i c a l s . O r b i t a l symmetry arguments suggest the [2+2] c y c l o a d d i t i o n should not be concerted, and experiments o f v a r i o u s types suggest that these r e a c t i o n s do,, indeed, i n v o l v e d i r a d i c a l s (9a,53-55). The arguments are p r i m a r i l y based on the l a c k of s t e r e o s p e c i f i c i t y i n c y c l o a d d i t i o n r e a c t i o n s . However, r e c e n t l y , s e v e r a l r e p o r t s have been published i n which 1 , 4 - d i r a d i c a l s have been trapped by added reagents. Table I I I presents these data. Most of the d i r a d i c a l s a r e produced photochemically,


3.

PRYOR

53


Table I I . Approximate Second Order Rate Constants f o r the Reaction of Ozone w i t h a S e r i e s of Organic Compounds i n CCU at 24° (44,57) k

1


Substrate

1

1

k(M" sec* )

rel

2,3-Dimethylbutane

0.2

Cyclohexane

0.02

0.1

(1)

te*»t-Butyl a l c o h o l

0.02

0.1

Cyclopentanol

1.3

6.

Benzene

0.05

0.2 185.

t e r t - B u t y l hydroperoxide

37.

1-Pentene

8 χ 10

Table I I I .

4 χ 10

4

Trapping of 1 , 4 - D i r a d i c a l s by Scavengers

Diradical

Scavenger

CÔFSCH—CH2CH2~CHC6Fs

C6FsCH CH2

Diradical h a l f l i f e ΤOO· + O

2

+

ΗO·

The spontaneous polymerization of pentafluorostyrene (PFS) has been s t u d i e d . I t appears u n l i k e l y that t h i s monomer undergoes an initiation by the D i e l s - A l d e r mechanism. Rather, we suggest it undergoes initiation by a mechanism i n v o l v i n g 1 , 4 - d i r a d i c a l s . Measurements of the k i n e t i c isotope e f f e c t s on the polymerization of PFS-ß,ß-d prove that the 1 , 4 - d i r a d i c a l is not converted to monoradicals by donating a hydrogen atom to another molecule of PFS. Instead, it appears that the 1 , 4 - d i r a d i c a l s undergo chain t r a n s f e r r e a c t i o n s w i t h oligomers that possess b e n z y l i c hydrogens. (See F i g u r e 1B.) I f t h i s is c o r r e c t , then the capture of thermal l y produced 1 , 4 - d i r a d i c a l s by t r a n s f e r agents is s u f f i c i e n t l y f a s t to compete with the c l o s u r e of the d i r a d i c a l s to form c y c l o b u t anes. 2

Acknowledgement The work described i n t h i s r e p o r t was done by a s e r i e s of talented graduate students and p o s t - d o c t o r a l f e l l o w s , and I want to acknowledge t h e i r considerable c o n t r i b u t i o n s . Without them, t h i s chapter would not have been w r i t t e n . I p a r t i c u l a r l y want to acknowledge the c o n t r i b u t i o n s of P r o f e s s o r Michael Kurz and Drs. W. David Graham, John G. Green, Masashi l i n o , and D a n i e l Church. T h i s research was supported by grants from the N a t i o n a l Science Foundation, the N a t i o n a l I n s t i t u t e s of Health, and Dow Chemical Company. I express my s i n c e r e thanks to those agencies f o r t h e i r continuing support.


60


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6. 6a. 6b. 7.

8. 9.

9a. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 21. 22.

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3.

PRYOR

23. 23a. 24. 25. 26. 27.


28. 29. 30. 31. 32. 33. 34. 34a. 35. 35a. 36. 37. 38. 39. 40. 41. 42. 43. 43a. 43b. 44. 44a. 45. 45a. 45b.


61

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RECEIVED December 23, 1977.


Radical Production from the Interaction of Closed-Shell Molecules

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