Oxidation of Iron(II) Chloride in Nonaqueous Solvents - Advances in

75 Oxidation of Iron(II) Chloride in Nonaqueous Solvents

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch075

G E O R G E S.

HAMMOND

and

CHIN-HUA

S.

WU

Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, Calif. 91109

Autoxidation of iron(II) chloride in nonaqueous solvents is much faster than in water. The rate is first order in oxygen, and under controlled conditions, second order in iron(II). Various additives have powerful catalytic or inhibitory effects. The inhibition by iron(III) disappears in the presence of excess lithium chloride, so inhibition is attributed to competition between iron(II) and iron(III) for chloride ions. Induced autoxidation of benzoin to benzil has the same rate-limiting step as the autoxidation of iron(II) without cosubstrate. The data can be accommodated by a mechanism in which the rate-limiting step is production of iron(IV) by dissociation of a binuclear complex having the composition Cl FeOOFeCl . In the presence of excess lithium chloride, intermediates containing more chloride bound to iron become involved. 2

A

2

n u m b e r of w o r k e r s (8, 13, 14, 15, 20, 21, 22, 23, 33, 34, 35)

have

r e p o r t e d studies of the rates of a i r o x i d a t i o n of i r o n ( I I ) to i r o n ( I I I ) i n aqueous s o l u t i o n a n d h a v e discussed t h e m e c h a n i s t i c i m p l i c a t i o n s of the k i n e t i c studies. O n l y P o u n d (28)

has r e p o r t e d t h e results of a p r e

l i m i n a r y s t u d y of the r e a c t i o n i n n o n a q u e o u s solvents. W e h a v e i n v e s t i g a t e d the r e a c t i o n i n o r g a n i c solvents because the faster rates are m o r e c o n v e n i e n t to f o l l o w a n d because a w i d e r range of o r g a n i c

cosubstrates

c a n b e i n c l u d e d . T h e u l t i m a t e a i m of this s t u d y is to increase o u r u n d e r s t a n d i n g of t h e v a r i o u s c a t a l y t i c a n d i n h i b i t o r y roles p l a y e d b y m e t a l l i c c o m p o u n d s i n autoxidations.

Experimental Materials. Iron ( I I ) chloride dihydrate was purified b y a slight vari a t i o n of t h e p r o c e d u r e d e s c r i b e d b y M o e l l e r (25). R e a g e n t grade 186 In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

75.

HAMMOND AND wu

Iron(II)

Chloride

187

F e C l • 2 H 0 (100 g r a m s ) , w a t e r (40 m l . ) , 5 grams i r o n p o w d e r , a n d 5 m l . 62V H C 1 w e r e h e a t e d u n d e r n i t r o g e n at 120 ° C . f o r one h o u r . T h e hot suspension w a s filtered, a n d the filtrate w a s c o o l e d , first at r o o m t e m p e r a t u r e a n d t h e n at ice t e m p e r a t u r e , a n d t h e g r e e n crystals of F e C l • 4 H 0 w e r e r e m o v e d b y filtration. A l l of the last three operations w e r e c a r r i e d out i n a n i t r o g e n box. T h e crystals w e r e d r i e d to constant w e i g h t over p h o s p h o r u s p e n t o x i d e i n a v a c u u m desiccator. T h e i v o r y c o l o r e d d i h y d r a t e w a s o b t a i n e d a n d stored i n v a c u u m - s e a l e d a m p o u l e s c o n t a i n i n g 2 - 3 grams. W h e n a m p o u l e s w e r e o p e n e d for use, the contents w e r e t r a n s f e r r e a to s c r e w - c a p vials a n d stored i n a v a c u u m desiccator. V a r i o u s samples w e r e a n a l y z e d f o r i r o n ( I I ) b y d i c h r o m a t e t i t r a t i o n (17). Samples stored as d e s c r i b e d s h o w e d a m a x i m u m loss i n titer of 1 % over five-month p e r i o d s . A n a l y s i s : c a l c u l a t e d for F e C l • 2 H 0 , F e ( I I ) , 34.31. F o u n d , 34.10-34.87. M e t h a n o l w a s first treated w i t h a l u m i n u m a m a l g a m (32) a n d t h e n d i s t i l l e d . Tests for c a r b o n y l c o m p o u n d s w i t h a l k a l i n e m e r c u r i c c y a n i d e (12) s h o w e d the presence of less t h a n 0 . 0 0 2 % of s u c h materials. E t h a n o l was p u r i f i e d i n the same w a y as m e t h a n o l . T h e f o r m e r was f o u n d to c o n t a i n less t h a n 0 . 0 0 5 % c a r b o n y l c o m p o u n d s . E t h y l e n e g l y c o l w a s p u r i f i e d b y d i s t i l l a t i o n f r o m s o d i u m (36). Pyridine, purified b y d i s t i l l a t i o n over pellets of p o t a s s i u m h y d r o x i d e , w a s s u p p l i e d b y James W a t e r s . D i m e t h y l sulfoxide, p u r i f i e d b y d i s t i l l a t i o n a n d stored over m o l e c u l a r sieves, w a s p r o v i d e d b y R o b e r t C . N e u m a n , Jr. R e a g e n t grade p i p e r i d i n e was d r i e d over s o d i u m a n d d i s t i l l e d ; the f r a c t i o n b o i l i n g at 105 ° C . w a s c o l l e c t e d f o r use. A c e t y l a c e t o n e w a s p u r i f i e d b y treatment w i t h s o d i u m h y d r o x i d e a n d d i s t i l l a t i o n (6, 7 ) . T h e f r a c t i o n c o l l e c t e d f o r use b o i l e d at 1 3 3 ° - 1 3 4 ° C . a n d i n i t i a l l y h a d n = 1.4525; the v a l u e c h a n g e d to n = 1.4512 after a f e w days. D i p i v a l o y l m e t h a n e (18), purified b y preparative vapor chromatography, was p r o v i d e d b y D o n a l d P a s k o v i c h . O t h e r materials w e r e reagent grade c h e m i c a l s u s e d as d r a w n f r o m the s t o c k r o o m . 2

2

2

2


2

D

D

2

2 5

2 5

K i n e t i c P r o c e d u r e s . T h e v o l u m e t r i c o x y g e n m o n i t o r i n g apparatus has b e e n d e s c r i b e d b y B o o z e r (2). T h e pressure of o x y g e n c a n b e k e p t w i t h i n =b2 m m . H g . T h e gas b u r e t reads w i t h a p r e c i s i o n ± 0 . 0 1 m l . The 0 u p t a k e is i n d e p e n d e n t of s t i r r i n g speed, a n d the p o s s i b i l i t y of photocatalysis has b e e n r u l e d out b y d a r k experiments. T h e r e a c t i o n vessel consists of a t h e r m o j a c k e t e d flask w h o s e s i d e a r m is e q u i p p e d w i t h a s n u g ground-glass c h a m b e r seat for i r o n ( I I ) sample. I r o n c h l o r i d e is w e i g h e d i n a s m a l l P t boat, p l a c e d i n the seat, t h e n the c h a m b e r is closed. U n d e r these circumstances, the s a m p l e is p r o t e c t e d f r o m solvent v a p o r d u r i n g degassing. T h e r e a c t i o n m i x t u r e is degassed, saturated w i t h 1 a t m . o x y g e n or air. O w i n g to the v o l a t i l i t y of m e t h a n o l , it is i m p o r t a n t to establish the near e q u i l i b r i u m b e t w e e n the o x y g e n gas a n d the l i q u i d b e f o r e the zero t i m e is m a r k e d . A p e r i o d of 15 to 20 m i n u t e s s h o u l d b e a l l o w e d . T h e n the i r o n s a m p l e is a d d e d . W i t h i n one to t w o m i n u t e s , a homogeneous r e a c t i o n m i x t u r e results, a n d at this t i m e t h e zero p o i n t of a r u n is m a r k e d . 2

A n a l y t i c a l M e t h o d s . I R O N ( I I ) A N D I R O N ( I I I ) . M e t h a n o l i c solutions of v a r y i n g concentrations of i r o n ( I I ) a n d i r o n ( I I I ) w e r e m a d e . Spectrop h o t o m e t r i c analyses w e r e p e r f o r m e d a n d c a l i b r a t e d a c c o r d i n g to the r e p o r t e d p r o c e d u r e (10, 37). They both obey Beers L a w . I n a kinetic

In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

188

OXIDATION OF ORGANIC COMPOUNDS

HI

run an aliquot was removed and quenched b y dilution w i t h methanol. T o t a l i r o n c o n c e n t r a t i o n w a s first o b t a i n e d b y r e d u c i n g the i r o n ( I I I ) b y hydroxylamine hydrochloride. T h e iron (II) o-phenanthroline complex w a s t h e n d e v e l o p e d at p H 2.9, a n d the o p t i c a l d e n s i t y at 5100 A . w a s m e a s u r e d . T h e same p r o c e d u r e w i t h o u t r e d u c t i o n gave i r o n ( I I ) [cor r e c t i o n w a s m a d e for a b s o r b a n c e o w i n g to i r p n ( I I I ) o - p h e n a n t h r o l i n e ] . B y t h i o c y a n a t e c o l o r i m e t r y at 4780 A . a n d p H 1.5, the i r o n ( I I I ) c o n c e n t r a t i o n w a s d e t e r m i n e d . T h e s u m of the i r o n ( I I ) a n d i r o n ( I I I ) c o n centrations w a s i n g o o d agreement w i t h the assay f o r t o t a l i r o n . B E N Z O I N A N D B E N Z I L . Beer's L a w was o b e y e d b y a m e t h a n o l i c s o l u t i o n o f b e n z o i n a n d b e n z i l at w a v e l e n g t h s 2500 to 2900 A . T h e r e l a t i o n ship c = c • M + c • M w a s established for a series of solutions c o n t a i n i n g these t w o c o m p o u n d s i n different p r o p o r t i o n s . T h e values of c, € , c are r e s p e c t i v e l y the e x t i n c t i o n coefficients of the m i x t u r e , b e n z o i n , a n d b e n z i l ; M a n d M are the m o l e fractions of b e n z o i n a n d b e n z i l . I n the k i n e t i c w o r k the o p t i c a l d e n s i t y m e a s u r e d w a s c o r r e c t e d f o r the c o n t r i b u t i o n s b y i r o n ( I I a n d I I I ) species w h i c h w e r e d e t e r m i n e d i n d e p e n d e n t l y . T h e o p t i c a l d e n s i t y w a s m e a s u r e d at m o r e t h a n one w a v e l e n g t h ( u s u a l l y 2700, 2800, 2600 A . ) , a n d the results w e r e r e p r o d u c i b l e a n d agreed w i t h i n ± 2 % error. A C a r y m o d e l 11 spectrophotometer w a s u s e d for this analysis.


N

N

N

L

L

L

N

L

D e t e c t i o n a n d D e t e r m i n a t i o n o f A l d e h y d e . T h e a m o u n t of f o r m a l d e h y d e i n m e t h a n o l i c r e a c t i o n m i x t u r e w a s estimated q u a n t i t a t i v e l y a c c o r d i n g to the p r o c e d u r e b y K o l t h o f f (16). A series of solutions c o n t a i n i n g v a r y i n g amounts ( 5 X 10" to 5 X 1 0 " M ) of f o r m a l d e h y d e as w e l l as the u n k n o w n sample, w i t h p H adjusted to 3 b y p h o s p h a t e - c i t r i c a c i d buffer, w a s treated w i t h 1.5 X 1 0 " M Schiff's reagent (31). Thirty m i n u t e s later, the o p t i c a l d e n s i t y at 5500 A . was d e t e r m i n e d b y a C o l e m a n J u n i o r spectrometer. T h e u n k n o w n c o n c e n t r a t i o n of f o r m a l d e h y d e w a s estimated b y i n t e r p o l a t i n g the k n o w n values. T h i s p r o c e d u r e w a s re p r o d u c i b l e for a u t o x i d a t i o n of ferrous c h l o r i d e i n m e t h a n o l . H o w e v e r , i n the presence of a reactive cosubstrate, s u c h as b e n z o i n , the color b e c a m e unstable, a n d the analysis w a s o n l y s e m i q u a n t i t a t i v e . It w a s possible to d e t e r m i n e a c e t a l d e h y d e q u a n t i t a t i v e l y i n e t h a n o l i c r e a c t i o n mixtures b y v a p o r c h r o m a t o g r a p h y u s i n g a d e c y l p h t h a l a t e c o l u m n at 66°-68°C. 5

4

3

D e t e c t i o n a n d D e t e r m i n a t i o n o f H y d r o g e n P e r o x i d e . T i t a n y l sulfate s o l u t i o n (30) w a s u s e d f o r q u a l i t a t i v e d e t e c t i o n of h y d r o g e n p e r o x i d e . C o n t r o l measurements s h o w e d that the sensitivity l i m i t w a s 1 X 1 0 " M . I o d o m e t r y w i t h starch i n d i c a t o r w a s also u s e d to detect h y d r o g e n p e r oxide. K o l t h o f F s (17) p r o c e d u r e was f o l l o w e d strictly w i t h respect to the e x c l u s i o n of a i r o x i d a t i o n a n d c o r r e c t i o n f o r b l a n k . 4

Results Rates w e r e f o l l o w e d b y m o n i t o r i n g o x y g e n u p t a k e at constant pres sure. I n the earlier stages of this w o r k , s o l i d i r o n ( I I ) samples w e r e c o n t a m i n a t e d b y i r o n ( I I I ) o w i n g to a slight p r e o x i d a t i o n at the surface d u r i n g degassing a n d t h e r m a l e q u i l i b r a t i o n . C a r e f u l d u p l i c a t i o n of ex-


75.

HAMMOND AND wu

Iron(II)

Chloride

189

p e r i m e n t s gave r e p r o d u c i b l e results a n d a l l o w e d s e m i q u a n t i t a t i v e obser v a t i o n of the effects of v a r i o u s a d d i t i v e s . I n the later e x p e r i m e n t a l stages, b y u s i n g a s p e c i a l l y d e s i g n e d r e a c t i o n vessel, p r e o x i d a t i o n w a s e l i m i n a t e d . D a t a f r o m these runs gave reasonably accurate values of the true i n i t i a l o x i d a t i o n rates.

I n the tables r e l a t i v e reactivities are c i t e d w h e n d a t a

are c o m p a r e d that are o n l y accurate to the o r d e r of m a g n i t u d e . S o l v e n t Effects. A major a i m of o u r s t u d y w a s to l e a r n a b o u t the effects of l i g a n d s (solvents a n d c h l o r i d e i o n s ) o n the r e a c t i v i t y of i r o n ( I I ) w i t h molecular oxygen. iron (II) water. Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch075

T a b l e I shows that the rate of o x i d a t i o n of

is three to 1000 times as fast i n these o r g a n i c solvents as i n T h i s increase i n rate, as w e l l as the large difference

between

m e t h a n o l a n d e t h a n o l , c a n b e i n f e r r e d or seen f r o m p r e v i o u s reports. L i g a n d effects i n w a t e r h a v e b e e n binding iron (III)

(8,

a t t r i b u t e d to t h e i r effects o n

13, 14, 15, 33, 34, 35).

that the a c c e l e r a t i o n of i r o n ( I I )

However, w e shall show

oxidation by chloride ion i n methanol

s o l u t i o n is c a u s e d m o s t l y b y its effect o n i r o n ( I I ) rather t h a n o n i r o n ( I I I ) . A s a c o r o l l a r y w e suggest that solvent effects are also l a r g e l y associated w i t h l i g a n d effects o n the r e a c t i v i t y of i r o n ( I I ) . T a b l e I.

O x i d a t i o n o f I r o n (II) C h l o r i d e i n V a r i o u s Solvents a t 39.8 ± 0.2°C. [Fe(W] , M

Relative

AFe(II)

Ro

AO,

0.176 0.136 0.145 0.123 0.176

C H COCOC H methanol 6

5

6

5

F i g u r e 7 shows the representative runs, a n d T a b l e V s u m m a r i z e s d a t a f o r a n u m b e r o f runs. T h e r e a c t i o n is zero o r d e r w i t h respect to b e n z o i n , a n d t h e specific rates of o x y g e n u p t a k e are w i t h i n e x p e r i m e n t a l error t h e same as t h e i n i t i a l rates i n solutions c o n t a i n i n g t h e same amounts of iron (II)

chloride without benzoin.

C o m p a r i s o n a m o n g runs gives a

g o o d i n d i c a t i o n of the d e p e n d e n c e of the i n i t i a l rate o n the c o n c e n t r a t i o n of i r o n ( I I ) since a l l of the runs w e r e c a r r i e d o u t b y o u r best p r o c e d u r e . A l t h o u g h t h e i n i t i a l slopes m a y s t i l l b e s o m e w h a t i n error, t h e a c c u r a c y is g o o d e n o u g h to i n d i c a t e that t h e rates decrease w i t h

concentration

m o r e r a p i d l y t h a n is p r e d i c t e d b y t h e second-order l a w . W e are f a i r l y w e l l c o n v i n c e d b y t h e k i n e t i c s o b t a i n e d i n t h e presence of excess c h l o r i d e that t h e r e a c t i o n is second order w i t h respect to i r o n ( I I ) , so w e n e e d to a c c o u n t f o r t h e decrease i n rate b y some other e x p l a n a t i o n . A l i k e l y


196

OXIDATION O F ORGANIC COMPOUNDS

1.00-

III

.04-


17% 8 6 . 2 % O

3 I >

.75-

.03-

.50-

.02-

.25-

.01-

8

Oxidation of Iron(II) Chloride in Nonaqueous Solvents - Advances in

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