66 Comments on the Mechanism of Ozone
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Rate Action with Hydrocarbons and Alcohols SIDNEY W.
BENSON
Department of Thermochemistry and C h e m i c a l Kinetics, Stanford Research Institute, M e n l o Park, Calif. 94025
The
ozone molecule has a bond dissociation energy of 24 kcal. for decomposition into an oxygen molecule and a ground state oxygen atom. Almost all of the gas-phase work I know of concerning ozone and organic materials (for Τ > 25°C.) can be explained in terms of an initiation step, producing oxygen atoms from the ozone, followed by chain-propagation steps, as follows:
Step 2 in this sequence is exothermic by about 66 kcal. If any significant share of this is left as excitation energy of the alkoxy radical, it w i l l prob ably dissociate further into a simple aldehyde or ketone, plus a smaller alkyl group. However, at dry ice temperatures or lower, the initiation step, i, is almost certainly too slow to account for the reaction rates which have been reported here. The suggestion—that ozone attacks alcohols and hydrocarbons by abstracting hydrogen to form the HO radical— appears equally unlikely. 3
The Η—Ο bond in the HO radical can be calculated to have a bond strength of about 61 ±2 kcal. (1). If the R—Η bond is taken as 91, as 3
74
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
66.
Ozone Rate
BENSON
Action
75
r e p o r t e d for 2 - p r o p a n o l or i n isobutane, t h e n R e a c t i o n 4 is e n d o t h e r m i c b y some 30 k c a l . A t d r y ice temperatures this is even s l o w e r b y m a n y p o w e r s of 10 t h a n the i n i t i a t i o n step, i , above.
Step 4 i n a l c o h o l s o l u t i o n
m a y benefit b y a heat of s o l u t i o n of the H C V r a d i c a l s to the extent of p o s s i b l y 10 or p e r h a p s 12 k c a l . T h i s w o u l d m a k e the o v e r - a l l r e a c t i o n e n d o t h e r m i c b y o n l y 18 or 19 k c a l . , if w e i g n o r e a n y possible s o l v a t i o n of ozone, w h i c h m a y a m o u n t a c t u a l l y to a f e w kcalories. Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 19, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch066
T h i s is still too h i g h a n a c t i v a t i o n e n e r g y to a c c o u n t for a s i m p l e one-step reaction, w h i c h has b e e n r e p o r t e d .
O n the other h a n d , if the
net i n c r e m e n t i n s o l v a t i o n energy is as large as 12 k c a l . , t h e n Step 4 c o u l d c o n c e i v a b l y be a n i n i t i a t i o n step i n a r e a s o n a b l y l o n g c h a i n m e c h a n i s m . If, h o w e v e r , a c h a i n r e a c t i o n is i n v o l v e d , the most l i k e l y next steps w o u l d be the attack of R - r a d i c a l s o n either o x y g e n or o n ozone. T h e o x y g e n r e a c t i o n is l i k e l y to b e d i f f u s i o n c o n t r o l l e d a n d w o u l d l e a d to p e r o x y - t y p e products.
H o w e v e r , attack o n ozone is m o r e l i k e l y to l e a d to f o r m a t i o n
of free a c i d a n d smaller a l k y l r a d i c a l s , a n d these are not o b s e r v e d as major p r o d u c t s . It thus appears as if a r a d i c a l m e c h a n i s m c a n n o t satisfy the observa tions. T h e alternative has b e e n suggested that Step 4 occurs as a h y d r i d e i o n a b s t r a c t i o n w i t h the f o r m a t i o n of a d i s s o c i a t e d i o n p a i r , H 0 " a n d R . 3
+
If this is the case, it w o u l d r e q u i r e u n u s u a l l y large e l e c t r o n affinities for the H 0 3
r a d i c a l or u n u s u a l l y s m a l l i o n i z a t i o n potentials for the
R-
r a d i c a l to a c c o u n t for the data. I don't b e l i e v e that i n d e p e n d e n t measure ments of these q u a n t i t i e s c a n be a d d u c e d to s u p p o r t s u c h a c o n t e n t i o n . Nevertheless, a n i n t i m a t e i o n p a i r m e c h a n i s m cannot be c o m p l e t e l y r u l e d out at the m o m e n t . T h e t h e r m o c h e m i s t r y of p o l y o x i d e r a d i c a l s a n d m o l e c u l e s c a n also b e u s e d to c o m m e n t o n the m e c h a n i s m of olefin o z o n i z a t i o n . O n e c a n ask if a d d i t i o n of ozone to the d o u b l e b o n d occurs via
a biradical with
the f o r m a t i o n of one b o n d at a t i m e or t h r o u g h a c o n c e r t e d process to p r o d u c e c y c l i c t r i o x i d e . If it p r o d u c e s one b o n d at a t i m e , it m u s t go t h r o u g h the f o r m a t i o n of a n R 0
3
• t y p e b i r a d i c a l as s h o w n i n R e a c t i o n 5.
H o w e v e r , the energetics of R e a c t i o n 5
(I)
s h o w that the
activation
energy w o u l d b e m u c h too h i g h f o r this r e a c t i o n to be a p p r e c i a b l e at d r y ice temperatures, w h e r e it has b e e n o b s e r v e d .
W e must then conclude
that a d d i t i o n of ozone to olefin at l o w temperatures, or p r o b a b l y also at h i g h e r t e m p e r a t u r e , is a c o n c e r t e d process l e a d i n g to the d i r e c t f o r m a t i o n of a c y c l i c t r i o x i d e , R e a c t i o n 5', a n d this is p r e t t y m u c h i n a c c o r d w i t h the d a t a recently r e p o r t e d .
H o w e v e r , the other d a t a r e p o r t e d o n the
s t a b i l i t y of these c y c l i c peroxides is i n a c c o r d w i t h o u r o w n estimates of the b o n d strength of the O — O b o n d i n s u c h trioxides. N o r m a l l y , it w o u l d be of the order of 21 k c a l . I n the case of the s t r a i n e d
five-membered
ring
w i t h strain energies a r o u n d 6 k c a l . , it s h o u l d have a b o n d strength of
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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OXIDATION
OF
ORGANIC
COMPOUNDS
III
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8
some 1 5 k c a l . T h u s the o p e n i n g of this r i n g to p r o d u c e the o x y p e r o x y b i r a d i c a l , as s h o w n b y R e a c t i o n 6 , w o u l d b e e x p e c t e d to be r e a s o n a b l y r a p i d at d r y ice temperatures.
O n e m i g h t expect this b i r a d i c a l to be the
p r e c u r s o r of the p r o d u c t s w h i c h are seen i n the subsequent d i s p l a c e m e n t r e a c t i o n w i t h c a r b o n y l a n d a l d e h y d e species or i n the s e c o n d a r y d e c o m p o s i t i o n of ozonides.
Literature Cited (1)
Benson, S.
W.,
Shaw,
R., ADVAN. C H E M . SER.
75,
288 (1968).
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
