6 Selectivity in the Liquid Phase Autoxidation of Olefins WILLIAM F. BRILL
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Petro-Tex Chemical Princeton, N. J.
Corp., FMC Chemical
Research and Development
Center,
Selectivity in the liquid phase autoxidation of olefins is determined by the influence of structure on the elementary abstraction and addition reactions. Epoxides, as well as hydroperoxides, are considered to be primary reaction products because of their detection at very early reaction stages and the stereochemistry of their formation. Cis- and trans-2-butene and 4-methyl-2-pentene produce mixed epoxides richer in the trans isomers. While the unrearranged hydroperoxide from trans olefin is produced with retention of geometric configuration, the production of trans hydroperoxide is favored from cis olefin. The isomeric 4-methyl2-pentenes show additional differences in behavior owing to the low reactivity of the tertiary hydrogen of the cis isomer.
y h e n a t u r e of t h e p r o d u c t s a n d t h e m e c h a n i s m b y w h i c h t h e y arise w h e n o x y g e n reacts w i t h u n s a t u r a t e d c o m p o u n d s has l o n g b e e n a subject of i n v e s t i g a t i o n i n s e v e r a l areas i n c h e m i s t r y . T h e o x i d a t i o n of olefins w i t h m o l e c u l a r o x y g e n is i n v o l v e d i n subjects as diverse as t h e d e g r a d a t i o n of p o l y m e r s a n d t h e biogenesis o f epoxides (11 ). T h e i n f o r m a t i o n r e v i e w e d h e r e stems f r o m a n interest i n p e t r o c h e m i c a l s a n d concerns t h e r e l a t i o n s h i p of olefin s t r u c t u r e to p r o d u c t s a n d t h e steric course of t h e o x i d a t i o n of cis a n d trans isomers. T h e p r o d u c t s o b t a i n e d w h e n l i q u i d olefins react w i t h o x y g e n m a y b e d i v i d e d i n t o t w o categories: ( 1 ) t h e p r i m a r y p r o d u c t s p r o d u c e d i n free r a d i c a l p r o p a g a t i o n steps w h i c h r e q u i r e t h e c o n s u m p t i o n of o x y g e n a n d , ( 2 ) t h e s e c o n d a r y p r o d u c t s w h i c h are p r o d u c e d b y s u b s e q u e n t i o n i c a n d 70 Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
6.
BRILL
Liquid Phase
71
Autoxidation
r a d i c a l reactions of t h e i n i t i a l p r o d u c t s .
Primary products m a y be recog
n i z e d b y t h e i r f a i l u r e to b e i n f l u e n c e d b y other t h a n d r a s t i c changes i n r e a c t i o n c o n d i t i o n s , b y t h e i r f o r m a t i o n at v e r y e a r l y stages i n h i g h t o t a l selectivities, a n d b y t h e e v e n t u a l a p p e a r a n c e of p r e d i c t a b l e products.
secondary
H y d r o p e r o x i d e s h a v e l o n g b e e n k n o w n as o x i d a t i o n p r o d u c t s
w h i c h fit these c r i t e r i a (14).
Epoxides have only more recently been
f o u n d to fit e q u a l l y w e l l ( 7 ) , t h e i r d e s c r i p t i o n as p r i m a r y p r o d u c t s b e i n g s u p p o r t e d also b y t h e stereochemistry of t h e i r f o r m a t i o n . whose occurrence polyperoxides
A third product,
is o n l y s u s p e c t e d f o r n o n c o n j u g a t e d
(10).
olefins, is t h e
W h a t e v e r c o n t r o l is p o s s i b l e i n i n f l u e n c i n g t h e
y i e l d s a n d selectivities to e n d p r o d u c t s i n o x i d a t i o n s is g e n e r a l l y d i r e c t e d Downloaded by CORNELL UNIV on October 31, 2016 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0051.ch006
t o w a r d s p r e s e r v i n g these p r o d u c t s a n d m o d i f y i n g s e c o n d a r y reactions.
Retoionship between Primary Products and Olefin Structure F i g u r e 1 depicts t h e r e l a t i o n s h i p b e t w e e n p r i m a r y p r o d u c t s a n d t h e n a t u r e o f t h e olefin.
E x a m i n i n g t h e olefins s h o w n a n d t h e r e l a t i v e y i e l d s
of p r i m a r y p r o d u c t s emphasizes t h e i m p o r t a n c e of steric factors. hexene a n d t h e u n c o n j u g a t e d 1,5-cyclo-octadiene hydroperoxide,
resembling cumene
i n this respect.
cr ο C=C-C=C
Cyclo-
p r o d u c e h i g h y i e l d s of T h e amount
of
oo
RoC=CR
RC=C
2
R-C-C=C
CUMENE
I
R
— *
Figure 1.
^-HYDROGEN ABSTRACTION-*
ADDITION-
Relationship
of structure to products
e p o x i d e p r o d u c e d b y o x i d i z i n g c y c l o h e x e n e varies f r o m a b o u t 3 % at l o w conversions to a m a x i m u m of 1 2 % at h i g h conversions. E p o x i d e h a s n o t b e e n i d e n t i f i e d i n t h e o x i d a t i o n p r o d u c t s of t h e u n c o n j u g a t e d c y c l o octadiene, a n d iodometric titration indicates that most of the oxygen r e a c t e d m a y b e a c c o u n t e d f o r as h y d r o p e r o x i d e at l o w c o n v e r s i o n . O n the o t h e r h a n d , cw-cyclo-octene, w h i c h h a s a m o r e s t r a i n e d r i n g a n d less favorable geometry t h a n cyclohexene a n d presumably cyclo-octadiene, p r o d u c e s e p o x i d e i n y i e l d s u p to 6 5 % of olefin r e a c t e d . I t s h o u l d b e
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
72
SELECTIVE OXIDATION PROCESSES
p o i n t e d out that a l l three compounds, representing epoxide yields r a n g i n g f r o m 3 % o r l o w e r t o 6 5 % , -possess f o r m a l l y s i m i l a r s e c o n d a r y a l l y l i c h y d r o g e n s , a n d p e r h a p s a l l c o u l d h a v e b e e n e x p e c t e d to p r o d u c e m o s t l y h y d r o p e r o x i d e b y r e a c t i o n at this p o s i t i o n .
T h e differences i n t h e r e l a
tive activity of the α-hydrogen a n d the double b o n d are apparently e n o u g h to cause a m a j o r c h a n g e i n p r o d u c t d i s t r i b u t i o n .
W h e n the overall
o x i d a t i o n rate of b o t h e i g h t - m e m b e r e d r i n g s w a s d e t e r m i n e d a n d c o r r e c tions w e r e m a d e o n t h e basis of a s e c o n d - o r d e r d e p e n d e n c y f o r t h e l a r g e r n u m b e r of a c t i v e a l p h a positions i n t h e d i e n e , b o t h olefins w e r e f o u n d to h a v e t h e same a c t i v i t y w i t h o x y g e n .
Thus, w h i l e the relative reactivity
of the a l p h a p o s i t i o n a n d o f the d o u b l e b o n d d e t e r m i n e t h e p r i m a r y p r o d Downloaded by CORNELL UNIV on October 31, 2016 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0051.ch006
u c t d i s t r i b u t i o n , r e a c t i o n at one site does n o t l e a d to a n i n h e r e n t l y faster rate t h a n r e a c t i o n at t h e other. T h e a c y c l i c olefins present w i d e differences i n p r o d u c t d i s t r i b u t i o n s a n d i n t e r e s t i n g differences
i n oxidation behavior.
Although
epoxide
y i e l d s , s h o w n f o r t h e l o w e r olefins i n T a b l e I , reflect t h e effect o f subse q u e n t r i n g o p e n i n g s as w e l l as t h e i r ease o f f o r m a t i o n a n d therefore a r e d e p e n d e n t to some extent o n c o n v e r s i o n levels, i t appears t h a t olefins w h i c h m a y b e o x i d i z e d t o epoxides i n t h e h i g h e s t y i e l d s are, i n g e n e r a l , the ones most s u b s t i t u t e d at t h e d o u b l e b o n d .
I n m a n y cases, a s i n g l e
a l d e h y d e a n d its c o r r e s p o n d i n g a c i d o r k e t o n e is p r o d u c e d i n g o o d y i e l d s f r o m e a c h olefin. i n g oxidative
T h e particular product m a y b e anticipated b y assum
fission
of t h e d o u b l e
bond.
T h u s , 2-butene
a c e t a l d e h y d e , a n d 1-butene p r o d u c e s p r o p i o n a l d e h y d e .
produces
Although only
the results f o r t h e cis-2-butene a r e s h o w n , £rans-2-butene y i e l d s essen t i a l l y t h e same p r o d u c t r e s u l t s — a p p r o x i m a t e l y a 5 0 % y i e l d o f e p o x i d e a n d a h i g h y i e l d of a c e t a l d e h y d e .
W h i l e t h e cis- a n d frans-2-butenes
s h o w n o significant difference i n p r o d u c t s o r rates, t h e cis- a n d trans-4methyl-2-pentenes
show
s u r p r i s i n g l y l a r g e differences
yields of epoxide a n d hydroperoxide obtained.
i n the relative
F o r this reason, t h e b e
h a v i o r o f these isomers w a s r e c e n t l y s t u d i e d i n d e t a i l , a n d t h e results w i l l b e d i s c u s s e d later. Hydroperoxide yields obviously are m u c h more dependent o n con v e r s i o n levels a n d reflect t h e a c c u r a c y o f a v a i l a b l e a n a l y t i c a l m e t h o d s , w h i c h are a l w a y s o p e n to q u e s t i o n .
F o r this reason, n o a t t e m p t w a s m a d e
to i n c l u d e t h e m f o r a l l olefins i n T a b l e I w h i c h shows results o b t a i n e d under practical synthetic conditions.
T h e relative amounts of h y d r o
p e r o x i d e a n d p r o d u c t s f o r m e d b y r e a c t i o n a t t h e d o u b l e b o n d is i m p o r tant to olefin o x i d a t i o n t h e o r y a n d h a s b e e n t h e p a r t i c u l a r object of a recent s t u d y ( J 9 ) .
E v e n a t h i g h conversions, h o w e v e r , w h e n t h e olefin
contains a t e r t i a r y a l l y l i c p o s i t i o n , t h e f o r m a t i o n o f h y d r o p e r o x i d e s a r e so f a v o r e d that t h e y m a y b e i d e n t i f i e d or, as f o r 2 - m e t h y l - l - b u t e n e , i s o l a t e d as the m a j o r p r o d u c t .
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
6.
BRILL
Liquid Phase Table I.
Effect of Structure in Olefin Oxidations Relative Rate
Olefin
Calc'd.
b
G—C=C G
I G—G=G G=C—C—G
73
Autoxidation
Observed
0.5
