9 Nitric Acid Oxidations N O R M A N C. D E N O
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The Pennsylvania State University, University Park, Pa. 16802
The common mechanism for nitration of aromatic hydrocarbons consists of replacement of H by NO and in this process, oxidation-reduction plays no part. However, there are reactions of nitric acid and its reduction products that do produce nitro compounds and do involve complex oxidations and reductions. Examples are (a) the conversion of alkanes to smaller nitroalkanes via fragmentations, (b) the conversion of alkanes to nitroalkanes, (c) the nitration of anisole in which nitrosation and oxidation of the nitroso to nitro can be very much faster than direct nitration,(1) (d) the oxynitration of benzene to 2,4-dinitrophenol in the presence of Hg(II), and (e) the additions to double bonds to form a confusing variety of nitro and nitroso compounds and nitrites and nitrates. +
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Several circumstances t r a n s p i r e t o e f f e c t an atmosphere of u n c e r t a i n t y about most of these r e a c t i o n s . Much of the work dates back before the days of gc and nmr and a s i g n i f i c a n t f r a c t i o n i s over 50 years o l d . Furthermore, a l a r g e body of the work i s i n Russian. Noteworthy are the extensive s t u d i e s of T i t o v which were reviewed by T i t o v , ( 2 ) but j u s t before Usp. Khim. began t o be t r a n s l a t e d i n t o E n g l i s h . F o r t u n a t e l y , there i s one e x c e l l e n t review of most of the work i n E n g l i s h and that i s i n a book by Sosnovsky.(3) F i n a l l y , the systems themselves are enormously complex with HN0 , N 0 , HN0 , N 0 , NO, and N 0 a l l present and a l l able t o f u n c t i o n as o x i d i z i n g agents. Recently, we have completed some s t u d i e s on the o x i d a t i o n of c a r b o x y l i c acids and a l c o h o l s which suggest that the mechanism of o x i d a t i o n o f acids i s very d i f f e r e n t than that o f a l c o h o l s . Both oxidations are conducted with 70$ aqueous HN0 at 90-100° under good s t i r r i n g . The o x i d a t i o n of c a r b o x y l i c acids w i l l be considered f i r s t . When hexadecanoic ( p a l m i t i c ) a c i d i s o x i d i z e d with 70$ HN0 , a mixture of predominantly C u - C d i c a r b o x y l i c acids i s produced f i r s t , (Table I).(4) T h i s i s only evident at low conversions because these d i a c i d s undergo f u r t h e r o x i d a t i o n . A noteworthy aspect of the r e a c t i o n i s that products that would 3
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In Industrial and Laboratory Nitrations; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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Table I. Products from O x i d a t i o n 7 0 * HNOg (105-110°)
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D i a c i d of Carbon Number
of P a l m i t i c ( C
10$ C o n v e r s i o n
4 5 6 7 8 9 10 11 12 13 14 15 16 (a)
3
1 6
) A c i d with
100$ Conve: 2 3 8 18 21 21 13 7 3 3 1 0 0
0 1 2 3 4 8 12 14 16 16 14 10 0
From r e f . 4 (b) These d i f f e r somewhat from values i n r e f . 4 because o f a s h o r t e r r e a c t i o n time and s l i g h t l y higher T.
a r i s e by o x i d a t i o n at the carboxyl end of the molecule ( C , C , etc. monoacids) are never p r e s e n t . ( 4 ) Not only i s the carboxyl group and i t s environs i n e r t t o the o x i d a t i o n , but the o x i d a t i o n i s not s i g n i f i c a n t on CH groups u n t i l they are 8-10 carbons removed from the carboxyl group. The o x i d a t i o n i s thus e s s e n t i a l l y the o x i d a t i o n of an alkane. T i t o v had shown i n the o x i d a t i o n of alkanes that no r e a c t i o n occurs i n the absence of N 0 and that the f i r s t step i s a b s t r a c t i o n of a H atom by N 0 . ( 2 , 3 ) S i m i l a r l y the o x i d a t i o n of c a r b o x y l i c acids f a i l s i f urea i s present t o destroy N 0 . Having e s t a b l i s h e d that i n i t i a l attack i s by N 0 , i t i s of i n t e r e s t t o draw a p a r a l l e l between the oxidations i n Table I and c h l o r i n a t i o n s by R N C 1 . ( 5 ) In both cases e l e c t r o n e g a t i v e groups s t r o n g l y r e p e l the a t t a c k i n g reagent, N 0 and R3NÎ r e s p e c t i v e l y , and t o a degree without precedent i n s y n t h e t i c organic chemistry. In both cases the a t t a c k i n g reagent i s a f r e e r a d i c a l with the unpaired e l e c t r o n on Ν o r p o t e n t i a l l y on Ν and the Ν i s s u b s t i t u t e d by powerfully electron-withdrawing groups. The f a c t o r s r e s p o n s i b l e f o r the great p o l a r s e l e c t i v i t y i n the HNOg o x i d a t i o n of c a r b o x y l i c acids can be assumed t o be the same as those a t t r i b u t e d t o the R N H C 1 and R N C 1 c h l o r i n a t i o n s . ( 5 ) At 100$ conversion, the products are l a r g e l y C - C dicarb o x y l i c acids (Table I ) . These form because the C ^ - C ^ d i a c i d s are o x i d i z e d at the midpoint i n the chain. A necessary conse quence o f t h i s path i s that MW i s augmented and the weight of products i s more (^10$) than the weight of the r e a c t a n t . This i s observed.(4) The C - C d i a c i d s are r e l a t i v e l y i n e r t t o f u r t h e r o x i d a t i o n because a l l CH groups are now w i t h i n 4-5 carbons of a carboxyl group. A v a r i e t y of other c a r b o x y l i c acids have been 1 5
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In Industrial and Laboratory Nitrations; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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INDUSTRIAL AND LABORATORY NITRATIONS
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o x i d i z e d by H N 0 and the products are i n accord with the above principles. These n i t r i c a c i d oxidations i n the l i q u i d s t a t e c o n t r a s t sharply with oxidations i n the vapor s t a t e . The r e a c t i o n of alkanes with H N 0 o r N 0 at 1 5 0 - 2 2 0 ° gives n i t r o a l k a n e s . ( 2 , 3 , 6 - 8 ) The l i q u i d s t a t e oxidations produce only t r a c e amounts of C - N 0 compounds as shown by the i n f r a r e d s p e c t r a . A l c o h o l s are r a p i d l y o x i d i z e d by H N 0 , and the r e a c t i o n i s complete i n an hour at 9 0 ° . Products from four r e p r e s e n t a t i v e cases are shown i n Table I I . 3
3
2
2
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Table I I .
Products from HNO3 Oxidation of A l c o h o l s
i Y i e l d o f RCOOH of Carbon Number'a
Substrate
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at !
5
1-octanol 2-octanol 1-nonanol
6
0
8
7
36
56
8
9
0
68
32
0
0
0
10
65
25
$ Y i e l d of HOOC(CH ) COOH o f Carbon Number 2
8
12-hydroxystearic (a)
acid 0
9
10
0
9
n
11
12
41
50
Analyzed by gc of methyl e s t e r s .
These o x i d a t i o n s of a l c o h o l s are not b e l i e v e d t o i n v o l v e H a b s t r a c t i o n by N 0 . The p a r a l l e l behavior of R3N+ and N 0 has already been noted i n the remote attack on c a r b o x y l i c a c i d s . Since the a l c o h o l f u n c t i o n i s i n e r t t o R N t ( 5 ) , i t i s presumed that i t would a l s o be i n e r t t o N 0 . This suggests that the o x i d a t i o n o f a l c o h o l s by H N 0 i s an i o n i c r e a c t i o n i n v o l v i n g an i n t e r n a l breakdown of a n i t r a t e e s t e r . One curious aspect o f the H N 0 o x i d a t i o n of a l c o h o l s i s that a small amount of a l l y l i c o x i d a t i o n accompanies the normal oxidation. F o r example, o x i d a t i o n o f 1 2 - h y d r o x y s t e a r i c a c i d produces some C d i a c i d along with the expected C J J and C d i a c i d (Table I I ) . A t one time t h i s might have been a s c r i b e d t o o x i d a t i o n a t the
