55 Air Oxidation of Alcohols to Esters via
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Bromine-Nitric Acid Catalysis E. F. LUTZ Shell Development Co., Emeryville, Calif. 94608
Primary aliphatic alcohols are air-oxidized to the corresponding esters in the presence of a bromine—nitric acid catalyst. Evidence supporting a mechanism, which is not a free radical chain process, is presented.
^ p h e bromine oxidation of alcohols ( I ) was first reported in 1901 by Bugarszky, who found that ethanol formed ethyl acetate in concen trated aqueous alcohol (76% weight E t O H ) and acetic acid in dilute solution (3, 4). The oxidation of acetaldehyde in aqueous solution, now known to proceed through the hydrate (6), gave acetic acid (5). T r i bromide ions, formed during the reaction, do not oxidize ethanol and are responsible for a decrease in reaction rate with increasing time (6). Bromination becomes an important side reaction at low p H (6). Sec ondary alcohols oxidize to ketones (2). A
C H — C H O H + B r - » C H — C H O + 2 HBr 3
2
2
3
C H — C H O + Br + R O H -> C H C O O R + 2 HBr 3
2
3
R = H — or C H C H — 3
2
The application of bromine oxidation in a catalytic fashion attracted our attention as a useful route to acids and esters. Results and Discussion H i g h valent ( > + 2) nitrogen oxides catalyze the air oxidation of hydrogen bromide to bromine (9), and this led to the development of an alcohol air-oxidation catalyst based on bromine and nitric acid. The higher aliphatic alcohols ( C — C ) , as summarized in Table I, are oxidized in high yield to the corresponding esters. 8
2 i
389
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
390
OXIDATION O F
ORGANIC COMPOUNDS
Π
Ο Brs—HNO.,
2 R—CH OH
-»
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2
+
11
R—C—O—CH —R 2
air
The oxidation is fast (even in a rocking autoclave), shows no induction period, and is essentially unaffected by the form in which the catalyst is added. N o oxidation is obtained when either of the catalyst components is used independently or when chlorine is substituted for bromine. Free acid is not obtained with the water-insoluble alcohols. In contrast, the alcohols of low molecular weight give poor, yields and a mixture of products. This can be explained by the intermediate aldehydes' water solubility, which favors enolization and hydration in the acidic medium, leading to catalyst consumption and a distribution of products.
Br —HN0 CH —CH —OH —2—. 2
3
3
2
[CH —CHO] 3
ROH, H© ^
OH ' I CH —C—Η ' I O—R S
r
|H O©
oxidation
3
OH
I-
I
-
CH —C—Η J
2
I
Br
Br
Br
Γ
/° Ί
CH —COOR
Η
—Γ% J 2
3
R = H — or Et—
I -HBr Br—CH —CHO 2
H® EtOH : • oxid.
Ο
" B r — C H — C — O — C H — C H + other products 2
2
3
H 0®oxid. 3
\
Br—CH —COOH + other products 2
The alcohols of higher molecular weight brominate to a minor extent and this is probably caused, at least in part, by thermal bromination during the initial exotherm. The oxidation of lauryl aldehyde gives lauric acid in 53.4% yield. This reaction is not exothermic and is much slower than that of the corresponding alcohol. Neopentyl glycol oxidizes to a low molecular weight polyester in 2 1 % yield. Upon completion of the oxidation, bromine was always found in the organic layer when phase separation occurred.
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
55.
LUTZ
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Mechanism of
391
Air Oxidation of Alcohols Oxidation
Our observations are summarized as follows: (1) no induction period, (2) fast alcohol oxidation in an oxygen-poor liquid phase, (3) no carboxylic acids from the higher alcohols, (4) slow oxidation of lauryl aldehyde to lauric acid in the presence of water, and (5) recovery of bromine in the organic phase on reaction completion. These data show that the reaction is not a radical chain process but rather a bromine oxida tion in which the halogen is continuously regenerated, as shown in Reactions 1 through 7.
R—CHoOH + Br
R—CH
R — C H O + 2 HBr
Br
(1)
2
R—CHO + R — C H — O H
H
OH I R—C—Η I O—CH
Ο
' +
2
Br
II
2
—Λ.
R—C—Ο—CH —R 2
2
+ 2 HBr
(2)
HBr + H N 0 = H 0 + N 0 B r
(3)
HBr + N 0 B r = Br + H N 0
(4)
3
2
2
2
2
2
3 H N 0 = HNO3 + 2 N O + H 0
(5)
2 NO + 0
(6)
2
2
2
= 2 N0
2
3 N 0 + H 0 = 2 H N 0 + NO 2
2
(7)
3
A cyclic transition state as proposed by Barker et al. (2) is most attractive for the initial bromine oxidation of the alcohol since the reaction probably takes place in the organic phase. Other transition states have also been proposed which are better suited to homogeneous oxidations in aqueous solution (8, 13). The regeneration of bromine from nitryl bromide and hydrogen bromide in the organic phase is analogous to the known formation of bromine from hydrogen bromide and N-bromosuccinimide (JO, 11). (The physical properties of nitryl bromide have not been described because of its fleeting existence, and the assumption is made that its solubility in the organic phase w i l l be similar to that of bromine.) Talbot and Thomas (14) have postulated that N O C 1 reacts with surface-adsorbed HC1 at 25° to 55°C. to form C l . 2
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
392
OXIDATION
O F ORGANIC
COMPOUNDS
Table I.
Alcohol, Mmoles
Ethanol, 200
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Alcohol
Temp., °
