Weygand/Hilgetag Preparative Organic Chemistry

Removal of carbon monoxide 1021
This decarbonylation mechanism is involved in the following reactions:
Oxalic acid decomposes to carbon monoxide, carbon dioxide, and water
when heated alone above 100° or when treated with concentrated sulfuric
acid.
Oxalyl chloride decomposes to carbon monoxide and phosgene when
gently warmed with aluminum chloride: 50
(COC1)2 + AICI3 • [ + O=C—COC1] [AICI4-] •
CO + [O=C—Cl] [A1CV] > AICI3 + COC12
Intermediates analogous to the carboxonium salt formulated in the preceding
example are also formed during Friedel-Crafts acylations and these can
undergo decarbonylation, so that reaction of an aromatic compound with
an acid chloride may result in alkylation instead of acylation; for example,
2,2-dimethylpropionyl chloride reacts with aromatic compounds in the presence
of aluminum chloride to some extent in accord with the formulation:
AlCb ArH
(CH3)3CCOC1 - 3 ^ (CH3)3C + • ArC(CH3)3
This side reaction is very largely repressed if the aromatic compound is very
reactive, since the greater its reactivity the more likely it is to react with the
acyl cation before decarbonylation of the latter can occur. Thus, reaction of
2,2-dimethylpropionyl chloride and aluminum chloride under the same
conditions with benzene, toluene, and anisole gives about 90%, 50%, and
10%, respectively, of carbon monoxide. 51
a-Hydroxy acids sometimes lose carbon monoxide and water when heated
alone or in presence of concentrated sulfuric acid, yielding an aldehyde or
ketone. Yields are often high (up to 90%), especially in the aliphatic series
from C5 upwards. Acetonedicarboxylic acid can be prepared from citric
acid by this method: 52
HOOCCH2—C—CH2COOH > HOOCCH2COCH2COOH
HO COOH
Acetonedicarboxylic acid: Finely powdered citric acid is added in portions to 20 % oleum
at 0-10°. It dissolves rapidly and completely. When the solution is warmed slowly to 30°,
carbon monoxide is evolved; when this evolution ceases, the liquid is cooled again to 0° and
crushed ice is added, the temperature being allowed to rise slowly to 25-30°. The mixture is
then cooled again to 0° and filtered through a fritted filter. The solid is moistened several
times with a little ethyl acetate, being sucked dry after each addition. This affords 92-97 % of
acetonedicarboxylic acid. The acid is stable only for a few hours and further reactions with
it should be carried out immediately.
When the degradation of an a-hydroxy acid to the aldehyde containing
one fewer carbon atoms is unsatisfactory, the acetoxy acid or, better, according
to Darzens 53 the a-alkoxy carboxylic acid may be used. This procedure was
used, for instance, for preparation of heptadecanal: 2-bromostearic acid,
50 H. Staudinger, Ber. Deut. Chem. Ges.9 41, 3566 (1908).
51 E. Rothstein and R. W. Saville, /. Chem. Soc, 1949, (1946).
52 R, Adams, H. M. Chiles, and C. F. Rassweller, Org. Syn., 5, 5 (1925).
53 G. Darzens, C. R. Hebd. Seances Acad. ScL, 196, 348 (1933).