Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

THE HOFMANN AND CURTIUS EBACTIONS 155
Both, processes furnish, the primary amine corresponding to the next
lower homologue of the series, and both lead to this result by way of the
same intermediate product, namely, the cyanic ester.
By the action of hypobromite on the —-CONH2-group one hydrogen
atom of the NH2-group is replaced by bromine. The first product of
the Hofmann reaction, the N-bromoamide, can be isolated in certain
cases.
By the action of alkali the bromoamide loses HBr, and the transient
radicle which is thus formed undergoes rearrangement to a cyanic ester
which, under the experimental conditions of the reaction, is decomposed
into a primary amine and CO2:
R.C:O
| —
NH2
R.C:O
HNBr
R.C:O
_ A _
—>• RN;C:O —•> R.NH, + C0
In this way acetamide furnishes meihylamine, benzamide yields
aniline, and urea, albeit in small amount, forms hydrazine.
Hydroxamic acids are transformed in a similar way by loss of water
into cyanates and so into amines.
The reaction of Curtius, which is especially to be preferred in the
case of the higher members on account of the favourable solubilities
of the intermediate products, involves as its first stage the preparation
of the hydrazide from an ester (or acid chloride). The hydrazide is
then converted, usually very readily, by the action of nitrous acid into
the azide. In many cases it is more convenient to prepare the azide by
treating an acid chloride with sodium azide previously activated with
hydrazine hydrate. 1 Azides easily undergo thermal decomposition, the
two " azo " nitrogen atoms being eliminated as elementary nitrogen.
In this way, however, the same radicle is formed as was invoked above to
explain the Hofmann reaction :
R.C:O
R.C:O
N
A
-N,
—> RN:C:O .
HN.NH,
N
A
N-N
Curtius usually decomposed the azides in alcohol, which at once
explains the formation of urethanes; these, by vigorous hydrolysis,
decompose into primary amine, CO2, and alcohol.
An important application of the Hofmann reaction occurs in the
first technical synthesis of indigo and consists in the degradation of
phthalimide to anthranilic acid. See p. 372.
1
J. Nelles, Ber., 1932, 65, 1345.