Carbonium Ions, Electron-deficient N and O Atoms
O—C—N—R
(XXII)
H.OHOaC—NH—R > 0 2 C+H 2 NR
(XXIII)corresponds closely to the ketene (XVII) obtained from the latter.
The reaction is completed by hydration of the isocyanate to yield the
carbamic acid (XXIII) which undergoes spontaneous decarboxyla
tion to the amine. The N-bromamide (XIX), its anion (XX) and the
isocyanate (XXII) postulated as intermediates can all be isolated
under suitable conditions.
TJj^rate-determiniig step of the reaction is the loss of Br® from
the ion (XX) but it is probable that the loss of Br® and the migration
of R take place simultaneously, i.e. effectively internal SN 2 once again.
It might be expected that the more electron-releasing R is, the more
rapid would be the reaction: this has been confirmed by a study of
the rates of decomposition of benzamides substituted in the nucleus
by electron-donating substituents.
There are two reactions very closely related to that of Hofmann,
namely the Curtius degradation of acid azides (XXIV) and the
Lossen decomposition of hydroxamic acids (XXV), both of which
also yield amines; all three reactions proceed via the isocyanate as a
common intermediate (see p. 95).
The Lossen reaction is, in practice, normally carried out not on the
free hydroxamic acids but on their O-acyl derivatives which tend to
give higher yields; the principle is, however, exactly analogous except
that now R'-COO® instead of HO® is expelled frojn the anion. In
the Curtius reaction, the azide is generated as required by the action
of sodium nitrite and acid on the hydrazide; if the reaction is carried
out in solution in an alcohol instead of in water (nitrous acid being
o o o
II BrOQ II ®OH II A
R-C-NH 3 > R-C-NH Br —A- R_C-N^Br
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(XIX) (XX)