A Guidebook to Mechanism in Organic Chemistry

Migration to Electron-deficient Nitrogen Atoms

Ag.O

H.O

-N,

RCH 2 C—OH RCH=C=0

In aqueous solution, the acid is obtained directly by addition of

water to the ketene but if the reaction is carried out in ammonia or

an alcohol the corresponding amide or ester, respectively, may be

obtained directly.

MIGRATION TO ELECTRON-DEFICIENT NITROGEN ATOMS

The reactions involving rearrangement of structure that we have

already considered all have one feature in common: the migration of

an alkyl or aryl group with its electron pair to a carbon atom which,

whether a carbonium ion or not, is electron-deficient. Another atom

that can similarly become electron-deficient is nitrogen in, e.g.,

R 2 N® or RN, and it might be expected that the nitrogen atoms in

such species should be able to induce migration to themselves as is

observed with R 3 C or R 2 C. This is indeed found to be the case.

(i) The Hofmann, Curtius and Lossen reactions

A typical example is the conversion of an amide to an amine con­

taining one carbon less by the action of alkaline hypobromite, the

Hofmann reaction (see p. 94).

It will be noticed that the species (XXI) has an electron-deficient

nitrogen atom corresponding exactly to the electron-deficient carbon

atom in the carbene (XVIII) from the Wolff rearrangement, and that

the isocyahate (XXII) obtained by the former's rearrangement

The intermediate (XVIII) is not a carbonium ion but it is never­
theless an electron-deficient species, known as a carbene, so the R
group migrates with its electron complement complete as in the cases
we have already considered. The diazoketone may be obtained by
the reaction of diazomethane, CH 2 N 2 , on the acid chloride and the
Wolff rearrangement is of importance because it constitutes part of
the Arndt-Eistert procedure by which an acid may be converted into
its homologue:

. ° ° °
II ' SOCl, II CH.N, II
RC-OH > R-C—CI —-> RC—CHNa