Reaction with acids The reaction is closely related to the
Curtius rearrangement except that in this reaction the acyl azide is produced by reaction of the carboxylic acid with hydrazoic acid via the protonated carboxylic acid, in a process akin to a
Fischer esterification. An alternative, involving the formation of an acylium ion, becomes more important when the reaction takes place in concentrated acid (>90%
sulfuric acid). (In the Curtius rearrangement, sodium azide and an acyl chloride are combined to quantitatively generate the acyl azide intermediate, and the rest of the reaction takes place under neutral conditions.) The carboxylic acid Schmidt reaction starts with
acylium ion
1 obtained from
protonation and loss of water. Reaction with
hydrazoic acid forms the protonated azido ketone
2, which goes through a
rearrangement reaction with the alkyl group R, migrating over the C-N bond with expulsion of nitrogen. The protonated
isocyanate is attacked by water forming
carbamate 4, which after deprotonation loses
carbon dioxide to the
amine.
Reaction with ketones In the reaction mechanism for the Schmidt reaction of
ketones, the carbonyl group is activated by protonation for
nucleophilic addition by the azide, forming azidohydrin
3, which loses water in an
elimination reaction to diazoiminium 5. One of the alkyl or aryl groups migrates from carbon to nitrogen with loss of nitrogen to give a nitrilium intermediate
6, as in the
Beckmann rearrangement. Attack by water converts
6 to protonated imidic acid
7, which undergoes loss of proton to arrive at the imidic acid
tautomer of the final
amide. In an alternative mechanism, the migration occurs at
9, directly after protonation of intermediate
3, in a manner similar to the
Baeyer–Villiger oxidation to give protonated amide
10. Loss of a proton again furnishes the amide. It has been proposed that the dehydration to
3 to give
5 (and, hence, the Beckmann pathway) is favored by nonaqueous acids like conc. H2SO4, while aqueous acids like conc. HCl favor migration from
9 (the Baeyer-Villiger pathway). These possibilities have been used to account for the fact that, for certain substrates like
α-tetralone, the group that migrates can sometimes change, depending on the conditions used, to deliver either of the two possible amides. ==Reactions involving alkyl azides==