Halogenation in base

Unlike its acid-catalyzed counteipart, a halogenation in base cannot nomnally be limited to monohalogenation. Methyl ketones, for example, undergo a novel polyhalo-genation and cleavage on treatment with a halogen in aqueous base. 

For this reaction the equilibrium constant is in each case very favorable, that is, 1027 for CIO", 1015 for BrO", and 1020 for IO". Thus the actual products obtained on dissolving the halogens in base depend on the rates at which the hypohalite ions initially produced undergo disproportionation, and these rates vary from one to the other and with temperature. 

Halogenation in base is much less useful, because it is often difficult to stop the reaction after addition of just one halogen atom to the a carbon. For example, treatment of propiophenone with Br2 and aqueous OH yields a dibromo ketone. 

The hypohalite ions can all be produced in principle by dissolving the halogens in base according to the general reaction X2 + 20H" X-+X0-+H20... 

In Summary Treatment of primary and secondary amides with base leads to deprotonation at nitrogen, giving amidate ions. Bases abstract protons from the a-carbon of tertiary amides. In the Hofmann rearrangement, primary amides react with halogens in base to furnish amines with one fewer carbon. In the conrse of this process an alkyl shift takes place, which converts an acyl nitrene into an isocyanate. 

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