Di-alpha-halogenation of ketones in acidic medium
Acid Catalysed Halogenation of ketone

In the halogenation of ketones, both in acidic and basic medium, enols are reactive intermediates. After addition of first halogen to double bond, deprotanation occurs immediately (Scheme 1). The second halogen does not add$\ce{^1}$.

Scheme 1



The introduction of second Bromine is slower then the first ,since cabocation is destabilized by the electron-attracting polar effect of two Bromines (**Scheme 2).

Scheme 2



Consequently, reprotonation of the monobrominated ketone does not occur.



Base Catalysed Halogenation of ketone

The mechanism of this reaction involves the formation of an enolate ion as a reactive intermediate. After mono bromination, the enolate ion of the alpha-bromo ketone is even more stable the enolate ion of the starting ketone.(Scheme 3). Consequently, a second bromination occurs$\ce{^2}$.

Scheme 3





As discussed above, in acid catalysed halogenation  the intermediate is unstable ,hence reprotonation to give dihalo products is less (If one equivalent of Bromine is taken).

Note$\ce{^3}$ :


Comparatively, in base calalysis the intermediate (Bromo enolate anion) is quite stable leading to di and tri bromination (provided acidic H is present).



Reference


Organic Chemistry, 5th Edition  by Prof. Marc Loudon 
Organic Chemistry, 5th Edition by Prof. Marc Loudon
Organic Chemistry, 2nd Edition ,Jonathan Clayden , Nick Greeves ,Stuart Warren.