α-HALOGENATION
Aldehydes
and ketones react with halogens under acid conditions, resulting in
halogenation at the α-carbon.
The
mechanism involves formation of the enol tautomer, which acts as a nucleophile.
A halogen atom is bound to the α-carbon and the final step involves loss of a
proton.
Treatment
of a methyl ketone with excess iodine and sodium hydroxide results in
tri-iodination of the methyl group. The resulting CI3 group is a
good leaving group and is displaced by the hydroxide ion to form a yellow
precipitate (CHI3).
Aldehydes and ketones react with chlorine,
bromine or iodine in acidic solution, resulting in halogenation at the α-carbon (Fig. 1).
Since acid conditions are employed, this
process does not involve an enolate ion. Instead, the reaction takes place
through the enol tautomer of the carbonyl compound. The enol tautomer acts as a
nucleophile with a halogen by the mechanism shown (Fig. 2). In the final step,
the solvent acts as a base to remove the proton.
α-Halogenation can also be carried out in the presence of base. The
reactionproceeds through an enolate ion which is then halogenated (Fig. 3). However, it is difficult to
stop the reaction at mono-halogenation since the resulting product is generally
more acidic than the starting ketone due to the electron-withdrawing effect of
the halogen. As a result, another enolate ion is quickly formed leading to
further halogenation.
This tendency towards multiple halogenation is
the basis for a classical test called the iodoform test which is used to
identify methyl ketones. The ketone to be tested is treated with excess iodine
and base and if a yellow precipitate is formed, a positive result is indicated.
Under these conditions, methyl ketones undergo α-halogenation three times (Fig.
4). The product obtained is then suscep-tible to nucleophilic substitution
whereby the hydroxide ion substitutes the tri-iodomethyl (−CI3) carbanion – a good leaving
group due to the three electron-withdrawing iodine atoms. Tri-iodomethane is
then formed as the yellow precipitate.
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