6.0 Haloform Reactions

  Aldehydes and Ketones

Chlorine or bromine replaces one or more $\alpha $-hydrogen atoms in aldehydes and ketones, e.g., acetone may be brominated in glacial acetic acid to give monobromoacetone :

The halogenation of carbonyl compounds is catalysed by acids and bases. Let us consider the case of acetone. In alkaline solution, tribromoacetone and bromoform are isolated. Thus, the introduction of a second and a third bromine atom is more rapid than the first. In aqueous sodium hydroxide, the rate has been shown to be independent of the bromine concentration, but first order with respect to both acetone and base i.e.,

$rate{\text{ }} = {\text{ }}k{\text{ }}\left[ {acetone} \right]{\text{ }}\left[ {O{H^ - }} \right]$

Another point that we shall discuss here is: How is it possible for a $C—H$ b­ond to be broken so readily? This has been explained as follows. Owing to the inductive effect of the carbonyl group, the electrons of the $C—H$ bonds on the $\alpha $-carbon atom are displaced towards the carbon atom, thus facilitating the release of a proton from . ${C_\alpha }$At the same time, the anion produced has greater resonance stabilization than the parent carbonyl compound, i.e., the conjugate base is stabilized with respect to its acid.

The inductive effect is very much weaker on a $\beta $-carbon atom since the I effect falls off rapidly from the source. Hence proton release is far less easy than on $\alpha $-carbon. Also, if a proton were released from the $\beta $—C, this negative atom would be `insulated’ from the CO group by the intervening saturated carbon atom, and consequently such an anion would not have increased resonance stabilization. Thus, because of the increased reactivity of hydrogen in a methylene group $( - C{H_2} - )$ or in a methyne group $\left( { = CH - } \right)$ when adjacent to a carbonyl group or any other strongly electron-attracting group, these groups are referred to as the `active’ methylene or methyne group.

Aldehydes and ketones with $\alpha $-hydrogen atoms readily react with sulphuryl chloride at room temperature in the absence of a catalyst, to replace $\alpha $-hydrogen atoms only, e.g.,