Haloform reaction

Compounds which have one of the following structural units, on heating with a halogen ( Cl₂, Br₂ or I₂) in the presence of a base like NaOH or KOH give rise to a haloform (chloroform, Iodoform).

Chloroform
It is prepared by heating bleaching powder [Cl₂ + Ca(OH)₂ ] and ethanol. It is Chloroform is formed as a colorless liquid. It is generally preserved in the presence of small amounts of ethanol (negative catalyst) to prevent the formation of carbonyl chloride that is phosgene which is highly poisonous.
Chloroform was used as an anesthetic during surgery; it is no longer used now because of severe side effects like liver and cardiac toxicity.  An anesthetic used at present is Halothane. ( F₃CCHBrCl).

Net reaction using NaOH as the base

Steps

Note:   1.  First step is oxidation of alcohol by the halogen.

Aldehydes and ketones with a methyl group next to the carbonyl group, will also give rise to haloform.
The difference is the oxidation step will not be there.

Substitution of the three α-hydrogens by halogen atoms take place in the second step.
The hydrogens replaced by halogens are all from the same side of the carbonyl group, in compounds like
CH₃COCH₃.  Once hydrogen is replaced by halogen the other hydrogen atoms at that carbon become more acidic causing further substitution at the same place.

Attack of the base on the trisubstituted compound resulting in haloform.