Question

Explain haloform reaction with suitable example.

Write a note on the Haloform reaction.

Answer 1

1. This reaction is given by acetaldehyde, all methyl ketones (CH₃–CO–R), and all alcohols containing CH₃(CHOH)– group.
2. When an alcohol or methyl ketone is warmed with sodium hydroxide and iodine, a yellow precipitate is formed. Here the reagent sodium hypoiodite is produced in situ.
3. During the reaction, the sodium salt of the carboxylic acid is formed which contains one carbon atom less than the substrate.
4. The methyl group is converted into haloform (CHX₃).
   e.g. Acetone is oxidized by sodium hypoiodite to give sodium salt of acetic acid and yellow precipitate of iodoform.
   \[\begin{array}{cc}
   \phantom{..}\ce{O}\phantom{.......................................}\ce{O}\phantom{..........................}\\
   \phantom{..}||\phantom{.......................................}||\phantom{..........................}\\
   \ce{\underset{\text{Acetone}}{CH3 - C - CH3} + \underset{\text{Sodium hypoiodite}}{3NaOI} ->[NaOH/I2][\Delta] \underset{\text{Sodium acetate}}{CH3 - C - O- Na+} + \underset{\text{Iodoform}}{CHI3 ↓} + 2NaOH}
   \end{array}\]