Question

Write a note on the Haloform reaction.

Answer 1

Sodium hypohalite, a combination of sodium hydroxide and halogen, oxidises a ketone with a –COCH₃ group. The outcome is a sodium salt of carboxylic acid with one fewer carbon atom than the ketone, while the methyl group is transformed into haloform.

\[\begin{array}{cc}
\ce{O}\phantom{....................................................}\ce{O}\phantom{....................................}\\
||\phantom{.....................................................}||\phantom{....................................}\\
\ce{\underset{Methyl ketone}{R - C - CH3} + \underset{hypohalite}{\underset{Sodium}{3NaOX}} ->[(NaOH/X2)][\Delta] \underset{carboxylic acid}{\underset{Sodium salt of}{R - C - \overset{-}{O}N\overset{+}{a}}} + \underset{Haloform}{CHX3} + 2NaOH}\\
\end{array}\]

Acetaldehyde is the only aldehyde which gives the haloform reaction.

A yellow precipitate of iodoform is produced when acetone is warmed with iodine and sodium hydroxide.

The iodoform reaction is used as a qualitative test to detect the CH₃CO– group in an organic compound.

Example:

\[\begin{array}{cc}
\ce{O}\phantom{.......................................................}\ce{O}\phantom{.................................}\\
||\phantom{........................................................}||\phantom{.................................}\\
\ce{\underset{Acetone}{CH3 - C - CH3} + \underset{hypohalite}{\underset{Sodium}{3NaOI}} ->[(NaOH/I2)][\Delta] \underset{Sodium acetate}{CH3 - C - \overset{-}{O}N\overset{+}{a}} + \underset{(Yellow precipitate)}{\underset{Iodoform}{CHI3\downarrow + 2NaOH}}}\\
\end{array}\]