Question

Write a note on the Wolf-Kishner reduction reaction.

Answer 1

The carbonyl group of aldehydes and ketones is reduced to a methylene group (–CH₂–) on treatment with zinc-amalgam and concentrated hydrochloric acid (Clemmensen reduction) or hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent like ethylene glycol (Wolf Kishner reduction).

\[\begin{array}{cc}
\phantom{..}|\phantom{....................}|\phantom{........................................}|\phantom{.........}\\
\ce{\underset{ketones)}{\underset{in aldehyde and}{\underset{(Carbonyl group}{- C = O}}} ->[H2N-NH2][-H2O] \underset{(Hydrazone)}{- C = N - NH2} ->[KOH, HO-CH2-CH2-OH][\Delta] \underset{(Methylene group)}{- CH2} + N2}\\
\end{array}\]

For example:

\[\ce{\underset{(Propanal)}{C2H5 - CHO} ->[H2N - NH2][-H2O] \underset{(Hydrazone)}{CH3 - CH2 - CH3 + N2} ->[KOH, HO-CH2-CH2-OH][\Delta] \underset{(Propane)}{CH3 - CH2 - CH2 + N2}}\]