Question

Write a note on Clemmensen reduction.

Answer 1

1. The carbonyl group C = O on reduction with zinc amalgam (Zn–Hg ) in concentrated hydrochloric acid is converted into methylene group (–CH₂ –).
   \[\ce{C = O + 4[H] ->[Zn–Hg][conc. HCl] = CH2- + H2O}\]
2. Aldehydes and ketones on reaction with Zn –Hg in concentrated HCl forms corresponding alkanes. This reduction is called Clemmensen reduction.
   \[\begin{array}{cc}
   \ce{O}\phantom{.............................}\\
   ||\phantom{.............................}\\
   \ce{\underset{Ketone}{R-C}-R + 4[H] ->[Zn –Hg][conc. HCl] \underset{Alkane}{R–CH2 –R}}
   \end{array}\]
3. Acetaldehyde on reduction with Zn–Hg in concentrated HCl forms ethane.
   \[\begin{array}{cc}
   \ce{O}\phantom{...................}\\
   //\phantom{.....................}\\
   \ce{CH3 - C + 4 [H] ->[Zn–Hg][conc. HCl] \underset{Ethane}{C2H6} + H2O}\\
   \backslash\phantom{.....................}\\
   \ce{\underset{Acetaldehyde}{H}}\phantom{....................}
   \end{array}\]