Question

Write short notes on the following:
i) Tischenko reaction
ii) Cannizzaro's reaction
iii) Clemmensen reduction

Hint:

- Tischenko: aldehyde → ester (via Al(OR)$_3$ catalyst).
- Cannizzaro: aldehyde (no $\alpha$-H) → alcohol + carboxylate (via base).
- Clemmensen: carbonyl → hydrocarbon (Zn(Hg)/HCl).

Answer 1

i) Tischenko Reaction:
- It is the disproportionation of aldehydes (without $\alpha$-hydrogen) in the presence of an alkoxide catalyst to give esters.
\[ 2R{-}CHO \xrightarrow{Al(OR)_3} RCH_2O{-}COR \] Example:
\[ 2CH_3CHO \xrightarrow{Al(OC_2H_5)_3} CH_3CH_2OCOCH_3 \] (Product: ethyl acetate)
ii) Cannizzaro's Reaction:
- This is the base-induced disproportionation of aldehydes without $\alpha$-hydrogen into alcohol and carboxylate salt.
\[ 2R{-}CHO \xrightarrow{conc.\ NaOH} RCH_2OH + RCOONa \] Example:
\[ 2C_6H_5CHO \xrightarrow{NaOH} C_6H_5CH_2OH + C_6H_5COONa \] iii) Clemmensen Reduction:
- It is the reduction of carbonyl compounds (aldehydes and ketones) to hydrocarbons using zinc amalgam and concentrated HCl.
\[ R{-}CO{-}R' \xrightarrow[\Delta]{Zn(Hg)/conc.\ HCl} R{-}CH_2{-}R' \] Example:
\[ C_6H_5COCH_3 \xrightarrow{Zn(Hg)/HCl} C_6H_5CH_3 \] (Product: toluene)
Thus, these three reactions are important methods: Tischenko (ester formation), Cannizzaro (disproportionation), and Clemmensen (reduction).