Question

Give reasons for the following.
Benzaldehyde reduces tollen’s but not the Fehling’s or benedict’s solution

Answer 1

Hint: Benzaldehyde has a carbonyl group and benzene. Benzene ring produces a +R effect on the carbonyl group. Due to +R effect of the benzene ring of the benzaldehyde, the electron density in the carbonyl group of benzaldehyde increases.

Complete answer:
Due to this, the electron density in the \[CH\] bond of the aldehyde group increases.
 As a\[CH\] result, the bond becomes stronger and hence only oxidising agent like tollens agent \[Ag{(N{H_3})_2}^ + \] which as can oxidise \[CH\] bond into \[COH\] bond, from carboxylic acids.
This is not possible for weaker oxidising agents like Fehling’s solution or Benedict's solution fail to oxidise benzaldehyde into benzoic acid.
In Fehling solution, it is an alkaline solution of copper sulphate which contains sodium potassium tartrate.
When Fehling’s solution is added to an aldehyde, it produces a red coloured precipitate. This red coloured precipitate is actually cuprous oxide. When we add Fehling’s solution with acetaldehyde, it gives acetic acid and cuprous oxide.
\[C{H_3} - CHO + 2CuO \to C{H_3}COOH + C{u_2}O\]
But when benzaldehyde is added, it rapidly converts into benzyl alcohol and sodium benzoate because of the presence of alkali. It occurs with the help of a Cannizzaro reaction. Here, there is no formation of cuprous oxide to give out a red precipitate.

Note:
In Fehling solution, it is an alkaline solution of copper sulphate which contains sodium potassium tartrate. This red coloured precipitate formed is actually cuprous oxide. Benzaldehyde has presence of both carbonyl group and benzene.