Question

Give reason for the following:
Benzaldehyde reduces Tollen's reagent but not the Fehling's or Benedict;s solution.

Answer 1

Due to $+R$ effect of benzene ring, the electron density in the carbonyl group of benzaldehyde increases. This in turn, increase the electron density in the $C-H$ bond of aldehyde group. As a result, the $C-H$ bond becomes stronger and hence only oxidising agent like Tolllens agent; $Ag\left(N{H}_3{)}_2^{+}\right(E_{A{g}^{+}/Ag}^o=0.8V)$ can oxidise $C-H$ to $C-OH$ to from carboxylic acids but weaker oxidising agent like Fehling's solution or Benedict's solution $(E_{Cu2+/C{u}^{+}}^o=0.18V)$ fail to oxidies benzaldehyde to benzoic acid.