Why does benzaldehyde not respond to Fehling's test?
The rate-limiting step of the Fehling’s test reaction with aldehydes is the formation of the corresponding enolate:



The subsequent reaction of the enolate with copper(II) proceeds through a single electron transfer mechanism.

Aldehydes that lack alpha hydrogens, such as benzaldehyde or pivalaldehyde (2,2-dimethylpropanal) cannot form an enolate and thus do not give a positive Fehling’s test result under usual conditions.