Question

why benzal dehyde doesnt respond fehlings test

Answer 1

The rate-limiting step of theFehling's test reaction with aldehydes is the formation of the corresponding enolate: ... Aldehydes that lack alpha hydrogens, such asbenzaldehyde or pivalaldehyde (2,2-dimethylpropanal) cannot form an enolate and thus do not give a positive Fehling's test result under usual conditions.

Answer 2

The rate-limiting step of the Fehling’s test reaction with aldehydes is the formation of the corresponding enolat

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.

OR U CAN WRITE




In benzaldehyde, the carbonyl group is an electron withdrawing group so the carbonyl group pulls the electron from the electron-rich benzene ring. Due to this, the polarity of the C−HC−H bond in the carbonyl group is reduced because the C−HC−H bond now has a higher electron density. Thus, the C−HC−H bond becomes stronger (the lesser the polarity of a bond, the stronger the bond ). So Fehling's solution (comparatively a weaker oxidizing agent than Tollen's reagent) can't oxidize benzaldehyde (an aromatic aldehyde). However, Fehling's solution can oxidize an aliphatic aldehyde.




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