Formaldehyde is the first member of the aldehyde series. It is present in green leaves of plants where its presence is supposed to be due to the reaction of \(CO_2\) with water in the presence of sunlight and chlorophyll. Commercially, formaldehyde is obtained by the controlled oxidation of methane in the presence of molybdenum oxide \((MoO)\) acting as a catalyst. Formaldehyde is a constituent of a number of polymers also called resins. These are non volatile solids or semi solids which are obtained directly from certain plants and can be synthesized in the laboratory.

Benzaldehyde is less reactive towards nucleophilic addition than formaldehyde because of:

-R effect of carbonyl group and steric factor due to phenyl group

The correct answer is option 2. -R effect of carbonyl group and steric factor due to phenyl group.

Let us break down the factors that influence the reactivity of benzaldehyde compared to formaldehyde in nucleophilic addition reactions.

Resonance Effect (\(-R\) Effect)

The carbonyl group (\(C=O\)) is the functional group involved in nucleophilic addition reactions. In benzaldehyde, the carbonyl group is attached to a phenyl group (a benzene ring).

Resonance in Benzaldehyde: The phenyl group can participate in resonance with the carbonyl group. The lone pair of electrons on the oxygen in the carbonyl group can delocalize into the phenyl ring through conjugation. This delocalization reduces the partial positive charge on the carbonyl carbon, making it less electrophilic (less reactive towards nucleophiles).

Benzaldehyde Resonance Structure:

In this resonance structure, the phenyl ring stabilizes the carbonyl group, decreasing its reactivity towards nucleophiles.

Formaldehyde: In contrast, formaldehyde  lacks any resonance stabilization because there is no group attached to the carbonyl carbon that can participate in resonance. Therefore, the carbonyl carbon in formaldehyde retains a higher partial positive charge, making it more electrophilic and more reactive towards nucleophilic attack.

Steric Hindrance:

Steric hindrance refers to the physical blocking of a reaction site by bulky groups, making it difficult for reactants to approach.

Benzaldehyde: The phenyl group is a large, bulky group that is attached directly to the carbonyl carbon. This bulkiness creates steric hindrance around the carbonyl carbon, making it harder for nucleophiles to approach and attack the carbonyl carbon. This steric factor further reduces the reactivity of benzaldehyde towards nucleophilic addition.

Formaldehyde: In formaldehyde, the carbonyl carbon is attached to two small hydrogen atoms. There is very little steric hindrance, allowing nucleophiles to easily approach and react with the carbonyl carbon. This lack of steric hindrance makes formaldehyde more reactive towards nucleophilic addition.

Conclusion: The combination of the \(-R\) effect from the phenyl group and the steric hindrance in benzaldehyde makes it significantly less reactive towards nucleophilic addition reactions compared to formaldehyde. Formaldehyde, lacking these factors, is much more reactive.

Therefore, the correct reason for benzaldehyde's lower reactivity compared to formaldehyde is due to the \(-R\) effect of the carbonyl group and steric factor due to the phenyl group. This is option 2 in the question provided.