Aldehyde and Ketone MCQ Questions & Answers in Organic Chemistry | Chemistry

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Benzaldehyde is less reactive than ethanal as the polarity of the carbonyl group is reduced in benzaldehyde due to resonance as shown below :

It is an example of intramolecular aldol condensation

It is an example of Cannizzaro reaction. One molecule of aldehyde is reduced and other is oxidised.

When acetone is treated with $${H_2}S{O_4},$$  three molecules get condensed to give mesitylene,

Here, $$C{H_3}Li$$   abstract is an active proton from cyclo pentanone forming methane leaving behind an intermediate lithium cyclopentanoyl anion.

Since the carbonyl carbon is electron deficient, so most susceptible to attack by nucleophilic reagents or base. A base increases the acidity of hydrogen atom attached to the $$\alpha - C$$  of the ketones or aldehydes. That’s why $$\alpha $$ - hydrogen is easily abstracted from ketones by a base, e.g. in aldol condensation reaction, $$\alpha $$ - hydrogen atom of aldehyde or ketone is abstracted by a strong base.

Remember that during reduction with $$NaB{H_4},$$  a hydride ion is transferred by $$NaB{H_4}$$  to carbonyl carbon and a proton ( or deutron ) is transferred from the solvent ( in the second step ) to carbonyl oxygen.

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\[{{C}_{6}}{{H}_{6}}+HCN+HCl\xrightarrow{anhy.\,ZnC{{l}_{2}}}\left[ {{C}_{6}}{{H}_{5}}CH=NH \right]\to {{C}_{6}}{{H}_{5}}CHO\]