Reaction of Aldehydes and Ketones - Practice Questions & MCQ

(i) Mechanism of nucleophilic addition reactions:
A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp² hybridised orbitals of carbonyl carbon. The hybridisation of carbon changes from sp² to sp³ in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu^– and H⁺ across the carbon oxygen double bond as shown in the figure below.

(ii) Reactivity
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.

Reduction to hydrocarbons:
The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc amalgam and concentrated hydrochloric acid (Clemmensen reduction) hydrazine hydrazone or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

Oxidation
Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.

It is the condensation taking place when two different aldehydes or two different ketones or one aldehyde and one molecule of ketone both containing -H atoms. A number of products due to self-condensation and cross condensation is obtained. The reaction occurs as follows.

For example,

The aldehydes and ketones undergo a number of reactions due to the acidic nature of -H, which in turn is due to the strong electron-withdrawing effect of the (C=O) group and resonance stabilisation of the conjugate base. When two molecules of the same aldehyde or ketone containing -H atom condense together in the presence of dilute alkali, such as NaOH, KOH, K₂CO₃, Na₂CO₃, or at least 2 -H-atoms to give a molecule of aldol or ketol, it is called aldol condensation. On heating, it loses a molecule of H₂O to give a molecule of α, β-unsaturated aldehyde or ketone.

For example,

When two different aldehydes lacking α-H atom are reacted in the presence of a strong base, they undergo disproportionation or redox reaction to give a molecule of alcohol and salt of an acid. Alcohol is obtained from the less reactive aldehyde and acid salt is obtained from the more reactive aldehyde. In this reaction, OH^- attacks at the C of (C=O) group of more reactive aldehyde and gives adduct anion from which H^- ion is transferred to the less reactive aldehyde. It gives acid ion from more reactive aldehyde and alcohol from less reactive aldehyde.

For example,

Two molecules of the same aldehyde lacking α-H atom undergo disproportionation or redox reaction in the presence of a strong base to give a molecule of alcohol and a molecule of the salt of an acid.

For example,