Question

Organic compounds containing carbonyl group as the functional group are aldehydes, ketones and carboxylic acids. The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen as compared to carbon. Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. They react with HCN, Sodium bisulphite, Grignard reagent and a number of ammonia derivatives like Hydroxylamine, Hydrazine, Semicarbazide, etc. Aldehydes and ketones undergo various reactions due to the acidic nature of \( \alpha \)-hydrogen. Aldehydes and ketones having at least one \( \alpha \)-hydrogen atom undergo a reaction in the presence of dilute alkali as a catalyst to form \( \beta \)-hydroxy aldehydes or \( \beta \)-hydroxy ketones. On the other hand, aldehydes which do not possess an \( \alpha \)-hydrogen undergo self-oxidation and reduction in the presence of concentrated alkali.

Why is \( \alpha \)-hydrogen of carbonyl group acidic in nature ?

Hint:

The C=O group withdraws electrons from the alpha C-H bond. The carbanion formed after losing H+ is resonance stabilised (enolate), so the alpha-H is acidic.

Answer 1

Step 1: Locate the \( \alpha \)-hydrogen:
It is the hydrogen on the carbon directly bonded to the carbonyl carbon.

Step 2: Reason from the carbonyl group:
The C=O group is strongly electron withdrawing, by the -I and -M effects.
It pulls electron density from the \( \alpha \)-C-H bond, so the bond is weakened and the H leaves easily as \( \mathrm{H^+} \).
The anion left behind is an enolate ion. Its negative charge is delocalised between the \( \alpha \)-carbon and the oxygen atom:
\[ \mathrm{{}^{-}CH_2-CHO \;\longleftrightarrow\; CH_2=CH-O^{-}} \]
This resonance stabilises the conjugate base, so the \( \alpha \)-H is acidic.

Final Answer:
The electron withdrawing C=O group polarises the C-H bond, and the carbanion formed is stabilised by resonance, so \( \alpha \)-hydrogen is acidic.