Give reasons for the following:
(I) Carboxylic acids do not give the characteristic reactions of the carbonyl group.
(II) Ethanoic acid is a stronger acid than ethanol.
(I) Carboxylic acids do not give the characteristic reactions of the carbonyl group.
(II) Ethanoic acid is a stronger acid than ethanol.
Solution and Explanation
(I) Reactivity of Carboxylic Acids vs. Aldehydes/Ketones
Carboxylic acids do not undergo the characteristic reactions of the carbonyl group (such as nucleophilic addition) because the carbonyl (\( {C=O} \)) is part of the larger carboxyl group (\( {-COOH} \)).
In carboxylic acids:
- The carbonyl carbon is less electrophilic because of resonance interaction with the hydroxyl group.
- Hydrogen bonding also affects the reactivity, reducing the accessibility of the carbonyl carbon.
As a result, typical carbonyl reactions (e.g., with \({NaHSO_3}\), HCN) do not occur readily in carboxylic acids.
(II) Why Ethanoic Acid is More Acidic Than Ethanol
Ethanoic acid (\({CH3COOH}\)) is a stronger acid than ethanol (\({CH3CH2OH}\)) due to the stability of their respective conjugate bases.
- The conjugate base of ethanoic acid is the acetate ion \({CH3COO^-}\), in which the negative charge is delocalized over two oxygen atoms via resonance: \[ {CH3COOH <=> CH3COO^- + H^+} \]
- In contrast, the conjugate base of ethanol, \({CH3CH2O^-}\), has the negative charge localized on a single oxygen atom, and there is no resonance stabilization.
Thus, ethanoic acid donates a proton more readily, making it a stronger acid than ethanol.
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