Concept:The acidic strength of unsubstituted aliphatic carboxylic acids decreases as the length of the alkyl chain increases due to the electron-donating inductive effect (\( +I \)) of alkyl groups, which destabilizes the conjugate base.
Formula:$$ \text{Longer Alkyl Chain} \rightarrow \text{Greater } +I \text{ Effect} \rightarrow \text{Weaker Acid} $$
Solution:- Formic acid (1 carbon) has no alkyl group \( \rightarrow \) strongest.
- Acetic acid (2 carbons) has a methyl group.
- Propanoic acid (3 carbons) has an ethyl group.
- Butanoic acid (4 carbons) has a propyl group. The larger propyl group exerts the strongest \( +I \) effect, destabilizing the carboxylate anion the most, rendering it the weakest acid of the bunch.
Why other options are incorrect:They all possess shorter carbon chains and therefore inherently less electron-donating interference than butanoic acid.
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