Concept:Melting points of carboxylic acids do not uniformly increase with chain length; they exhibit an alternating (sawtooth) effect. Furthermore, smaller molecules often pack exceptionally well into crystal lattices.
Solution:- Ethanoic acid (acetic acid) has a relatively high melting point (16.6 \(^\circ\text{C}\)) due to its small, symmetrical size allowing incredibly tight and efficient packing in the solid state, alongside strong dimeric hydrogen bonding.
- In contrast, propanoic acid (-20.8 \(^\circ\text{C}\)), butanoic acid (-5 \(^\circ\text{C}\)), and pentanoic acid (-34.5 \(^\circ\text{C}\)) have significantly lower melting points because their longer, flexible alkyl chains disrupt efficient crystal packing.
Why other options are incorrect:Because of the packing efficiency of the tiny methyl group, ethanoic acid dominates the melting points of the immediate lower aliphatic series.
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