Melting points of carboxylic acids

At room temperature, the Cj to C7 carboxylic acids are liquids and acids with more than 7 carbons are solids. Boiling points of carboxylic acids increase with increasing molar mass. However there is no regular change in the melting points of carboxylic acids. Carboxylic acids with an even number of carbon atoms melt at higher temperatures than similar acids with an odd number of carbon atoms. 

Problem 15.18. Compare the melting points of carboxylic acids with their sodium carboxylate salts. 

B. The melting point of carboxylic acids are higher than that of comparable alcohols. 

Unsaturation lowers the melting points of carboxylic acids, especially if the configuration around the double bond is cis. These unsaturated acids are bent molecules because of the geometry around the double bonds. The bends hinder efhcient molecular packing, so London forces in unsaturated fatty acids are weaker than those in saturated fatty acids. 

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