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Why do carboxylic esters usually have lower boiling points than the corresponding carboxylic acids?
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carboxylic acid
Alcohols may be oxidized to give aldehydes, ketones, and carboxylic acids. The oxidation of organic compounds generally increases the number of bonds from carbon to oxygen, and it may decrease the number of bonds to hydrogen.
The primary reason carboxylic esters have lower boiling points than their corresponding carboxylic acids is the inability of ester molecules to form hydrogen bonds with one another.2
In contrast, carboxylic acid molecules are capable of forming strong hydrogen bonds, often existing as dimers (pairs of molecules) in both liquid and gaseous states. Because these hydrogen bonds must be broken to transition the substance into a gas, significantly more heat energy is required to boil a carboxylic acid compared to an ester of similar molecular weight.12
For example, acetic acid has a boiling point of 118 °C (244 °F), whereas the corresponding ester, ethyl acetate, boils at only 77 °C (171 °F).2