Some important physical properties of carboxylic acids are given below,
As the size of the alkyl group increases, the solubility of the acid decreases and polarity is reduced.
2. Boiling points
Due to intramolecular hydrogen bonding dimerization of acid takes place and boiling point of carboxylic acid is higher than expected.
3. Melting points
The melting points of aliphatic carboxylic acids do not show a regular pattern. The first ten members show a alteration effect, i.e. the melting point of an acid containing even number of carbon atoms is higher than the next lower and next higher homologues containing odd number of carbon atoms.
Illustration 1. On the basis of H-bonding explain that the second ionization constant K2 for fumaric acid is greater than for maleic acid.
Solution: We know that H-bonding involving acidic H has an acid weakening effect and H-bonding in conjugate base has an acid strengthening effect.
Both dicarboxylic acids have two ionisable hydrogen atoms. Considering second ionization step.
Since the second ionisable H of the Maleate ion participates in H-bonding more energy is needed to remove this H because the H-bond must be broken. The maleate mono anion is, therefore, the weaker acid.
(ii) Acetic acid in the vapour state shows a relative molecular weight of 120.
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