Q.Explain the isomerism exhibited by carboxylic acids.

Carboxylic acids exhibit chain isomerism, functional-group isomerism (with esters), and — for substituted acids — position isomerism of the substituent, since the $-COOH$ carbon is fixed as C-1.

1. Chain isomerism

Arises from different arrangements (branching) of the carbon skeleton while the molecular formula and the functional group ($-COOH$) remain the same.

Example: $C_4H_8O_2$

• $CH_3CH_2CH_2COOH$ — butanoic (n-butyric) acid (straight chain)
• $(CH_3)_2CHCOOH$ — 2-methylpropanoic (isobutyric) acid (branched chain)

2. Functional isomerism

Carboxylic acids ($-COOH$) and esters ($-COOR$) both fit the general formula $C_nH_{2n}O_2$, so an acid and an ester of the same molecular formula are functional isomers even though their functional groups (and chemical properties) are completely different.

Example: $C_2H_4O_2$

• $CH_3COOH$ — acetic acid
• $HCOOCH_3$ — methyl formate (methyl methanoate)

Another example: $C_3H_6O_2$

• $CH_3CH_2COOH$ — propanoic acid
• $CH_3COOCH_3$ — methyl acetate
• $HCOOC_2H_5$ — ethyl formate

3. Position isomerism (of a substituent, not of $-COOH$)

Since the carboxyl carbon must always be numbered C-1, the $-COOH$ group itself cannot show position isomerism. However, if the acid carries another substituent (e.g. a halogen), that substituent can occupy different positions along the chain, giving position isomers.

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