Which of the following is a pair of functional isomers?

Which of the following is a pair of functional isomers?
  1. $\mathrm{CH}_3 \mathrm{COCH}_3, \mathrm{CH}_3 \mathrm{CHO}$
  2. $\mathrm{C}_2 \mathrm{H}_5 \mathrm{CO}_2 \mathrm{H}, \mathrm{CH}_3 \mathrm{CO}_2 \mathrm{CH}_3$
  3. $\mathrm{C}_2 \mathrm{H}_5 \mathrm{CO}_2 \mathrm{H}, \mathrm{CHCO}_2 \mathrm{C}_2 \mathrm{H}_5$
  4. $\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}, \mathrm{CH}_3 \mathrm{CHO}$

Solution

Carboxylic acid and esters show functional group isomerism. When two compounds have same molecular formula but different functional groups, then functional isomerism arises. e.g., $\mathrm{C}_2 \mathrm{H}_5 \mathrm{COOH} \text { and } \mathrm{CH}_3 \mathrm{COOCH}_3$

Asked in: AP EAMCET 2005

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