NEET 2021 Chemistry Question 84 Solution

According to NCERT, metamerism is a type of constitutional isomerism that arises due to different alkyl chains attached to the same polyvalent functional group (e.g., ether $-O-$, thioether $-S-$, secondary amine $-NH-$, or ketone $-CO-$) in the molecule.

The molecular formula $C_4H_{10}O$ corresponds to saturated mono-oxygenated compounds (degree of unsaturation $DBE = 0$). Among the isomeric ethers, we can distribute the 4 carbons on either side of the divalent oxygen atom in multiple distinct ways: 1. Ethoxyethane (Diethyl ether): $CH_3-CH_2-O-CH_2-CH_3$ (symmetrical $2 + 2$ split) 2. 1-Methoxypropane (Methyl propyl ether): $CH_3-O-CH_2-CH_2-CH_3$ ($1 + 3$ linear split) 3. 2-Methoxypropane (Methyl isopropyl ether): $CH_3-O-CH(CH_3)_2$ ($1 + 3$ branched split) These compounds belong to the same homologous family and have identical functional groups, but differ in the alkyl chains flanking the oxygen atom. They are metamers.

- $C_5H_{12}$: Saturated pentane; exhibits only chain isomerism (pentane, isopentane, neopentane) as it contains no polyvalent heteroatom. - $C_3H_8O$: Ethers require dividing 3 carbons across oxygen; the only possible division is $1 + 2$ ($CH_3-O-C_2H_5$). No second ether isomer exists. - $C_3H_6O$: Ketone is propan-2-one ($CH_3-CO-CH_3$), which cannot have unequal alkyl groups since only 2 carbons remain to flank the carbonyl carbon.