Homologous series and structural/stereoisomerism: NEET MCQs with answers

Answer: (B) Thioethers

Metamerism arises due to unequal distribution of alkyl groups on either side of a polyvalent functional group or heteroatom. Thioethers contain a bivalent sulfur atom (-S-) and with four carbon atoms, diethyl sulfide (C₂H₅-S-C₂H₅) and methyl propyl sulfide (CH₃-S-C₃H₇) act as metamers. Monovalent functional groups like -OH, -CHO, and -COOH cannot exhibit metamerism.

Answer: (A) Both Statement I and Statement II are correct.

Tautomerism involves the rapid interconversion of constitutional isomers via proton migration accompanied by a switch of double bonds, existing in dynamic equilibrium. Acetone has α-hydrogens adjacent to the carbonyl group, which are essential for forming the enol tautomer. Both statements are true.

Answer: (C) 9 σ-bonds, 1 π-bond, and 2 lone pairs

The enol form of acetone is prop-1-en-2-ol, CH₂=C(OH)-CH₃. Counting single bonds: 2 C-H bonds in =CH₂, 1 C=C bond (1 σ, 1 π), 1 C-O bond, 1 O-H bond, 1 C-C bond, and 3 C-H bonds in -CH₃. Total σ-bonds = 2 + 1 + 1 + 1 + 1 + 3 = 9. There is 1 π-bond in C=C. The oxygen atom has 2 lone pairs of electrons. Hence, it contains 9 σ-bonds, 1 π-bond, and 2 lone pairs.

Answer: (D) 3

1,2-Dichlorocyclobutane has two stereocenters. The cis-isomer possesses an internal plane of symmetry and is therefore an optically inactive meso form. The trans-isomer lacks a plane or centre of symmetry and exists as a pair of enantiomers (d- and l-forms). Thus, there are 1 (cis-meso) + 2 (trans enantiomers) = 3 stereoisomers.

Answer: (C) Pentan-2-one and pentan-3-one

Metamerism arises due to unequal distribution of alkyl groups on either side of a polyvalent functional group. Pentan-2-one (methyl propyl ketone) and pentan-3-one (diethyl ketone) have different alkyl chains attached to the carbonyl group, making them metamers.

Answer: (D) 5

The cyclic isomers with formula C₅H₁₀ are: cyclopentane (5-membered), methylcyclobutane (4-membered), 1,2-dimethylcyclopropane, 1,1-dimethylcyclopropane, and ethylcyclopropane (3-membered). This gives exactly 5 cyclic structural isomers.

Answer: (A) 3

The structural ethers of formula C₄H₁₀O are: (1) 1-methoxypropane [CH₃-O-CH₂CH₂CH₃], (2) 2-methoxypropane [CH₃-O-CH(CH₃)₂], and (3) ethoxyethane [CH₃CH₂-O-CH₂CH₃]. Thus, exactly 3 constitutional ether isomers exist.

Answer: (D) Cis-but-2-ene and trans-but-2-ene

Diastereomers are stereoisomers that are not mirror images of each other. Cis-but-2-ene and trans-but-2-ene are geometrical isomers and non-mirror image stereoisomers, hence they are diastereomers. The pairs in the other options are non-superimposable mirror images of each other (enantiomers).

Answer: (A) 2

The formula C₃H₆O corresponds to a degree of unsaturation of 1. The structural carbonyl isomers (containing C=O) are propanal (an aldehyde) and propan-2-one (acetone, a ketone). Thus, exactly 2 carbonyl structural isomers are possible.

Answer: (A) 3

For the molecular formula C₄H₁₀O, the acyclic ether isomers are: (1) 1-methoxypropane (CH₃-O-CH₂CH₂CH₃), (2) 2-methoxypropane (CH₃-O-CH(CH₃)₂), and (3) ethoxyethane (CH₃CH₂-O-CH₂CH₃). Thus, there are exactly 3 structural ether isomers.

Answer: (B) 4

The acyclic monochloro derivatives of butane (C₄H₁₀) can be derived from the two carbon skeletons: n-butane gives 1-chlorobutane and 2-chlorobutane; isobutane (2-methylpropane) gives 1-chloro-2-methylpropane and 2-chloro-2-methylpropane. Hence, there are exactly 4 structural isomers.

Answer: (C) Butan-1-ol and 2-methylpropan-1-ol

Chain isomers have the same functional group and molecular formula but differ in the carbon skeletal framework. Butan-1-ol has a straight continuous 4-carbon chain, whereas 2-methylpropan-1-ol has a branched 3-carbon parent chain. Pentan-2-one and pentan-3-one are position isomers (or metamers); 1-chloropropane and 2-chloropropane are position isomers.