Take timed practice tests on Isomerism for JEE Main and JEE Advanced with session-wise drills, score review, and explanation-led revision.
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1. Butane (C₄H₁₀) and 2-methylpropane (isobutane) are examples of:
Explanation: Chain isomers have the same molecular formula but different carbon skeleton (chain length/branching). n-butane (straight chain) and isobutane (branched) are chain isomers.
2. Ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃) are:
Explanation: Functional group isomers have the same molecular formula (C₂H₆O) but different functional groups (−OH vs −O−). They show distinctly different chemical properties.
3. Cis-trans isomers arise due to:
Explanation: Geometrical (cis-trans) isomerism occurs because C=C bonds cannot rotate freely. The cis isomer has same groups on the same side of the double bond; trans has them on opposite sides.
4. Which compound shows cis-trans isomerism?
Explanation: For cis-trans isomerism, each doubly-bonded carbon must have two DIFFERENT substituents. In but-2-ene: C2 has H and CH₃; C3 has H and CH₃ — both meet the condition. Propene: C1 has two H → no isomerism. But-1-ene: C1 has two H → no isomerism.
5. A chiral carbon (asymmetric carbon) is bonded to:
Explanation: A chiral (asymmetric) carbon has four different substituents. It is a stereogenic centre — its two mirror image configurations (R and S) are non-superimposable = enantiomers.
6. Enantiomers are mirror images that are:
Explanation: Enantiomers: mirror-image stereoisomers that cannot be superimposed on each other (like left and right hands). They have identical physical properties except optical rotation: one rotates plane-polarised light clockwise (+), the other anticlockwise (−).
7. The R/S configuration is assigned based on:
Explanation: CIP rules: assign priority (1–4) to groups based on atomic number (higher = higher priority). If 1→2→3 is clockwise with group 4 pointing away → R. Anticlockwise → S.
8. Diastereomers are stereoisomers that are:
Explanation: Diastereomers are stereoisomers that are NOT mirror images. They differ in configuration at some (but not all) stereocentres. Unlike enantiomers, they have different physical properties (different mp, bp, solubility).
9. The number of stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid) is:
Explanation: Tartaric acid has 2 chiral carbons. Maximum stereoisomers = 2² = 4. But the (R,S) form = meso compound (internally compensated, optically inactive). Actual stereoisomers: (R,R), (S,S) (enantiomers), and meso (R,S) = 3 total.
10. Conformational isomers (conformers) of ethane differ in:
Explanation: Conformers arise from rotation about C−C single bonds. Staggered (anti/gauche) conformations are lower energy than eclipsed. Newman projections are used to visualise conformers.
11. Meso compounds are optically inactive because:
Explanation: Meso compounds have chiral carbons but possess an internal plane (or point) of symmetry that makes the two halves mirror images of each other. The optical rotations of the two halves cancel out — net rotation = 0 (optically inactive).
12. Position isomers differ in:
Explanation: Positional isomers have the same skeleton and functional group, differing only in the position of the functional group. E.g., 1-propanol vs 2-propanol.
13. Which of the following pairs are tautomers?
Explanation: Tautomers interconvert rapidly through H-atom transfer. Keto-enol tautomerism is most common: −CH₂−C=O ⇌ −CH=C−OH. Acetaldehyde (keto) ⇌ vinyl alcohol (enol).
14. The maximum number of stereoisomers of a compound with n chiral carbons is:
Explanation: Maximum stereoisomers = 2ⁿ where n = number of chiral carbons. Actual number may be less if meso forms exist or if some configurations are geometrically impossible.