JEE Main Chemistry: Stereochemistry / Isomerism questions with solutions

Q1. Among the following compounds, how many can exhibit geometric (cis-trans) isomerism? (i) HO-C(CH3)=CH-C(F)(CH3)2 (ii) CH3-C(CH3)=C(F)(CH3) (iii) CH3-N(=O)-OH (iv) H-N-C(CH3)=CH-CH(CH3)2 (v) CH3-C(=O)Cl (vi) (CH3)2C=CH-CH3 (vii) A cyclohexane ring bearing two different substituents on the two carbons of an endocyclic C=C bond (viii) CH3-C(CH3)=C(CH2CH3)2 (ix) CH3-C(Cl)=C(CH3)2

1. 1
2. 2
3. 3
4. 4

Answer: 4

(i) C(CH3)(OH)=CH-: left C bears OH and CH3 (different); right C bears H and C(F)(CH3)2 (different) -> YES. (ii) C(CH3)(CH3)=: two identical CH3 on left C -> NO. (iii) CH3-N(=O)-OH: N=O with N carrying CH3 and OH (different) -> shows syn/anti isomerism -> YES. (iv) C(CH3)=CH-: left C bears CH3 and N-group (different); right C bears H and CH(CH3)2 (different) -> YES. (v) C=O: one side is only O -> NO. (vi) C(CH3)2=: two CH3 on one carbon -> NO. (vii) Two different substituents on each double-bond carbon in the ring -> YES. (viii) C=C(CH2CH3)2: right C has two identical ethyl groups -> NO. (ix) C=C(CH3)2: right C has two CH3 -> NO. Count: (i), (iii), (iv), (vii) = 4.

Q2. Which of the following compounds will show geometrical (cis-trans) isomerism?

1. (A) A cyclopentane ring with identical T substituents on two adjacent upper carbons and a geminal dichloro group (Cl, Cl) on the topmost carbon.
2. (B) A cyclohexene ring with identical D substituents on the two carbon atoms adjacent to and on the same side as the double bond.
3. (C) A spiro arrangement of four cyclopropane rings with a central cyclopropane connected to three others at its vertices.
4. (D) A cyclohexadiene ring with identical D substituents on two adjacent carbons and two double bonds within the ring.

Answer: (B) A cyclohexene ring with identical D substituents on the two carbon atoms adjacent to and on the same side as the double bond.

In cyclohexene (option B), the two sp3 carbons each carry one D and one H substituent; since neither carbon has two identical groups, restricted rotation in the ring fixes their relative orientation, giving rise to cis and trans isomers. The geminal dichloro group in option A prevents G.I. at that center; the spiro polycyclopropane in option C has no appropriate stereocenters; the diene in option D places the D-substituted carbons on sp2 centers that cannot show classical cis-trans ring isomerism.