JPJC 2026 Intro to Organic Chemistry and Isomerism Annotated SLS MCQ w Answer

Introduction to Organic Chemistry – SLS MCQs Lecture 1 1 Which of the following compounds have the same molecular formula? 1 O O CH3 2 O OH CH3 3 OH O A 1 and 2 only B 1 and 3 only C 2 and 3 only D All of the above Ans: D These are skeletal formulae where all C—C bonds and C—H bonds are hidden. MF: C7H12O2 MF: C7H12O2 MF: C7H12O2

2 Rosmarinic acid occurs in herbs such as rosemary, sage and thyme. Which of these functional groups are present in a molecule of rosmarinic acid? 1 phenol 2 ester 3 alkene 4 ketone A 1 and 3 only B 1, 2 and 3 only C 1, 3 and 4 only D All of the above Ans: B (no ketone) ester No ketone phenol phenol alkene carboxylic acid

3 Which pairs of organic compounds contain the same number of hydrogen atoms in each of their molecules? 1 2-aminopropane and 2-bromo-2-methylpropane 2 ethylpropanoate and butane-1,2-diol 3 butanenitrile and 2-methylpropanal A 1, 2 and 3 B 1 and 2 only C 1 only D 2 only Ans: B 1✓ 2✓ 3

Lecture 2 1 Compound Z is used in perfumes to give a fragrance of sandalwood or musk. It can be reduced by hydrogen gas with nickel catalyst to form compound Q. How many stereoisomers (both cis-trans isomers and enantiomers) exist for Z and for Q? Z Q A 4 4 B 4 8 C 8 4 D 8 8 Ans: B Stereoisomerism includes cis–trans isomerism and enantiomerism. Criteria for cis-trans isomerism: • Restricted free rotation about C=C bond • 2 different groups on each C of C=C bond Criteria for enantiomerism: • Presence of chiral C atom (with 4 different groups attached) • No plane of symmetry Total no. of stereoisomers in Z = 2(1+1) = 4 Total no. of stereoisomers in Q = 23 = 8

Note: The C=C in the 5–membered ring does not exhibit cis–trans isomerism as C=C is part of the rigid ring structure. The trans-isomer will result in ring strain on the molecule, making the molecule unstable.

Lecture 3 Q 1 to 3 are fill-in-the-blank summary from MOE Library 4 Propyne, C3H4 has the following structure. H—CC—CH3 Which row correctly describes the bonding and hybridisation in a molecule of propyne? number of  bonds number of sp C atoms number of sp2 C atoms A 1 1 1 B 2 2 0 C 2 2 1 D 3 3 0 Ans: B • 2 sp C atoms, 0 sp2 C atom and 1 sp3 C atom • In a multiple bond, one bond is  and the rest are .  a triple bond is made up of 1  and 2  bonds H—CC—CH3 sp hybridised (2 bond pairs & 0 lone pair) sp3 hybridised (4 bond pairs & 0 lone pair)