JPJC 2026 Introduction to Organic Chemistry, Alkanes, Alkenes Revision Answer

1 2026 JC1 H2 Chemistry (9476) Topical Revision for Organic Chemistry Topics: Introduction to Organic Chemistry, Alkanes & Alkenes (Suggested Answers) FOUNDATION KIT Multiple-Choice Questions Answers Question 1 2 3 4 5 6 7 Answer B C C C A C C Question 8 9 10 11 12 13 14 15 Answer C D C D A A C A Question 16 17 18 19 20 21 22 23 24 Answer D A C B B C B B C Multiple-Choice Questions – Worked Solution [PJC Promo 2013/P1/Q16] 1 The thyroid gland concentrates iodine and uses it to produce thyroxine, which is a hormone that controls the metabolic rate. OH I I O I I C C N H C O OH H H H H p q r s thyroxine What are the values of the bond angles p, q, r and s to the nearest degree? p q r s A 180o 90o 180o 90o B 105o 110o 107o 120o C 180o 90o 120o 180o D 105o 90o 107o 180o p: O atom has 2 LP s, 2 BP s.  Bent, 104.5o  105o q: C atom has 4BPs, 0 LP .  Tetrahedral, 109.5o  110o r: N atom has 3BPs, 1 LP.  Trigonal pyramidal, 107o s: C atom has 3BPs, 0 LP.  Trigonal planar, 120o

2 [PJC Promo 2013/P1/Q17] 2 Which of the following gives the correct number of  and  bonds present in the following molecule shown below? CH2 C C CH3 O OH Number of  bonds Number of  bonds A 5 2 B 7 0 C 11 2 D 13 0 C H H C C H H H C O O H [PJC Promo 2010/P1/Q16] 3 How many esters are there with the molecular formula C4H8O2? A 2 B 3 C 4 D 5 σ σ σ σ σ σ σ σ σ σ π π σ In a multiple bond, 1 bond is sigma, the rest are pi.

3 [PJC Promo 2011/P1/Q19] 4 The drug cortisone has the formula shown. O CH3 CH3 OHC CH2OH O O How many chiral centres are there in one molecule of cortisone? A 4 B 5 C 6 D 7 [PJC Promo 2012/P1/Q13] 5 Many drugs show enantiomerism. The diagrams below show the structure of three drugs. What is the total number of chiral carbon centres in these three structures? N C OH CH2 N CH2CH3 CH2CH3 lidocaine CH2 C CH3 H NH2 amphetamine N H O N O O CH3 H phenobarbital derivative A 1 B 2 C 3 D 4 [PJC Promo 2013/P1/Q18] 6 Which of the following structures does not exhibit enantiomerism? A CH3 CH2CH3 Br B CH3 CH3 CH2 C NH NH O CH3 O O D NH2 NH2NH2 * * * * * * * * * * * No chiral C centre No chiral C centre plane of symmetry plane of symmetry Only option C has no chiral carbon & a plane of symmetry. The rest have at least one chiral centre with no plane of symmetry.

4 [PJC Promo 2010/P1/Q17] 7 The diagram shows the structure of the insect attractant methoprene. C CH3 CH2 CH2 CH2 CH CH3 CH2 C H C C C C O O CH CH3 CH3 CH3 H H OCH3 CH3 Total no. of stereoisomers (enantiomers & cis-trans isomers) = 23 = 8. What is the total number of stereoisomers ( both cis-trans isomers and enantiomers) of this molecule? A 2 B 4 C 8 D 16 Alkanes 8 [PJC Prelim 2010/P1/Q19] Which of the following is a propagation step in the reaction between propane and chlorine when they are irradiated with ultraviolet light? A CH3CH2CH2 + Cl• → CH3CH2CH2Cl B CH3CH2CH2Cl + Cl• → CH3CH2CH2 + Cl2 C CH3CH2CHCl + Cl2 → CH3CH2CHCl2 + Cl• D CH3CH2CH2 + Cl2 → CH3CH2CHCl + HCl 9 [PJC Promo 2011/P1/Q18] Why does the reaction between propane and chlorine not give a high yield of 2-chloropropane? A A mixture of chlorine and propane cannot be prevented from exploding. B Disubstitution by chlorine takes place preferentially to monosubstitution. C The central carbon atom of propane is too weakly nucleophilic. D The chlorine replaces any of the hydrogen atoms in propane. This is not a good method of obtaining halogenoalkanes (RX) because the reaction involves free-radical substitution. The substitution of H atoms is random (i.e. not selective), and a mixture of mono -substituted and poly -substituted chlorinated products is formed in addition to 2 -chloropropane, hence lowering the yield of the desired product, 2-chloropropane. * Exhibit cis-trans isomerism since each C of C=C bond has 2 different groups attached to it. • • • • •

5 10 [PJC Prelim 2013/P1/Q22] Ethane reacts with bromine in the presence of ultraviolet light to form a mixture of products. Which of the following about the reaction mechanism is incorrect? A Both initiation and propagation steps involve homolytic fission. B Both initiation and propagation steps produce bromine atoms. C Both propagation and termination steps produce hydrogen bromide molecules. D Both propagation and termination steps involve C−Br bond formation. HBr is only produced in the propagation step ONLY. See the free-radical substitution below. 11 [PJC Promo 2012/P1/Q15] How many dichlorinated constitutional (structural) isomers can be formed by the chlorination of pentane in the presence of sunlight? A 6 B 7 C 8 D 9 Dichlorinated constitutional isomers: C-C-C-C-C C-C-C-C-C C-C-C-C-C C-C-C-C-C C-C-C-C-C C-C-C-C-C C-C-C-C-C C-C-C-C-C C-C-C-C-C Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl

6 12 [PJC JC2 MYE 2010/P1/Q19] Pentane was reacted with limited bromine in the presence of uv light. Assume that only mono-bromination takes place and the reaction occurs at the same rate at all carbon atoms. What is the ratio of the products obtained? 1-bromopentane 2-bromopentane 3-bromopentane A 3 2 1 B 1 2 3 C 1 3 2 D 3 1 2 Assuming that the substitution process is purely random , there are 3 types of replaceable H atoms in pentane and hence, 3 different mono–brominated cpds are formed. To form 1-bromopentane, there are 6 possible Ha atoms to be substituted. To form 2-bromopentane, there are 4 possible Hb atoms to be substituted. To form 3-bromopentane, there are 2 possible Hc atoms to be substituted. Hence, the ratio of 1-bromopentane : 2-bromopentane : 3-bromopentane is 3 : 2 : 1 13 [HCI Prelim 2017/P1/Q15] When heated with chlorine, the following hydrocarbon undergoes free radical substitution. In the propagation step, the free radical Q• is formed by the loss of one hydrogen atom. Ignoring any stereoisomers, how many different forms of Q• are theoretically possible? A 3 B 4 C 6 D 8 a a Hb Hb Hb Hb Hb Hb Hc Hb Hb Hc CH2 • CH3 CH3 • CH3 CH3 • CH3 3 types of replaceable H atoms, giving three different forms of Q•.

7 14 [RI Prelim 2017/P1/Q29] Deuterium, D, is a heavy isotope of hydrogen. 1,3 -Dideuteriopropane is reacted with limiting chlorine under UV light, so that monochlorination takes place. 1,3-dideuteriopropane Assuming that a C –D bond is broken as easily as a C –H bond, what proportion of the monochlorinated products will contain a chiral centre? A 33% B 40% C 50% D 66% 4 possible C—Ha bonds (out of 8 possible C —H/D bonds) that can be broken to give the product with a chiral carbon centre. 15 [RVHS Prelim 2020/P1/Q24] In the reaction of methylpropane with chlorine under ultraviolet light, which of the following compounds can be formed? Methylpropane has two types of replaceable H atoms, giving two different forms of radicals. When two of these radicals meet, a  covalent bond is formed between them, giving the following products. 1 (✓) 2 (✓) 3 (✓) A 1, 2 and 3 B 1 and 2 only C 1 and 3 only D 2 and 3 only Only product that contains chiral carbon! • • • • • • • • a a a a b b c c Cl *

8 Alkenes 16 [PJC Prelim 2010/P1/Q20] Fly paper is used as a non -toxi