RI 2024 Introduction to Organic Chemistry Tutorial Answers (Practice Questions) for uploading

1 Raffles Institution Year 5 H2 Chemistry 2024 Tutorial 9 – Introduction to Organic Chemistry 6. C5H10 can either be an alkene (CnH2n) or a cycloalkane (CnH2n). Steps to draw isomers of alkenes Structures (1) Draw positional isomers of pentene. II exhibits cis-trans isomerism. Draw its cis-isomer III. I II III (2) Draw positional isomers of the branched chain IV V VI Steps to draw isomers of cycloalkanes Structures (1) Draw the cyclopentane (with the biggest number of carbon in the ring) and methylcyclobutane . VII VIII (2) Draw cyclopropane with one ethyl group and two methyl groups on the same C. IX X (3) Draw stereoisomers of 1,2-dimethylcyclopropane. This compound has 2 chiral carbons and will have a maximum of 22 = 4 stereoisomers. However, cis-1,2-dimethylcyclopropane (XIII) has a plane of symmetry and is a meso compound. Thus, there are only 3 stereoisomers for 1,2-dimethylcyclopropane. XI XII XIII Type of isomerism Examples of isomers which exhibit the isomerism Constitutional isomerism I, IV, V, VI, VII, VIII, IX, X, (one of XI, XII or XIII) and (one of II or III) Cis-trans isomerism II and III ; XI (or XII) and XIII Enantiomerism XI and XII 7a b Suggested Solutions to Tutorial 9: Introduction to Organic Chemistry * * C O C C N H C H C H H H H H H O H C H H C C O H C H H H H H H

2 7c 8 (a) (b) Cl (c) 9(a) (b) (c) 10. (a) substitution (b) reduction (c) addition (d) elimination (e) substitution (f) elimination (g) hydrolysis (h) step 1: addition step 2: substitution O O OH sp3 tetrahedral sp2 trigonal planar sp2 trigonal planar * * * Nootkatone (Grapefruit oil) C CH2 CH3 O CH3 H3C

3 11 cis,cis-isomer trans,cis-isomer cis,trans-isomer trans,trans-isomer Note: • As each C atom in the C=C bonds has two different groups attached to it, the maximum number of cis-trans isomers is 22 = 4.] • When drawing cis-trans isomers, the trigonal planar geometry around each sp2 C in the C=C bonds must be shown. 12(a) Cis-trans isomerism refers to the existence of two compounds with the same molecular and structural formulae but with different spatial arrangements of atoms due to restricted rotation about a double bond (e.g. C=C, C=N or N=N) or a bond in a ring structure . To exhibit cis- trans isomerism, the compound either has a double bond with two different groups