RI 2024 Arenes Tutorial Self-Check Answers

-1- RAFFLES INSTITUTION YEAR 5 H2 CHEMISTRY 2024 Tutorial 13 – Arenes (Suggested Answers) ______________________________________________________________________________ Self-Check Questions - Suggested Answers 1(a) Isomeric arenes with molecular formula C9H12: 1(b)(i) 4-bromo-1,2-dimethylbenzene (ii) (2-chloroethyl)benzene (iii) 2-ethyl-6-methylbenzoic acid (iv) phenylethene 1(c) (ii) (iii) 2 In decreasing order of reactivity towards nitration: ethylbenzene > benzene > benzoic acid Ethylbenzene : The ethyl group is an electron-donating group which increases the electron density of the benzene ring, making it more susceptible to electrophilic attack by NO 2+ than an unsubstituted benzene ring. Benzoic acid : The -COOH group is an electron -withdrawing group which decreases the electron density of the benzene ring, making it less susceptible to electrophilic attack by NO 2+ than an unsubstituted benzene ring.

-2- 3 4 (a) Test Add Br2 in CCl4 drop-wise to each compound in a test-tube at room temperature. Observation • For cyclohexene, there is (rapid) decolourisation of orange-red Br2. • For benzene, there is no decolourisation of orange-red Br2. (b) Test Add two drops of KMnO4 acidified with dilute sulfuric acid to each compound in a test-tube, and heat each reaction mixture in a hot water bath. (Note: DO NOT heat under reflux) Observation • For ethylbenzene, there is decolourisation of purple KMnO 4(aq) and effervescence of CO2(g) which gives a white precipitate with limewater. • For benzene, there is no decolourisation of purple KMnO 4(aq). 5 Free-radical Substitution Step 1: Initiation Step 2: Propagation Then (a), (b), (a), (b), … Step 3: Termination 2 Br• ⎯→ Br2 CH2CH3 CH CH3 Br + Br2 uv light + HBr CH2CH3(a) + Br CHCH3 + HBr (b) CHCH3 + Br2 Br CH CH3 + Br + BrCHCH3 Br CH CH3 CHCH32