RI 2024 Alkenes Answer Key

-1- Answers to Practice Questions 1(a)(i) Molecular formula of Q is C5H8. 1(a)(ii) Isomers of Q 1(b) Squalene has 6 π bonds per molecule. 1(c) cis-pent-2-ene > trans-pent-2-ene > 2-methylbut-1-ene 2(a) A: C C CH3 H CH3 H B: C C H CH3 CH3 H C: C C H H CH2CH3 H cis-but-2-ene trans-but-2-ene but-1-ene D: C C H H CH3 CH3 E: F: CH3 2–methylpropene cyclobutane methylcyclopropane 2(b) G: C C * H C H H Br C H H H H H Br 1,2-dibromobutane H: C C H H H CH H H H CH H H 2-methylpropane 2(c) A (i.e. cis-but-2-ene) is slightly polar. In addition to instantaneous dipole -induced dipole attractive forces present between the molecules, there are also permanent dipole -permanent dipole attractive forces. B (i.e. trans-but-2-ene) is non-polar. Only instantaneous dipole-induced dipole attractive forces exist between the molecules. Since more energy is needed to overcome the stronger intermolecular forces in A during boiling, the boiling point of A is higher than that of B. 2(d) Reagent Alkene (i) cold, alkaline KMnO4(aq) (ii) hot KMnO4(aq) made alkaline with NaOH(aq) (iii) hot KMnO4(aq) acidified with H2SO4(aq) A C C CH3 H CH3 H OHHO CH3CO2−Na+ CH3CO2H B C C H CH3 CH3 H OHHO CH3CO2−Na+ CH3CO2H C C C H H CH2CH3 H OHHO CH3CH2CO2−Na+ CH3CH2CO2H

-2- D C C H H CH3 CH3 OHHO CH3COCH3 CH3COCH3 3(a) Cl Br Br KOH in ethanol heat Br2 in CCl 4 3(b) OH BrBr KOH in ethanol heat Br2(aq) 3(c) OH OHexcess conc H2SO4 heat 1. cold conc H2SO4 2. H2O, heat 3(d) OH O OHexcess conc H2SO4 heat KMnO4(aq) H2SO4(aq) heat 4(a) Bubble a sample of each gas through Br2(aq) in separate test-tubes at room temperature. For ethene, there is (rapid) decolourisation of the orange Br2(aq). For ethane, there is no decolourisation of the orange Br2(aq). Equation: CH2=CH2 + Br2 + H2O → HOCH2CH2Br + HBr OR Add two drops of Br 2 (in CCl4) to each sample of gas already collected in a stoppered test -tube at room temperature and in the dark. Shake the re-stoppered test-tube. For ethene, there is (rapid) decolourisation of the orange-red Br2. For ethane, there is no decolourisation of the orange-red Br2. Equation: CH2=CH2 + Br2 → BrCH2CH2Br Other possible chemical tests: • KMnO4(aq) acidified with H2SO4(aq), heat (Note: DO NOT heat under reflux) • KMnO4(aq) and NaOH(aq), cold 4(b) Add two drops of KMnO 4 acidified with dilute sulfuric acid to each sample in a test -tube, and heat each reaction mixture in a hot water bath. (Note: DO NOT heat under reflux) For pent-2-ene, there is decolourisation of purple KMnO4. For pent-1-ene, there is decolourisation of purple KMnO 4 and evolution of a colourless gas (CO 2) which gives a white precipitate with limewater / Ca(OH)2(aq). Equations: + 4[O] KMnO4(aq), H2SO4(aq) heat O OH O OH +

-3- + 5[O] KMnO4(aq), H2SO4(aq) heat + O OH + CO2 H2O