ACJC Prelim P3 Answers

1 © ACJC 2018 9729/03/Prelim/2018 Section A Answer all questions in this section. 1 (a) (i) Due to reactivity of A lCl3 and PCl5 in water, the following reactions are carried out under anhydrous conditions. Write equations showing the reactions of aluminium chloride and phosphorus pentachloride in water. State the pH of resulting solutions. [3] A lCl3 + 6H2O [A l(H2O)6]3+ + 3Cl– [Al(H2O)6]3+ [Al(H2O)5(OH)]2+ + H+ pH 3 PCl5 + H2O POC l3 + 2HCl POCl3 + 3H2O H 3PO4 + 3HCl OR PCl5 + 4H2O H 3PO4 + 5HCl pH 2 (ii) Chlorobenzene and 2-chloro-2-methylbutane are separately boiled with aqueous sodium hydroxide and acidified with nitr ic acid before aqueous silver nitrate is added. State and explain the difference in observations. Write equations for the reactions that occur. [3] 2-chloro-2-methylbutane forms a white ppt while chlorobenzene does not. The C–C l bond in chlorobenzene is strengthened by partial double bond character due to the overlap of p-orbital on C l atom and π-orbital of benzene. Hence chlorobenzene is resistant to hydrolysis. CH 3C(CH3)ClCH2CH3 + OH– CH 3C(CH3)OHCH2CH3 + Cl – Ag+ + Cl – AgC l (b) (i) To study the kinetics of the reaction between 2-chloro-2-methylbutane and sodium hydroxide in alcoholic medium, two experiments with different initial concentrations of 2-chloro-2-methylbutane were carried out at constant temperature. A [NaOH]–time graph was plotted using the results obtained from the experiments.

2 © ACJC 2018 9729/03/Prelim/2018 (i) Deduce the order of reaction with respect to 2-chloro-2-methylbutane and NaOH. [2] Using Reaction 1 (or 2), when [NaOH] decreases, gradient remains constant, hence rate of reaction is constant. Rate of reaction is independent of [NaOH], hence the reaction is zero order wrt [NaOH]. Comparing reactions 1 and 2, when [2-chloro-2-methylbutane] doubles from 0.25 to 0.50 mol dm -3, the gradient doubles, showing that the rate of reaction doubles from 1.60 × 10 -4 to 3.15 × 10 -4 mol dm -3 min-1. Rate of reaction ∝ [2- chloro-2-methylbutane], hence the reaction is first order wrt [2-chloro-2- methylbutane]. (ii) Hence, state the rate equation of this reaction. [1] Rate = k[2-chloro-2-methylbutane] Reaction 1: 0.25 mol dm-3 of 2-chloro-2-methylbutane Reaction 2: 0.50 mol dm-3 of 2-chloro-2-methylbutane 0 0.002 0.004 0.006 0 5 10 15 20 25 0.008 0.010 time / minutes [NaOH] / mol dm-3

3 © ACJC 2018 9729/03/Prelim/2018 (iii) The proposed mechanism of 2-chloro-2-methylbutane with alcoholic sodium hydroxide is given below. Using your answer in (b)(ii) and the above steps, suggest the complete mechanism by • drawing curly arrows, • showing lone pair(s) of electrons and • identifying the slow step. [3] (c) Chlorobenzene has the following resonance structures which represent the alternative distribution of electrons on the molecule. Using these structures, suggest why the electrophilic substitution of chlorobenzene is 2,4-directing. [2] The negative charges are located on positions 2 and 4 with respect to the chloro- group, hence the electrophilic attack is more likely to occur at these positions OR electrophile is more likely to react at these positions. (d) When butane is reacted with chlorine gas under strong UV rays, a mixture of compounds is formed. (i) Assuming that primary and secondary hydrogen atoms have similar reactivity, draw the structures of alkyl radicals formed during the reaction and state their likely ratio of formation. [2] and 3 : 2

4 © ACJC 2018 9729/03/Prelim/2018 (ii) During the termination stage, the alkyl radicals in (d)(i) form three structures with the molecular formula C 8H18. Suggest their structures and the ratio of these structures. [4] • CH3CH2CH2CH2CH2CH2CH2CH3 3/5 x 3/5 = 9/25 • CH3CH2CH(CH3)CH(CH3)CH2CH3 2/5 x 2/5 = 4/25 • CH3CH2CH2CH2CH(CH3)CH2CH3 2 (3/5 x 2/5) = 12/25 • Ratio = 9 : 4 : 12 (e) Chlorofluoroalkanes, CFCs, were once used as refrigerant fluids and aerosol propellants. In many applications, they have now been replaced by alkanes. This is because CFCs contribute to the destruction of the ozone layer. (i) Suggest one reason why CFCs were originally used for this purpose. [1] CFCs have high heat capacity OR They are not easily flammable. (ii) Suggest one potential hazard of using alkanes instead of CFCs. [1] Alkanes are flammable. [Total: 22]

5 © ACJC 2018 9729/03/Prelim/2018 2 Recreational drugs like methamphetamine and methcathinone can be synthesised easily from pseudoephedrine, a common nasal decongestant found in cough medicine. (a) (i) The structure of methcathinone and methamphetamine with pKb values at 298 K, are shown in the table below. compound p K b methcathinone 5.98 methamphetamine 3.79 Explain why methcathinone has a higher pKb value than methamphetamine. [2] Methcathinone is a weaker base (higher p Kb) as it has a carbonyl (C=O) functional group two carbons away from secondary amine functional group, which is electron withdrawing. Hence the lone pair of electrons on N for methcathinone is less available for coordination with H + ion. (dative bonding with H+ ion) O H N H N

6 © ACJC 2018 9729/03/Prelim/2018 (ii) Suggest the structure of intermediate X. [1] (b) Commercially, methcathinone is sold in its salt form as solid methcathinone hydrochloride. Methcathinone hydrochloride undergoes the following reaction to form a possible analogue, Y, with hallucinogenic properties. Name the type of reaction and describe its mechanism, showing curly arrows, charges and any relevant lone pairs. [3] Nucleophilic addition (c) Hydrogen iodide, H I, is listed as a controlled substance as it is often used illegally to produce popular recreational drugs like methamphetamine. δ– δ+ δ– δ+

7 © ACJC 2018 9729/03/Prelim/2018 The commercial preparation of hydrogen iodide, H I, involves the reaction of iodine I2, with hydrazine, N2H4, which has ammonia-like properties. N2H4(l) + 2I2(aq) N 2(g) + 4HI(g) (i) During the preparation, gaseous hydrogen iodide produced must be removed immediately. Suggest a reason why this is done.