RI 2024 Arenes Tutorial Answers

-1- RAFFLES INSTITUTION YEAR 5 H2 CHEMISTRY 2024 Tutorial 13 – Arenes ______________________________________________________________________________ Suggested Solutions to Practice Questions 1 Electrophile Major Organic Product(s) (i) Br+ (ii) (iii) NO2+ (iv) (v)

-2- 2(a) Reaction Reagents and conditions Type of reaction Unknown Reactant / Product I CH3CH2Cl, AlCl3 electrophilic substitution -- II H2, Ni catalyst, heat reduction A: III KMnO4(aq), dilute H2SO4, heat / heat under reflux oxidation C : IV limiting Br2, uv light/heat free radical substitution -- V Br2, FeBr3 electrophilic substitution -- VI conc. HNO3, conc. H2SO4, 30 C electrophilic substitution -- VII Br2 in CCl4, dark and room temperature electrophilic addition B : 2(b) 2(c) Positional isomers of D: isomer D1 isomer D2 D1 is more likely to be formed along with D. This is because the –CH2CH3 group is 2,4-directing (and not 3-directing). CH2CH3 CO2H + CO2 + 2H2O+ 6[O] CH2CH3Br Br CH2CH3

-3- 2(d) (i) Reaction of alkyl side chain vs alkene. Reaction IV requires energy (in the form of UV light) to be provided while reaction VII does not. Compare regions of high / low electron density • Reaction IV involves the reaction of the alkyl group which does not possess any regions of high or low electron density. • Reaction VII involves reaction at the highly electron rich C=C which polarises the Br 2 electron cloud to generate the Br + electrophilic site / attracts the Br 2 electrophile. Comparing bonds broken • The C–H bonds broken in Reaction IV are relatively strong. Hence, UV light is needed to generate the reactive bromine radicals which make the substitution reaction thermodynamically feasible. • The slow step of Reaction VII involves breaking of the relatively weak  bond of C=C. (ii) Reaction V requires the use of a Lewis acid catalyst while reaction VII does not. In reaction V, the conversion of ethylbenzene to D involves disrupting the resonance stabilisation due to the deloca lisation of the 6  electrons of benzene . Hence a n electrophile stronger than Br 2 is required for the electrophilic substitution. A Lewis acid such as FeBr 3 is required since it reacts with Br2 to generate the stronger Br+ electrophile. However, in reaction VII, the addition of Br 2 across the C=C bond does not disrupt any resonance stabilisation of the benzene ring. Also, the electron rich C=C bond of the alkene is able t o polarise the Br 2 electron cloud to generate the Br+ electrophilic site. Hence, no lewis acid is required.

-4- 2(e) Reaction I Electrophilic Substitution CH3CH2Cl + AlCl3 ⇌ CH3CH2+ + [AlCl4]– Step 1: Step 2: Reacti