2023 EJC H2 Chemistry Prelims Paper 3 (Solutions with comments)

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2023 General Certificate of Education Advanced Level Higher 2 Paper 3 Suggested Solutions with Marker’s comments Section A 1 (a) Solvolysis is a reaction in which the solvent is one of the reagents and is also present in great excess of that required for the reaction. Usually, the electron -rich solvent molecule substitutes an atom or group in the reactant molecule. One example of solvolysis is the reaction of compound A, C6H5CH2Br, with ethanoic acid as the solvent. This reaction happens via an SN1 mechanism. (i) Describe the mechanism for the reaction between compound A with ethanoic acid. Show relevant lone pairs of electrons, dipoles and curly arrows. [4] (ii) Explain why this reaction mainly proceeds via the SN1 mechanism. [1] (iii) Given that the enthalpy change of the reaction is endothermic, draw a labelled reaction pathway diagram for the reaction involving compound A and ethanoic acid. [2] Another reaction was carried out using C6H5CHBrCH3 with ethanol as the solvent, in the presence of trace amount of sodium hydroxide. This reaction undergoes the same mechanism as the reaction between compound A and ethanoic acid. (iv) Predict the major organic product formed from the reaction of optically pure C6H5CHBrCH3 with ethanol, and comment on the optical activity of the product formed. Briefly explain your answer. [3] (v) By-products are formed during the preparation of many organic compounds. This usually occurs because the reagents can react in more than one way. Suggest a possible by-product, in full structural formula, that can be formed from the above reaction. [1]

2 © EJC 9729/03/J2PE/23 ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... (i) partial charges, slow step, correct carbocation all correct arrows and lone pairs of electrons deprotonation and formation of HBr (ii) Heterolytic fission of C-Br bond in (bromomethylbenzene) forms a stable benzylic carbocation intermediate, , which is stabilised by the delocalisation of the positive charge into the benzene ring (iii) (iv) The carbocation is trigonal planar about the positively charge C, thus the ethanol nucleophile can attack from either side of the plane with equal probability, leading to formation of an equimolar mixture of two enantiomeric products.

3 © EJC 9729/03/J2PE/23 [Turn Over ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... Comments: (i) is generally well attempted as st udents showed some understanding of S N1 mechanism. (ii) is poorly done. Many students were not able to provide the correct answer. (iii) is poorly done. While students were able to draw the overall shape of an energy profile diagram, many lose marks due to insufficient labelling. (iv) is poorly done. Some students showed a lack of understanding of the context of the question i.e. solvolysi s. In addition, w hile students showed knowledge of the properties of SN1 mechanism, many students were rather vague in their answers and used the wrong terminology. (v) is generally well attempted. • Question analysis and references (where applicable) (i) is testing on application of SN1 mechanism when the nucleophile is a carboxylic acid. Students should also keep in mind that once the bond is formed between O of the acid with C of compound A, there should also be a deprotonation step to remove the H+ bonded to O of the carboxylic acid functional group. (ii) Students should answer why SN1 is occurring and not explain why SN2 will not occur. Students should answer directly to the question of why SN1 proceeds. (iii) A reaction pathway diagram is an energy profile diagram. Given that it is an endothermic reaction and with a slow step (SN1), students should keep in mind the number of peaks in the diagram. (iv) The starting material has changed. Apply solvolysis (the same mechanism) to the reaction between C6H5CHBrCH3 and ethanol to derive the product. (v) Given that C6H5CHBrCH3 and ethanol are reacted in the presence of trace amount of NaOH, NaOH can serve as base (elimination) or a competition nucleophile (nucleophilic substitution). • Common mistakes: (i) – did not show the removal of H + after the protonated produce is formed from the second step. – Br‒ formed at the end of slow step extracted H + from the acid. This will not occur as Br– is not a strong enough base to cause