HCI 2021 Prelim Paper 2 Solutions

2021 HCI C2 H2 Chemistry Prelim / Paper 2 1 (a) (i) The carboxylic acid functional group is acidic because the negative charge on its conjugate base anion is delocalized equally over two highly electronegative O atoms [1], stabilizing the conjugate base [1]. This is a recall question. Refer to Section 4.1.1 of Carboxylic Acid & Derivatives lecture notes. Do read the question carefully. You were asked to explain the acidity of the carboxylic acid functional group. You should not be explaining the effect of the NH2 group on the acidity of the carboxylic acid group Also, be very specific in the use of terms. You should explain why the conjugate base is stable by considering how well the negative charge is dispersed due to delocalization of negative charge of the conjugate base over 2 highly electronegative atoms. It is incorrect to say that the dispersion of the charge is through inductive effect. (ii) alkene [1] primary amine [1] QRWH>ò@IRU³SULPDU\´>ò@IRU³DPLQH´ This question was well done but many students did not state the type of amine. Do note that N atom is only attached to one carbon chain which makes it a primary amine. (iii) Hypoglycin A does not show cis-trans isomerism because one of the C=C carbon is attached to two identical groups / two hydrogen atoms. [1] You should apply the criteria for cis-trans isomerism and explain clearly the structural feature of hypoglycin A that makes it unable to exhibit cis-trans isomerism. In this case, there is a C=C that has restricted rotation but the molecule does not exhibit cis-trans isomerism as one of the carbon atoms is attached to two hydrogen atoms. Be direct in your answer and avoid stating what is missing. For instance, explanation such as “there is no two different groups on each side of C=C” is WRONG and confusing. You should be considering the two groups attached to each C of C=C, i.e. each end of C=C and not each side. It is also incorrect to say “hypoglycin A does not show cis- trans isomerism as there is no restricted rotation of bonds” because one of the criteria for cis-trans isomerism is in fact the presence of such bonds which is present in hypoglycin A (there is restricted rotation about C=C). Some students also recognize that there is restricted rotation about the 3 -membered ring. However, one of the carbon atoms of this 3-membered ring has 2 H atoms attached to it and hence cis-trans isomerism is not possible. Do note that “the presence of chiral carbon” does not give rise to cis -trans isomerism. Instead, it may exhibit enantiomerism. HWA CHONG INSTITUTION 2021 C2 H2 CHEMISTRY PRELIM PAPER 2 SUGGESTED SOLUTIONS

(b) P [1] Q [1] R [1] When hypoglycin A is heated with CH3OH in the presence of conc. H2SO4 to form P, many students were able to recognize that esterification took place and used the carboxylic acid group in hypoglycin A to form an ester with CH3OH. The presence o