ACJC 2026 Prelim P2 QP

2 ACJC2026 9476/Preliminary Examinations/2026 [Turn over Answer all the questions in the spaces provided. 1 Butenedioic acid, HOOCCH=CHCOOH, is a weak diprotic Brønsted-Lowry acid that exists as two stereoisomers: cis-butenedioic acid (maleic acid) and trans-butenedioic acid (fumaric acid). It has two acidic hydrogen atoms, and this is illustrated by the following equations 1.1 and 1.2. equation 1.1 HOOCCH=CHCOOH + H2O HOOCCH=CHCOO− + H3O+ equation 1.2 HOOCCH=CHCOO− + H2O −OOCCH=CHCOO− + H3O+ (a) (i) Identify a conjugate acid-base pair from the equations above. acid: ………………………………….. conjugate base: ………………………………… [1] (ii) Draw and label the structures of the cis and trans isomers of butenedioic acid. [2] (b) Table 1.1 shows the acid dissociation constants for both isomers at 298 K. isomer pKa1 pKa2 cis-butenedioic acid 1.93 6.23 trans-butenedioic acid 3.02 4.23 Table 1.1 (i) Draw the displayed formula of the conjugate base of cis-butenedioic acid. [1]

3 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (ii) With reference to your answer in (b)(i), suggest why the pKa1 of cis-butenedioic acid is significantly lower than that of trans-butenedioic acid. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ………………………………………………………………………………………………... [2] (c) A student adds a small portion of 0.100 mol dm -3 sodium hydroxide, NaOH(aq) to 20 cm 3 of 0.100 mol dm-3 cis-butenedioic acid. The student uses a pH meter to measure the pH of the mixture. (i) Using pKa1 value, calculate the initial pH of the cis-butenedioic acid solution before any NaOH(aq) is added. [2] (ii) State the pH of the mixture after 10 cm 3 of NaOH(aq) has been added to 20 cm 3 of 0.100 mol dm-3 cis-butenedioic acid. ………………………………………………………………………………………………... [1]

4 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (iii) Calculate the pH of the mixture at the second equivalence point , when 40 cm 3 of NaOH(aq) has been added to 20 cm3 of 0.100 mol dm-3 cis-butenedioic acid. [3] (iv) Table 1.2 provides information about some acid-base indicators. Table 1.2 pH indicator working range pKa methyl orange 3.1 – 4.4 3.5 bromothymol blue 6.0 – 7.6 7.1 phenolphthalein 8.2 – 10.0 9.3 thymolphthalein 9.3 – 10.5 9.8 Identify the most suitable indicator for the detection of the second equivalence point. Explain your answer. ……………………………………………………………………………………………………. ……………………………………………………………………………………………...… [2]

5 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (d) trans-butenedioic acid reacts with steam in the presence of a concentrated H3PO4 catalyst at high temperature and pressure to form product Y via the electrophilic addition mechanism. (i) Draw the structure of product Y. [1] (ii) State the number of chiral centres in a molecule of product Y. ………………………………………………………………………………...……………… [1] [Total: 16]

6 ACJC2026 9476/Preliminary Examinations/2026 [Turn over BLANK PAGE

7 ACJC2026 9476/Preliminary Examinations/2026 [Turn over 2 (a) The halogens Cl2, Br2, and I2 are often used in the synthesis of halogenoalkanes. Describe and explain the trend in volatility of the halogens down Group 17. …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... ……………………………………………………………………………………………………….. [2] (b) The hydrolysis of 2 -bromo-2-methylbutane, CH3CH2CBr(CH3)2, with aqueous sodium hydroxide produces an alcohol. The data for the kinetics of the reaction is shown in Table 2.1. Table 2.1 experiment initial [CH3CH2CBr(CH3)2] / mol dm–3 initial [NaOH] / mol dm–3 initial rate / mol dm–3 s–1 1 0.10 0.10 2.0 × 10–4 2 0.20 0.10 4.0 × 10–4 3 0.10 0.20 2.0 × 10–4 (i) Deduce the order of reaction with respect to CH3CH2CBr(CH3)2 and OH–. Hence, write the rate equation. [2]

8 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (ii) With reference to your answer in (b)(i), state the type of mechanism for this reaction. …………………………………………………………………………………………………[1] (iii) Describe the mechanism for the reaction of 2-bromo-2-methylbutane with hot aqueous NaOH. Show relevant lone pairs of electrons, dipoles and curly arrows. [2] (c) A student was given two unlabelled bottles, one containing 2-bromo-2-methylbutane and the other containing bromobenzene. Describe a chemical test to distinguish between the two compounds. …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... ……………………………………………………………………………………………………….. [2]

9 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (d) 2-bromobutane is a chiral molecule. (i) Draw the structures of the two enantiomers of 2-bromobutane. [1] (ii) If a single pure enantiomer of 2 -bromobutane undergoes the mechanism stated in (b)(ii), the resulting product mixture is optically inactive. Explain this observation. …………………………………………………………………………………………………... ………………………………………………………………………………………………….. …………………………………………………………………………………………………... ………………………………………………………………………………………………….. ………………………………………………………………………………………………... [2] (e) 1-bromobutane can be used as a starting material to synthesis pentanoic acid via a n intermediate in a two-step synthetic route. Complete Fig. 2.1 by drawing the structure of the intermediate and state the reagents and conditions needed for the conversion. Fig. 2.1 step 1: ………………………………………………………………………………………………..... step 2: …………………………………………………………………………………………………. [3] [Total: 15]

10 ACJC2026 9476/Preliminary Examinations/2026 [Turn over 3 (a) Mushrooms are an excellent source of essential amino acids such as phenylalanine. The amino acid tyrosine can then be synthesised from phenylalanine by enzymes in the body. The structures of phenylalanine and tyrosine are shown in Fig. 3.1. Fig. 3.1 (i) Aqueous bromine can be used to distinguish between phenylalanine and tyrosine. State the observations for phenylalanine and tyrosine and w rite an equation for the reaction that is positive. observations for phenylalanine observations for tyrosine …………………………………………………………………………………………………[2] (ii) In the presence of enzymes, amino acids can form dipeptides. Draw the structure of the dipeptide phe-tyr at pH 1. [1]

11 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (b) D