MI_H2_CHEM_P3_Answer Prelim

1. Lactic acid (2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. (a) (i) A is an isomer of lactic acid. It rotates plane -polarised light and reacts with Fehling’s solution to give a reddish brown precipitate. 0.01 mol of A reacts with sodium to give 0.24 dm 3 of hydrogen gas under room temperature and pressure conditions. Suggest a structural formula of A, giving reasons for your answer. A is ; A has a chiral centre as it rotates planepolarised light. ; A is an aldehyde since it reacts with Fehling’s solution to give a reddish brown precipitate. ; Amount of hydrogen evolved with sodium per mol of A = 0.24 0.0124  = 1 mol Hence, there are two OH group. ; (ii) Compare the acidity of isomer A and lactic acid. Explain your answer. A is less acidic than lactic acid. ; A is an alcohol. The stability of its anion is lower than the stability of the anion of lactic acid as the anion is destabilised by the electrondonating alkyl group. ; Lactic acid is a much stronger acid as its anion forms two equivalent resonance struc tures with the negative charge on O delocalised over the carbon and two electronegative O atoms. ; [7] (b) Lactic acid can undergo a condensation reaction in the presence of hot concentrated sulfuric acid to form a cyclic diester B, C6H8O4.

The system exists in dynamic equilibrium. (i) Draw the structure of diester B. ; (ii) Explain what is meant by the term dynamic equilibrium. rate of forward reaction equals to rate of reverse reaction ; (iii) Write an expression for the equilibrium constant, K C, for the reaction, stating its units. [ ][ ] [ ] ; units = mol dm-3 ; (iv) In an experiment, 2.0 mol of lactic acid was allowed to reach equilibrium. Given that 95% of lactic acid has reacted at equilibrium and the total volume of the mixture remained constant at 200 cm 3, calculate the K C for this reaction. 2 lactic acid ⇌ B + 2H2O initial mol 2 0 0 change in mol -0.95(2) +0.95 +0.95(2) eqm mol 0.1 0.95 1.9 ICE table ; ( )( ) ( ) mol dm-3 ; (v) In the reverse reaction, i.e. the hydrolysis of the diester B, the rate of reaction gradually increases at first, but subsequently it decreases. Explain why this is so. The rate increases at first when the carboxylic acid is first formed

because it partially dissociates to give H + which catalyses the hydrolysis of ester. ; The rate decreases as the reaction proceeds due to decrease in the concentration of the ester, leading to decrease in frequency of effective collisions. ; [8] (c) Lactic acid, along with ammonium hydrogen carbonate, (NH 4)HCO3, is used in mosquito attractant. Ammonium hydrogen carbonate is a solid which can decompose into ammonia, carbon dioxide and water. (i) Write an eq