DHS Prelim P3_SuggestedSolutions

DUNMAN HIGH SCHOOL Preliminary Examination 2014 H2 CHEMISTRY (9647/03) 1 (a) Lawsone is the dye that is extracted from the henna plant, Lawsonia inermis . Although its natural colour is yellow, lawsone reacts with the proteins in hair and skin to produce the characteristic brown henna colour. Compound A is a derivative of lawsone. O O OH lawsone O O OH A (i) Name two functional groups in A, other than the phenyl group. ketone, secondary alcohol (ii) Describe a reaction, with reagent and condition(s), to confirm one of the functional groups in A. Describe the observations you would make. Reagent & conditions A Add 2,4–dintrophenylhydrazine. Orange ppt seen. Add PCl5. White fumes is seen. (iii) Only one organic compound can be formed when lawsone is reacted with aqueous Br2, through the loss of one water molecule. Suggest the structural formula of this compound. O O O Br or O O Br Br O O Br OH or [4] (b) Compound B can be oxidised to lawsone by acidified K 2Cr2O7 involving two moles of electrons lost per mole of B. O O OH OH OH OH lawsoneB (i) With the use of the Data Booklet , construct a balanced half–equation for the oxidation of B and a balanced equation for the overall reaction. You are to use the molecular formulae of lawsone (C 10H6O3), B (C 10H8O3) and Cr 2O7 2– in your equations.

DHS Prelim Exam 2014 H2 Chemistry Paper 3 – Mark Scheme © DHS 2014 9647/03 2 [O]: C10H8O3  C10H6O3 + 2e + 2H+ Overall: Cr2O7 2– + 8H+ + 3C10H8O3  2Cr3+ + 7H2O + 3C10H6O3 (ii) Calculate the concentration of a solution of compound B given that 20.0 cm 3 of this solution required 12.50 cm3 of 0.050 mol dm–3 K2Cr2O7 solution to reach end–point. [4] No. of moles of B = 1000 12.50 x 0.050 x 3 = 0.001875 mol [B] = 100020 0.001875 = 0.0938 mol dm–3 (c) When lawsone is reacted under suitable condition, compound C is produced. Reacting C with ethanoyl chloride produces a neutral compound D, with molecular formula C 12H8O4. C12H8O4 O O O - C CH3 Cl O D (i) The production of D occurs via a two–step mechanism. Step I: B and ethanoyl chloride reacts to form an intermediate E as shown. O O O CH3 O Cl EStep II: A chloride ion is eliminated to form D. Outline the two mechanism steps by drawing relevant curly arrows, partial charges and lone pair of electrons in your answer. Label the steps clearly. O O O CH3 O Cl   O O O CH3 O Cl step I step II D

DHS Prelim Exam 2014 H2 Chemistry Paper 3 – Mark Scheme © DHS 2014 9647/03 3 (ii) Hence or otherwise, draw the structural formula of D. [3] O O O O CH3 (d) Another compound F, in addition to D, is also produced in the above reaction involving C and ethanoyl chloride. O O O CH3 O F (i) Draw the structure of the nucleophile, G, involved in the formation of F. O O O (ii) Hence, suggest with the use of curly arrows, to show how G is formed from C. [2] O O