NYJC H2 Chem 2012 Prelim P3 Soln

2 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over Answer any four questions 1 Aldehydes and ketones react with primary amines in slightly acidic solution to form imines which have the C=N functional group. For example, propanal reacts with methylamine to give N-methyl-1-propanimine. CH3CH2 C H O CH3NH2 H+ N CH3 H CH3CH2 C propanal N-methyl-1-propanimine (a) Aldehydes and ketones also react with 2,4-dinitrophenylhydrazine (2,4-DNPH) to give compounds that are closely related to imines. (i) Write a balanced equation for the reaction between benzaldehyde and 2,4- DNPH. NO2 O2N H2N N H +C H O H2O+NO2 O2N N N H C H (ii) State what you would observe in the reaction with 2,4-DNPH. What is the type of reaction? orange precipitate condensation [ 4]

3 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over (b) The mechanism for the reaction between benzaldehyde and methylamine is shown below. + H2O – C O H + CH3NH2 C O H N H H CH3 + C OH H N H CH3 carbinolamine H+ C OH 2 H N H CH3 + G + step I step II step III step IV −− −− H+ −− −−imine iminium ion G step V (i) What is the type of reaction in step I? nucleophilic addition (ii) The positive charge on the iminium ion, G+, does not reside on a carbon atom. Suggest the structure of G+. CH3C H N H +

4 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over (iii) Write equations for step I and step IV to show the movement of electrons, using curved arrow notation. Show the lone pairs of electrons, if any, that are involved in each step. (iv) The maximum rate of formation of imines occurs at a pH of about 4.5. Explain why the rate is slow under very acidic conditions and under alkaline conditions. acidic: amine protonated; no nucleophile (Step I) alkaline: very low H+ conc; no catalyst (Step III) [ 6] (c) Acrylamide is a carcinogen. It is formed when potato chips are heated to above 120 °C. Its formation has been linked to the presence of glucose (C 6H12O6) and an α -amino acid L found in relatively high amounts in potato. C C H CONH2 H H H C C OH H C OH H C OH H C OH H C OH H H O acrylamide glucose Glucose reacts with L to form an imine M. At high cooking temperatures, M decomposes to produce only three compounds in equimolar amounts: acrylamide, carbon dioxide and compound N (C6H13NO5). (i) Name the functional groups in acrylamide. alkene , amide

5 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over (ii) Deduce the structure of L. Show clearly how you obtained your answer. L : H2N C CH2CONH2 H CO2H (asparagine). Accept other possible alternatives. [5] (d) Suggest the reagents and conditions required to synthesise the following imine derivative. Identify all the intermediate compounds. O N N O OH aq NaBH4 xs conc. H2SO4 heat Br OH NH2 OH aq Br2 xs ethanolic NH3 heat in sealed tube NH2acidified K2Cr2O7 warm