ACJC_H2_CHEM_P3 Prelim

2 Answer any four questions. 1 (a) Samples of 2-bromo-2-methylpropane were dissolved in dilute aqueous ethanol (80% ethanol and 20% water by volume) and reacted with sodium hydroxide solution. Several experiments were carried out at constant tem perature. The initial rate of reaction was determined in each case. Expt [(CH 3)3CBr] / mol dm-3 [OH-] / mol dm-3 Rate / mol dm-3 s-1 1 0.020 0.010 20.2 2 0.020 0.020 20.2 3 0.040 0.030 40.4 Calculate a value for the rate constant. Hence write the rate equation for the reaction. [3] (b) The hydrolysis of 2-bromopropane is now investigated. Samples were dissolved in dilute aqueous ethanol (80% ethanol and 20% water by volume) and reacted with sodium hydroxide solution. The rate equation is found to be as follows: Rate = 0.24 x 10-5 [CH(CH3)2Br] + 4.7 x 10-5 [CH(CH3)2Br] [OH¯] (i) The rate equation obtained indicates that th e hydrolysis of 2-bromopropane exhibits a mixture of both first order and second order kinetics. Suggest why this may be so. [2] (ii) By deriving an expressi on in terms of [OH -], show that the % rate due to S N2 is - - 4.7[OH ] X 100%4.7[OH ] + 0.24 . [1] (iii) Using the expression derived from (b)(ii), calculate the % rate due to SN2 for various [OH-] of 0.01M, 0.1M and 1.0M. [1] (iv) In light of your answer to (b)(iii) , state how the % rate due to S N2 depends on the concentration of [OH-] and explain why it varies in that manner. [2] (v) By comparing the magnitude of the two rate constants in the rate equation given in (b), comment on the ease of hydrolysis by the two different pathways. The Arrhenius Equation is given as follows: k = A e –Ea / RT [2] (vi) Draw a labelled Maxwell-Boltzmann Curve for this reaction to illustrate the distribution of energy. Based on your answer to (b)(v), indicate the activation energies clearly on the diagram for the two different reaction pathways. [3] ACJC 2011 9647/03/Aug/11 [Turn over

3 (vii) On the same axes, draw the energy profile illustrating both the S N1 kinetics and the SN2 kinetics components observed for this reaction. Indicate the activation energies clearly on the diagram, for the two reaction pathways. [2] (viii) Explain how the rate constant will change if CH(CH 3)2Cl is used instead of CH(CH3)2Br. [2] (ix) Draw the structures of two possible organic by-products that may be formed in this reaction, other than CH(CH3