18 6 A bromoalkane, RBr, reacts with aqueous hydroxide ions in a nucleophilic substitution reaction. RBr + OH− → R OH + Br− This reaction is first order with respect to the bromoalkane and the rate equation is rate = k[RBr]1[OH−]x where x is the order of the reaction with respect to hydroxide ions. In an experiment, a sample of the bromoalkane was added to a large excess of aqueous sodium hydroxide and the concentration of the bromoalkane was determined at regular time intervals. Results Time / s [RBr] / mol dm–3 0 0.100 30 0.065 60 0.042 90 0.028 120 0.019 150 0.014 (a) This experiment is carried out using the bromoalkane dissolved in ethanol and the hydroxide ions dissolved in water. Give a reason why a solution of hydroxide ions dissolved in pure ethanol should not be used. (1) ..................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 19 Turn over (b) Plot a graph of [RBr] against time. (3) (c) Explain how the graph shows that the reaction is first order with respect to RBr. Include the values of two consecutive half-lives. You must show your working for the half-lives on the graph. (2) ..................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 20 (d) The experiment was repeated using equal concentrations of RBr and varying the concentration of hydroxide ions. A graph was plotted of the results. Rate / mol dm−3 s−1 [OH−] / mol dm−3 (i) Deduce the value of x in the rate equation. rate = k[RBr]1[OH−]x (1) ..................................................................................................................................................................................................................................................................................... (ii) Give the mechanism for the reaction that is consistent with the orders of reaction with respect to R Br and hydroxide ions. Include curly arrows and relevant lone pairs. (3) 21 Turn over (e) 2-bromobutane can react with aqueous hydroxide ions by an SN1 mechanism. Explain why the butan-2-ol produced from a single optical isomer of 2-bromobutane, using this mechanism, is not optically active. 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(Total for Question 6 = 13 marks)