7.4.3 Kinetics

Section A

Section B

1. Ingold was awarded a Nobel Prize for his investigations into the kinetics of the hydrolysis of bromoalkanes in alkaline aqueous ethanol.

RBr + OH^- ROH + Br^-

(a) The hydrolysis of bromoalkanes is an example of nucleophilic substitution. Explain this term using the equation shown.

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(b) Outline how you could determine the rate of hydrolysis of a bromoalkane. Practical details are not required.

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(c) Ingold obtained the following rate constants for the hydrolysis of bromoalkanes.

CH₃Br C₂H₅Br CH₃CHBrCH₃ (CH₃)₃CBr

First order - - 2.4 x 10⁴ 1.0 x 10²

Second order 2.1 x 10² 7.1 x 10³ 4.7 x 10⁵ -

(i) Deduce the units for a first order rate constant.

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(ii) Write a rate equation for the hydrolysis of bromoethane.

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(iii) Deduce the initial rate of hydrolysis of bromoethane, if 50 cm³ of a 0.1

mol dm^-3 ethanolic solution of bromoethane is completely mixed with 50 cm³ of alkaline aqueous ethanol which has a concentration of 0.05 mol dm^-3 with respect to hydroxide ions.

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(iv) How would the rate , calculated in part (iii), be affected if the concentration of hydroxide ions was doubled?

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(v) Suggest why CH₃CHBrCH₃ should have both a first and second order rate constant.

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(vi) Draw a flow scheme to illustrate the mechanism of the hydrolysis of (CH₃)₃Br.

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