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Chapter 11: Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations: 11-2 The Reaction Book Title: Organic Chemistry Printed By: Joetta Lloyd ([email protected]) © 2016 Cengage Learning, Cengage Learning
11-2 The Reaction
In every chemical reaction, there is a direct relationship between the rate at which the reaction occurs and the concentrations of the reactants. When we measure this relationship, we measure the kinetics (Referring to reaction rates. Kinetic measurements are useful for helping to determine reaction mechanisms.) of the reaction. For example, let’s look at the kinetics of a simple nucleophilic substitution—the reaction of with to yield
plus .
With a given temperature, solvent, and concentration of reactants, the substitution occurs at a certain rate. If we double the concentration of , the frequency of encounters between reaction partners doubles and we find that the reaction rate also doubles. Similarly, if we double the concentration of , the reaction rate again doubles. We call such a reaction, in which the rate is linearly dependent on the concentrations of two species, a second-order reaction (A reaction whose rate-limiting step is bimolecular and whose kinetics are therefore dependent on the concentration of two reactants.) . Mathematically, we can express this second-order dependence of the nucleophilic substitution reaction by setting up a rate equation. As either or changes, the rate of the reaction changes proportionately.
where
A mechanism that accounts for both the inversion of configuration and the second-order kinetics that are observed with nucleophilic substitution reactions was suggested in 1937 by the British chemists E. D. Hughes and Christopher Ingold, who formulated what they called the SN 2 reaction (A bimolecular nucleophilic substitution reaction.) —short for substitution, nucleophilic, bimolecular. (Bimolecular means that two molecules, nucleophile and alkyl halide, take part in the step whose kinetics are measured.)
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The essential feature of the mechanism is that it takes place in a single step, without intermediates, when the incoming nucleophile reacts with the alkyl halide or tosylate (the substrate) from a direction opposite the group that is displaced (the leaving group). As the nucleophile comes in on one side of the substrate and bonds to the carbon, the halide or tosylate departs from the other side, thereby inverting the stereochemical configuration. The process is shown in Figure 11-3 for the reaction of (S)-2-bromobutane with to give (R)-2-butanol.
Figure 11-3
Mechanism
The mechanism of the reaction. The reaction takes place in a single step when the incoming nucleophile approaches from a direction away from the leaving halide ion, thereby inverting the stereochemistry at carbon.
As shown in Figure 11-3, the reaction occurs when an electron pair on the nucleophile forces out the group , which takes with it the electron pair from the former
bond. This occurs through a transition state in which the new bond is partially formed at the same time that the old bond is partially broken and in which the negative charge is shared by both the incoming nucleophile and the outgoing halide ion. The transition state for this inversion has the remaining three bonds to carbon in a planar arrangement (Figure 11-4).
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Figure 11-4
The transition state of an reaction has a planar arrangement of the carbon atom and the remaining three groups. Electrostatic potential maps show that negative charge is delocalized in the transition state.
The mechanism proposed by Hughes and Ingold is fully consistent with experimental results, explaining both stereochemical and kinetic data. Thus, the requirement for a backside approach of the entering nucleophile ( away from the departing group) causes the stereochemistry of the substrate to invert, much like an umbrella turning inside- out in the wind. The Hughes–Ingold mechanism also explains why second-order kinetics are observed: the reaction occurs in a single step that involves both alkyl halide and nucleophile. Two molecules are involved in the step whose rate is measured.
Problem 11-2
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What product would you expect to obtain from reaction of with (R)-2-bromobutane? Show the stereochemistry of both the reactant and product.
Problem 11-3
Assign configuration to the following substance, and draw the structure of the product that would result from nucleophilic substitution reaction with
:
Chapter 11: Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations: 11-2 The Reaction Book Title: Organic Chemistry Printed By: Joetta Lloyd ([email protected]) © 2016 Cengage Learning, Cengage Learning
© 2019 Cengage Learning Inc. All rights reserved. No part of this work may by reproduced or used in any form or by any means - graphic, electronic, or mechanical, or in any other manner - without the written permission of the copyright holder.
