OC307-6-11-Ch6.2 - Organic Chemistry Nucleophilic...

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Organic Chemistry Nucleophilic Substitution H. D. Roth 1 ORGANIC CHEMISTRY 307 LECTURE POSTING VI Alkyl Halides, Nucleophilic Substitution, Elimination When we discussed the S N 2 reaction, we had considered two possible mechanisms, involving either a single step (nucleophilic substitution with second order kinetics, S N 2) or two steps, i.e., 1. R–LG R + + LG 2. R + + Nu R–Nu The reactions you learned about so far followed the one- step mechanism; you learned about the kinetic order and about the role of nucleophile, leaving group, the structure of the substrate, and of the solvent. All these reactions proceeds via a
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Organic Chemistry Nucleophilic Substitution H. D. Roth 2 The second mechanism we considered was a two-step reaction: in the first step, the haloalkane undergoes ionic dissociation forming a carbocation; in the second step reaction with a nucleophile completes the substitution. This mechanism would have first-order kinetics: the relative slow formation of the carbocation, R + , would determine the rate (k α [R–LG]); once the cation is generated, it would capture the nucleophile rapidly, forming the product. Does this alternative type of nucleophilic substitution, involving two steps, actually occur? We can answer this question by comparing the reactions of two nucleophiles with two different substrates: R–Br + Cl + H 2 O CH 3 –Br 1200 1 CH 3 CH 2 –Br 40 1 (CH 3 ) 2 CH–Br 1 12 (CH 3 ) 3 CH 2 –Br - 1200000 a) the reaction of iodide ion (Na + I ) with bromoalkanes in acetone; bromomethane reacts much faster with Na + I than 2-bromo-2-methylpropane; this result is exactly what you learned to expect from an S N 2 reaction; b) the reaction of water, H 2 O, with the same bromoalkanes; this reaction proceeds much faster for 2-bromo-2- methylpropane than for bromomethane; the reversed
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Organic Chemistry Nucleophilic Substitution H. D. Roth 3 order of reactivity is incompatible with an S N 2 reaction and requires a different mechanism. A more detailed study shows that the reaction with water follows a first order rate, as expected for the two-step reaction. The reaction also has different stereochemical consequences: the stereochemistry at the reacting carbon is randomized, i.e., iodide ion converts enantiomerically pure 3-bromo-3- methylhexane to racemic 3-iodo-3-methylhexane. This result requires an achiral intermediate , identified as the planar carbocation (cf., Figure 6.10). The two-step mechanism of these reactions and the nature of the intermediate causes a range of important features: 1. the reaction proceeds fastest for tertiary substrates; 2. the reaction requires a good leaving group, LG; 3. the solvent influences the rate significantly;
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Organic Chemistry Nucleophilic Substitution H. D. Roth 4 4. the nucleophile does not play a significant role [because it isn’t involved in the rate-determining step]. 1. The fast reaction of tertiary substrates is related to
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This note was uploaded on 11/17/2011 for the course ORGANIC CH 307 taught by Professor Boikes during the Fall '09 term at Rutgers.

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OC307-6-11-Ch6.2 - Organic Chemistry Nucleophilic...

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