20994115-CH7 - CHAPTER 7 Further Reactions of Haloalkanes:...

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CHAPTER 7 Further Reactions of Haloalkanes: Unimolecular Substitution and Pathways of Elimination
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Solvolysis of Tertiary and Secondary Haloalkanes 7-1 The rate of S N 2 reactions decrease dramatically as the reaction center changes from primary to secondary to tertiary. This is true only for bimolecular substitution. Secondary and tertiary halides can be substituted, however, the mechanism is different. Consider the reaction of 2-bromo-2-methylpropane and water. When a substrate undergoes substitution by the solvent, the process is called solvolysis . If the solvent is water, the process is hydrolysis . Water is the nucleophile, even though its nucleophilic capacity is low.
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2-Bromopropane undergoes a similar reaction, but more slowly. 1-Bromopropane, bromoethane, and bromomethane do not react well under these conditions. Solvolysis also occurs in alcohol solvents:
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The following kinetic data demonstrates that the order of reactivity is reversed from that found under typical S N 2 conditions: If the order of reactivity is different, a mechanism other than S N 2 must be involved.
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Unimolecular Nucleophilic Substitution 7-2 Solvolysis follows first-order kinetics. The rate law for the solvolysis of 2-bromo-2-methylpropane by water in formic acid (polar, low nucleophilicity) has been determined by varying the concentrations of the two reactants and measuring the rate of solvolysis. The rate law consistent with the kinetic data depends only on the halide concentration, not the water concentration: Rate = k[(CH 3 ) 3 CBr] mol L -1 s -1 To account for this kind of behavior, it is necessary to postulate a mechanism consisting of 2 or more steps having an initial rate- determining step , or slowest step which does not involve a water molecule. The sum of all of the steps in the proposed mechanism must add up to the observed overall reaction as shown in the stoichiometric equation.
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The mechanism of solvolysis includes carbocation formation. The mechanism for the hydrolysis of 2-bromo-2-methylpropane occurs by a unimolecular nucleophilic substitution, or S N 1 reaction. Only the haloalkane participates in the rate determining step. The mechanism consists of three steps: Step 1: Dissociation of the haloalkane to an alkyl cation and bromide:
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Step 2: An immediate reaction of the 1,1-dimethylethyl cation (a powerful electrophile) with a water molecule (a nucleophilic attack on the electron deficient carbon). The product is an alkyloxonium ion, which is the conjugate acid of the final product, 2-methyl-2-propanol.
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Step 3: The alkyloxonium ion (a strong acid) is finally deprotonated by another water molecule to produce the final product.
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Comparing the potential energy diagrams of an S N 2 reaction and an S N 1 reaction, one can see that the S N 2 reaction has only one transition state, whereas the S N 1 reaction has three: Of the three transition states in the S N 1 reaction, the first has the highest energy because it represents a charge separation. The remaining steps have lower transition state energies and occur more rapidly than the rate-limiting first step.
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20994115-CH7 - CHAPTER 7 Further Reactions of Haloalkanes:...

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