392 CHAPTER 10 Substitution Reactions of Alkyl Halides
Table 10.8 The Effect of the Polarity of the Solvent on the Rate
of Reaction of tert-Butyl Bromide in an SN 1 ReactionSolvent Relative rate100% water 1200
80% water / 20% ethanol 400
50% water / 50% ethanol 60
20% water / 80% ethanol 10
100% ethanol 1increasing the polarity of the solvent will increase the rate of the reaction
(Figure 10.8 and Table 10.8).In Chapter 12, we will see that compounds other than alkyl halides undergo re-
actions. As long as the compound undergoing an reaction is neutral, increasing
the polarity of the solvent will increasethe rate of the reaction because the polar
solvent will stabilize the dispersed charges on the transition state more than it will sta-
bilize the relatively neutral reactant (Figure 10.8). If, however, the compound under-
going an reaction is charged, increasing the polarity of the solvent will decrease
the rate of the reaction because the more polar solvent will stabilize the full charge on
the reactant to a greater extent than it will stabilize the dispersed charge on the transi-
tion state (Figure 10.7).SN 1SN 1SN 1SN 1‡reactant transition state productsCC CX XX+ −δ+δ−δ+δ−rate-determining step of an SN1 reactionSN 1The Effect of the Solvent on the Rate of an Reaction
The way in which a change in the polarity of the solvent affects the rate of an re-
action depends on whether the reactants are charged or neutral, just as in an
reaction.
Most reactions of alkyl halides involve a neutral alkyl halide and a charged nu-
cleophile. Increasing the polarity of a solvent will have a strong stabilizing effect on
the negatively charged nucleophile. The transition state also has a negative charge, but
the charge is dispersed over two atoms. Consequently, the interactions between the
solvent and the transition state are not as strong as the interactions between the solvent
and the fully charged nucleophile. Therefore, a polar solvent stabilizes the nucleophile
more than it stabilizes the transition state, so increasing the polarity of the solvent will
decrease the rate of the reaction (Figure 10.7).
If, however, the reaction involves an alkyl halide and a neutral nucleophile, the
charge on the transition state will be larger than the charge on the neutral reactants, so
increasing the polarity of the solvent will increase the rate of the substitution reaction
(Figure 10.8).
In summary, the way in which a change in solvent affects the rate of a substitution
reaction does not depend on the mechanism of the reaction. It depends onlyon
whether the reactants are charged: If a reactant in the rate-determining step is
charged, increasing the polarity of the solvent will decrease the rate of the reaction. If
none of the reactants in the rate-determining step is charged, increasing the polarity of
the solvent will increase the rate of the reaction.
In considering the solvation of charged species by a polar solvent, the polar sol-
vents we considered were hydrogen bond donors (protic polar solvents) such as water
and alcohols. Some polar solvents—for example,N,N-dimethylformamide (DMF), di-
methyl sulfoxide (DMSO), and hexamethylphosphoric acid triamide (HMPA)—are
not hydrogen bond donors (they are aprotic polar solvents) (Table 10.7).SN 2SN 2SN 1SN 2SN 2