CHAPTER 7    Further Reactions of Haloalkanes: Unimolecular Substitution and Pathways of Elimination

 

7-1     Solvolysis of Tertiary and Secondary Haloalkanes

              In polar protic solvents, secondary and especially tertiary halides undergo substitution. The rate limiting step involves only the alkyl halide, where the bond from carbon to halogen is cleaved heterolytically to produce a carbocation and halide ion.

7-2     Unimolecular Nucleophilic Substitution

              The rate-limiting step involves only the alkyl halide. Therefore, the reaction is characterized as SN1 (substitution nucleophilic 1st order). The solvent facilitates the reaction by stabilizing the halide ion.

7-3     Stereochemical Consequences of SN1 Reactions

              Carbocations are planar (VSEPR) and, as a result, stereochemistry that may be present at the carbon of the alkyl halide is lost in the reaction.

7-4     Effects of Solvent, Leaving Group, and Nucleophile on Unimolecular Substitution

              The solvent facilitates the reaction by stabilizing the halide ion.

7-5     Effect of the Alkyl Group on the SN1 Reaction: Carbocation Stability

              Carbocations are stabilized by hyperconjugation from attached alkyl groups. Methyl and primary cations are never observed in solution. Tertiary cations are very much more stable than are secondary carbocations.

7-6     Unimolecular Elimination: E1

              Elimination 1st order competes with subsititution. How much of each type of reaction occurs depends upon the nature of the substrate, the nucleophile, and even the reaction conditions. For alkyl halides and sulfonate esters, an E2 reaction is preferred over an E1 reaction.

7-7     Bimolecular Elimination: E2

              E2 reactions occur with strong bases, the best of which is potassium t-butoxide. With primary (and to some extent with secondary) substrates, substitution competes with elimination. The reaction strongly favors a transition state wherein the hydrogen being removed is anti-periplanar to the leaving group.

7-8     Competition between Substitution and Elimination: Structure Determines Function

      The rate of substitution reactions decreases with increasing substitution at the reacting carbon atom. On the other hand, the rate of elimination reaction does not change significantly with the degree of substitution. Thus, increasing substitution results in more elimination and less substitution.

     The use of a strong base that is a weak nucleophile favors elimination over substitution. A good nucleophile that is a weak base favors substitution over elimination.

7-9     Summary of Reactivity of Haloalkanes