422 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and EliminationTable 11.5 Relative Reactivities of Alkyl HalidesIn an reaction: In an reaction:
In an E2 reaction: 3 ° 72 ° 71 ° In an E1 reaction: 3 ° 72 ° 71 °SN 2 1 ° 72 ° 73 ° SN 1 3 ° 72 ° 71 °11.8 Competition Between Substitution
and Elimination
You have seen that alkyl halides can undergo four types of reactions: E2,
and E1. At this point, it may seem a bit overwhelming to be given an alkyl halide and
a nucleophile/base and asked to predict the products of the reaction. So we need to or-
ganize what we know about the reactions of alkyl halides to make it a little easier to
predict the products of any given reaction.
First we must decide whether the reaction conditions favor or re-
actions. (Recall that the conditions that favor an reaction also favor an E2 reaction
and the conditions that favor an reaction also favor an E1 reaction.) The decision
is easy if the reactant is a primaryalkyl halide—it undergoes only reactions.
Primary carbocations are too unstable to be formed, so primary alkyl halides cannot
undergo reactions.
If the reactant is a secondaryor a tertiaryalkyl halide, it may undergo either
or reactions, depending on the reaction conditions. reactions
are favored by a high concentration of a good nucleophile/strong base, whereas
reactions are favored by a poor nucleophile/weak base (Sections 10.9 and
11.4). In addition, the solvent in which the reaction is carried out can influence the
mechanism (Section 10.10).
Having decided whether the conditions favor reactions or reac-
tions, we must next decide how much of the product will be the substitution product
and how much will be the elimination product. The relative amounts of substitution
and elimination products will depend on whether the alkyl halide is primary, sec-
ondary, or tertiary, and on the nature of the nucleophile/base. This is discussed in the
next section and is summarized later in Table 11.6.Conditions
Let’s first consider conditions that lead to reactions (a high concentration of a
good nucleophile/strong base). The negatively charged species can act as a nucleo-
phile and attack the back side of the -carbon to form the substitution product, or it
can act as a base and remove a -hydrogen, leading to the elimination product. Thus,
the two reactions compete with each other. Notice that both reactions occur for the
same reason—the electron-withdrawing halogen gives the carbon to which it is bond-
ed a partial positive charge.The relative reactivities of alkyl halides in and E2 reactions are shown in
Table 11.5. Because a primaryalkyl halide is the most reactive in an reaction and
the least reactive in an E2 reaction, a primary alkyl halide forms principally the substi-
tution product in a reaction carried out under conditions that favor reactions.
In other words, substitution wins the competition.SN 2 >E2SN 2SN 2substitution
productCH 3HOCH 2 Br CH 3 CH 2 OH + Br−−elimination
productCH 2HOCH 2 Br CH 2 CH 2H+ H 2 O + Br−−baSN 2 >E2SN 2 /E2
SN 2 >E2 SN 1 >E1SN 1 >E1SN 2 >E2 SN 1 >E1 SN 2 >E2SN 1 >E1SN 2 >E2SN 1SN 2SN 2 >E2 SN 1 >E1SN2,SN1,BRUI11-400_436r3 26-03-2003 10:20 AM Page 422