432 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and EliminationPROBLEM 28Design a multistep synthesis to show how each of the following compounds could be pre-
pared from the given starting material:a.b.c.d.e.CH 2 CH 3
BrCH 2 CH 2 CH 2 CH 2 BrCH 3 CH 2 CH 2 CH 2 Br CH 3 CH 2 CCH 2 CH 2 CH 3OCH CH 2 CH 2 CHOOOHSummary
In addition to undergoing nucleophilic substitution reac-
tions, alkyl halides undergo : The
halogen is removed from one carbon and a proton is re-
moved from an adjacent carbon. A double bond is formed
between the two carbons from which the atoms are elimi-
nated. Therefore, the product of an elimination reaction is
an alkene. Removal of a proton and a halide ion is
called dehydrohalogenation. There are two important
reactions, E1 and E2.
An E2 reactionis a concerted, one-step reaction; the
proton and the halide ion are removed in the same step, so
no intermediate is formed. In an E1 reaction, the alkyl
halide dissociates, forming a carbocation. In a second step,
a base removes a proton from a carbon that is adjacent to
the positively charged carbon. Because the E1 reaction
forms a carbocation intermediate, the carbon skeleton can
rearrange before the proton is lost.
Primary alkyl halides undergo only E2 elimination re-
actions. Secondary and tertiary alkyl halides undergo both
E2 and E1 reactions. For alkyl halides that can undergo
both E2 and E1 reactions, the E2 reaction is favored by the
same factors that favor an reaction—a high concen-
tration of a strong base and an aprotic polar solvent—and
the E1 reaction is favored by the same factors that favor an
reaction—a weak base and a protic polar solvent.
An E2 reaction is regioselective. The major product is the
more stable alkene, unless the reactants are sterically hin-
dered or the leaving group is poor. The more stable alkene isSN 1SN 2b-eliminationB-elimination reactionsgenerally (but not always) the more substituted alkene. The
more substituted alkene is predicted by Zaitsev’s rule: It is
the alkene formed when a proton is removed from the
-carbon that is bonded to the fewest hydrogens.
An E2 reaction is stereoselective: If the -carbon has two
hydrogens, both Eand Zproducts will be formed, but the
one with the bulkiest groups on opposite sides of the double
bond is more stable and will be formed in greater yield. Anti
eliminationis favored in an E2 reaction. If the -carbon has
only one hydrogen, only one alkene is formed, since there is
only one conformer in which the groups to be eliminated are
anti. If the reactant is a cyclic compound, the two groups to
be eliminated must be trans to one another; in the case of
six-membered rings, both groups must be in axial positions.
Elimination is more rapid when H and X are diaxial in the
more stable conformer.
An E1 reaction is regioselective. The major product is
the most stable alkene, which is generally the most substi-
tuted alkene. An E1 reaction is stereoselective. The major
product is the alkene with the bulkiest groups on opposite
sides of the double bond. The carbocation formed in the
first step can undergo both syn and anti elimination; there-
fore, the two groups to be eliminated in a cyclic compound
do not have to be trans or both in axial positions. Alkyl sub-
stitution increases the stability of a carbocation and de-
creases the stability of a carbanion.
Predicting which products are formed when an alkyl
halide undegoes a reaction begins with determining whetherbbbBRUI11-400_436r3 26-03-2003 10:20 AM Page 432