in which the developing negative charge is on a secondary carbon. Because the
transition state leading to 1-pentene is more stable, 1-pentene is formed more rapidly
and is the major product of the E2 reaction of 2-fluoropentane.
The data in Table 11.2 show that as the halide ion increases in basicity (decreases in
leaving ability), the yield of the more substituted alkene product decreases. However,
the more substituted alkene remains the major elimination product in all cases, except
when the halogen is fluorine.
We can summarize by saying that the major product of an E2 elimination reaction
is the more stable alkeneexcept if the reactants are sterically hindered or the leaving
group is poor(e.g., a fluoride ion), in which case the major product will be the less
stable alkene. In Section 11.6, you will see that the more stable product is not always
the major product in the case of certain cyclic compounds.PROBLEM 3Give the major elimination product obtained from an E2 reaction of each of the following
alkyl halides with hydroxide ion:a. d.b. e.c. f.PROBLEM 4Which alkyl halide would you expect to be more reactive in an E2 reaction?a.b.c.d.11.3 The E1 Reaction
The second kind of elimination reaction that alkyl halides can undergo is an E1
elimination. The reaction of tert-butyl bromide with water to form 2-methyl-
propene is an example of an E1 reaction; “E”stands for elimination and “ 1 ”stands
for unimolecular.CH 2 CHCH 2 CH 3BrCH 2 CH 2 CHCH 3BrorCH 3 CH 2 CH 2 CHCH 3 orBrCH 3 CH 2 CHCH 2 CH 3BrBr
BrorCH 3 CHCHCH 2 CH 3BrCH 3CH 3 CHCH 2 CHCH 3BrCH 3orCH 3 CHCHCH 2 CH 3FCH 3CH 3 CHCHCH 2 CH 3ClCH 3BrCH 3 CHCH 2 CH 3FCH 3 CHCH 2 CH CH 2ClCH 3 CHCH 2 CH 3Cl408 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and EliminationBRUI11-400_436r3 26-03-2003 10:20 AM Page 408