55 Reactions
55.1 Preparation
There are several methods for creating alkenes.^1 Some of these methods, such as the Wittig
reaction, we’ll only describe briefly in this chapter and instead, cover them in more detail
later in the book. For now, it’s enough to know that they are ways of creating alkenes.
55.1.1 Dehydrohalogenation of Haloalkanes
Figure 147 Synthesis of alkene by dehydrohalogenation
Alkyl halides are converted into alkenes by dehydrohalogenation: elimination of the
elements of hydrogen halide. Dehydrohalogenation involves removal of the halogen atom
together with a hydrogen atom from a carbon adjacent to the one bearing the halogen. It
uses the E2 elimination mechanism that we’ll discuss in detail at the end of this chapter The
haloalkane must have a hydrogen and halide 180° from each other on neighboring carbons.
If there is no hydrogen 180° from the halogen on a neighboring carbon, the reaction will
not take place. It is not surprising that the reagent required for the elimination of what
amounts to a molecule of acid is a strong base for example: alcholic KOH.
In some cases this reaction yields a single alkene. and in other cases
yield a mixture. n-Butyl chloride, for example, can eliminate hydrogen only from
C-2 and hence yields only 1-butene. sec-Butyl chloride, on the other hand, can
eliminate hydrogen from either C-l or C-3 and hence yields both 1-butene and
2-butene. Where the two alkenes can be formed, 2-butene is the chief product.55.1.2 Dehalogenation of Vicinal Dibromides
Figure 148 Synthesis of alkene via debromination of vicinal dihalides using Sodium
Iodide
Figure 149 Synthesis of alkene via debromination of vicinal dihalides using Zinc
1 IIT Chemistry by Dr.O.P.Agrawal and Avinash Agrawal