Reactions
The dehalogenation of vicinal dihalides (halides on two neighboring carbons, think ”vicin-
ity”) is another method for synthesizing alkenes. The reaction can take place using either
sodium iodide in a solution of acetone, or it can be performed using zinc dust in a solution
of either heated ethanol or acetic acid.
This reaction can also be performed with magnesium in ether, though the mechanism is
different as this actually produces, as an intermediate, a Grignard reagent that reacts with
itself and and causes an elimination, resulting in the alkene.
55.1.3 Dehydration of alcohols
Figure 150 Synthesis of alkene by dehydration of an alcohol
An alcohol is converted into an alkene by dehydration: elimination of a molecule of water.
Dehydration requires the presence of an acid and the application of heat. It is generally
carried out in either of two ways, heating the alcohol with sulfuric or phosphoric acid to
temperatures as high as 200, or passing the alcohol vapor over alumina, Al 2 O 3 , at 350-400,
alumina here serving as a Lewis acid. Ease of dehydration of alcohols : 3° > 2° > 1° Where
isomeric alkenes can be formed, we again find the tendency for one isomer to predominate.
Thus, sec-butyl alcohol, which might yield both 2-butene and 1-butene, actually yields
almost exclusively the 2-isomer The formation of 2-butene from n-butyl alcohol illustrates
a characteristic of dehydration that is not shared by dehydrohalogenalion: the double bond
can be formed at a position remote from the carbon originally holding the -OH group. This
characteristic is accounted for later. It is chiefly because of the greater certainty as to where
the double bond will appear that dehydrohalogeation is often preferred over dehydration as
a method of making alkenes.
55.1.4 Reduction of Alkenes
Reduction of an alkene to the double-bond stage can unless the triple bond is at the end of
a chain yield either a cis-alkene or a trans-alkene. Just which isomer predominates depends
upon the choice of reducing agent. Predominantly trans-alkene is obtained by reduction of
alkenes with sodium or lithium in liquid ammonia. Almost entirely cis-alkene (as high as
98%) is obtained by hydrogenation of alkenes with several different catalysts : a specially
prepared palladium called Lindlar’s catalyst; or a nickel boride called P-2 catalyst. Each of
these reactions is, then, highly stereoselective. The stereoselectivity in the cis-reduction of
alkynes is attributed, in a general way, to the attachment of two hydrogens to the same side
of an alkyne sitting on the catalyst surface; presumably this same stereochemistry holds for
the hydrogenation of terminal alkenes which cannot yield cis- and trans-alkenes.
55.1.5 Wittig Reaction
Figure 151 Synthesis of alkene via Wittig reaction