H
H
H
H
H
H
H
H
H
H
H
H
H
.
C
H
.
. .
:
C
H
. H
.
.
.
C
C
C
C
Three σ bonds are formed using
three p orbitals from carbon and
s orbitals from hydrogen atoms
pi bond is formed by the sideways
overlap of the parallel p orbital
of each carbon atom
Ethene
1.08Å
1.33Å
4.4.4 Industrial uses of alkenes
Alkenes are useful intermediates in organic synthesis, but their main
commercial use is as precursors for polymers. For example, styrene
polymerizes to polystyrene.
Ph
Ph
Ph
n
H
H
H
Ph
Polystyrene
Polymerize
Styrene
4.4.5 Preparation of alkenes and cycloalkenes
Alkenes are obtained by the transformation of various functional groups,
e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl
halides (see Section 5.4.5) and dehalogenation or reduction of alkyl
dihalides (see Section 5.4.5). These reactions are known as elimination
reactions. An elimination reaction results when a proton and a leaving group
are removed from adjacent carbon atoms, giving rise to a p bond between
the two carbon atoms.
C C
H OH
C C
H X
C C
X X
Alkyl halide
or Zn, AcOH
H 2 SO 4
Alcohol
Alkyl dihalide
KOH (alc)
Heat
NaI, Acetone
Heat
Alkene
106
CH4 ORGANIC FUNCTIONAL GROUPS