Chemistry Reference
In-Depth Information
3
Alkenes and Alkynes
Chapter Summary
Alkenes
have a carbon-carbon double bond and
alkynes
have a carbon-carbon triple
bond. Nomenclature rules are given in Sec. 3.2. Each carbon of a double bond is
trigonal
,
and connected to only
three
other atoms, all of which lie in a plane with bond angles of 120
o
.
Ordinarily, rotation around double bonds is restricted. All six atoms of
ethylene
lie in a
single plane. The C=C bond length is 1.34 Å, shorter than a C-C bond (1.54 Å). These facts
can be explained by an orbital model with three
sp
2
hybrid orbitals (one electron in each)
and one
p
orbital perpendicular to these (containing the fourth electron). The double bond is
formed by end-on overlap of
sp
2
orbitals to form a
σ
bond and lateral overlap of aligned
p
orbitals to form a
π
bond
(Figures 3.4 and 3.5). Since rotation around the double bond is
restricted,
cis
-
trans
isomerism
is possible if each carbon atom of the double bond has two
different groups attached to it.
Alkenes react mainly by
addition
. Typical reagents that add to the double bond are
halogens, hydrogen (metal catalyst required), water (acid catalyst required), and various
acids. If either the alkene or the reagent is
symmetrical
(Table 3.2), only one product is
possible. If
both
the alkene and reagent are unsymmetrical, however, two products are
possible, in principle. In this case,
Markovnikov's rule
(Secs. 3.8-3.10) allows us to predict
the product obtained.
Electrophilic
additions
occur by a two-step mechanism. In the first step, the
electrophile
adds in such a way as to form the most stable
carbocation
(the stability order
is tertiary > secondary > primary). Then the carbocation combines with a
nucleophile
to
give the product.
The energetics of electrophilic additions, and all other reactions, can be described
using
reaction
energy
diagrams
(Figures 3.10-3.12). Such diagrams show each step in
the reaction mechanism, and indicate the relative energies of
reactants
,
products
,
intermediates
, and
transition
states
. They indicate whether the
enthalpy
of a step in a
reaction is
exothermic
or
endothermic
, or whether the step has a high or low
energy
of
activation
. In general, reactions that are exothermic and have low energies of activation
proceed at relatively fast rates (Secs. 3.11 and 3.12).
Conjugated
dienes
have alternating single and double bonds. They may undergo
1,2-
or
1,4-addition
.
Allylic
carbocations
, which are stabilized by resonance, are
intermediates in both the 1,2- and 1,4-additions (Sec. 3.15a). Conjugated dienes also
undergo cycloaddition reactions with alkenes (
Diels-Alder
reaction
), a useful synthesis of
six-membered rings (Sec. 3.15b).
Addition to double bonds may also occur by a
free-radical mechanism
.
Polyethylene can be made in this way from the
monomer
ethylene.
