unsaturated groups are called the vinyl group (CH 2 À À
À À CHÀ À) and the allyl
group (CH 2 CHCH 2 À À).
CH CHCH 2 CH 3
C
H 3
C
H 2 CH
C
H 2 CHCH 2
Vinylic carbons
Allylic carbons
Vinyl group
Allyl group
Cycloalkenes must have eight or more carbons before they are large enough
to incorporate a trans double bond. Thus, cycloalkenes are considered to be
cis unless otherwise specified. A bridged bicyclic (two ring) compound
cannot have a double bond at a bridgehead position unless one of the rings
contains at least eight carbon atoms. A bridgehead carbon is part of both
rings. A bridged bicyclic compound has at least one carbon in each of the
three links between the bridgehead atoms.
trans-Cyclodecene
Bicyclic
Bridged bicyclic
4.4.2 Physical properties of alkenes
As with alkanes, the boiling points and melting points of alkenes decrease
with increasing molecular weight, but show some variations that depend on
the shape of the molecule. Alkenes with the same molecular formula are
isomers of one another if the position and the stereochemistry of the double
bond differ. For example, there are four different acyclic structures that can
be drawn for butene (C 4 H 8 ). They have different b.p. and m.p. as follows.
C C
CH 2 CH 3
H
H
H
C C
CH 3
H
H
CH 3
C C
CH 3
C
H 3
H
H
C C
H
C
H 3
H
CH 3
2-Methylpropene
b.p = -7
m.p = -144
1-Butene
b.p = -6
m.p = -195
cis-2-Butene
b.p = +4
m.p = -139
trans-2-Butene
b.p = +1
m.p = -106
1
2 3
4
1
2 3
4
1 2
3
4
1 2
3
4
4.4.3 Structure of alkenes
In ethene (C 2 H 4 ), each carbon atom has three s-bonding electron pairs in its p
orbitals to form three s bonds, one with the carbon and the other two with two
hydrogen atoms. The p bond in C 2 H 4 is formed from the sideways overlap of a
parallel p orbital on each carbon atom. The CÀ ÀC bond in ethene is shorter and
stronger than in ethane, partly because of the sp
2 –sp
2 overlap being stronger
than sp
3 –sp
3
, but especially because of the extra p bond in ethene.
4.4 ALKENES AND THEIR DERIVATIVES
105
À À CHÀ À) and the allyl
group (CH 2 CHCH 2 À À).
CH CHCH 2 CH 3
C
H 3
C
H 2 CH
C
H 2 CHCH 2
Vinylic carbons
Allylic carbons
Vinyl group
Allyl group
Cycloalkenes must have eight or more carbons before they are large enough
to incorporate a trans double bond. Thus, cycloalkenes are considered to be
cis unless otherwise specified. A bridged bicyclic (two ring) compound
cannot have a double bond at a bridgehead position unless one of the rings
contains at least eight carbon atoms. A bridgehead carbon is part of both
rings. A bridged bicyclic compound has at least one carbon in each of the
three links between the bridgehead atoms.
trans-Cyclodecene
Bicyclic
Bridged bicyclic
4.4.2 Physical properties of alkenes
As with alkanes, the boiling points and melting points of alkenes decrease
with increasing molecular weight, but show some variations that depend on
the shape of the molecule. Alkenes with the same molecular formula are
isomers of one another if the position and the stereochemistry of the double
bond differ. For example, there are four different acyclic structures that can
be drawn for butene (C 4 H 8 ). They have different b.p. and m.p. as follows.
C C
CH 2 CH 3
H
H
H
C C
CH 3
H
H
CH 3
C C
CH 3
C
H 3
H
H
C C
H
C
H 3
H
CH 3
2-Methylpropene
b.p = -7
m.p = -144
1-Butene
b.p = -6
m.p = -195
cis-2-Butene
b.p = +4
m.p = -139
trans-2-Butene
b.p = +1
m.p = -106
1
2 3
4
1
2 3
4
1 2
3
4
1 2
3
4
4.4.3 Structure of alkenes
In ethene (C 2 H 4 ), each carbon atom has three s-bonding electron pairs in its p
orbitals to form three s bonds, one with the carbon and the other two with two
hydrogen atoms. The p bond in C 2 H 4 is formed from the sideways overlap of a
parallel p orbital on each carbon atom. The CÀ ÀC bond in ethene is shorter and
stronger than in ethane, partly because of the sp
2 –sp
2 overlap being stronger
than sp
3 –sp
3
, but especially because of the extra p bond in ethene.
4.4 ALKENES AND THEIR DERIVATIVES
105