Chapter 6:
Alkenes: Structure and Reactivity
Prof. Sivaguru Jayaramam,
North Dakota State University
6-1
Unsaturated Hydrocarbons
• Unsaturated hydrocarbon:
hydrocarbon contains one or more
carbon-carbon double or triple bonds
• alkene:
alkene contains a carbon-carbon double bond and has
the general formula CnH2n
H
H
C
C
H
H
Ethene
(an alkene)
Prof. Sivaguru Jayaramam,
North Dakota State University
6-2
Cis-Trans Isomerism
• Because of restricted rotation about a C-C double
bond, groups on adjacent carbons are either cis
or trans to each other
H
H
C
H3 C
C
CH3
C
CH3
cis-2-Butene
mp -139°C, bp 4°C
Prof. Sivaguru Jayaramam,
North Dakota State University
H
H3 C
C
H
trans-2-Butene
mp -106°C, bp 1°C
6-3
Cis-Trans Isomerism
• trans alkenes are more stable than cis alkenes because
of nonbonded interaction strain between alkyl
substituents of the same side of the double bond
trans-2-Butene
Prof. Sivaguru Jayaramam,
North Dakota State University
cis-2-Butene
6-4
Configuration: Cis-Trans
• The cis,trans system: configuration is determined
by the orientation of atoms of the main chain
H
C H2 C H3
C
C H3 CH 2
C
H
trans-3-Hexene
Prof. Sivaguru Jayaramam,
North Dakota State University
H
1
H3 C
2
C
3
C
C H3
4
C H ( C H3 ) 2
cis-3,4-Dimethyl-2-pentene
6-5
Configuration: E,Z
• Assign a priority to the substituents on each
carbon of the double bond
• if the groups of higher priority are on the same side of
the double bond, the configuration is Z (German:
zusammen, together)
• if the groups of higher priority are on opposite sides of
the double bond, the configuration is E (German:
entgegen, opposite)
higher
higher
C
lower
C
lower
C
lower
Z (zusammen)
Prof. Sivaguru Jayaramam,
North Dakota State University
higher
lower
C
higher
E (entgegen)
6-6
Configuration: E,Z
• Priority rules
1. Priority is based on atomic number; the higher the
atomic number, the higher the priority
(1)
(6)
(7)
(8)
(16)
(17)
-H
-CH3
-N H2
-OH
-SH
-Cl
(35) (53)
-Br
-I
Increasing priority
2. If priority cannot be assigned on the basis of the atoms
bonded directly to the double bond, look to the next set
of atoms; priority is assigned at the first point of
difference
(1)
- C H 2 -H
(6)
- C H 2 -C H 3
(7)
- C H 2 -N H2
(8)
- C H 2 -O H
Increasing priority
Prof. Sivaguru Jayaramam,
North Dakota State University
6-7
Configuration - E,Z
3. Atoms participating in a double or triple bond are
considered to be bonded to an equivalent number of
similar atoms by single bonds
-CH=CH2
O
-CH
is treated as
is treated as
C
C
-CH-CH2
O
C
C
O
H
Prof. Sivaguru Jayaramam,
North Dakota State University
6-8
Configuration - E,Z
•
Example: name each alkene and specify its configuration
by the E,Z system
Cl
Cl
(a)
(d)
(b)
Cl
(c)
(e)
Br
Prof. Sivaguru Jayaramam,
North Dakota State University
6-9
Cis-Trans Isomerism
• Dienes, trienes, and polyenes
• for an alkene with n carbon-carbon double bonds, each
of which can show cis-trans isomerism, 2n cis-trans
isomers are possible
• consider 2,4-heptadiene; it has four cis-trans isomers,
two of which are drawn here
Double bond
C2 -C3 C4 -C5
trans
trans
trans
cis
cis
trans
cis
cis
Prof. Sivaguru Jayaramam,
North Dakota State University
2
2
4
trans,trans-2,4heptadiene
4
trans,cis-2,4heptadiene
6-10