The Alkenes (1.7.3)
a. state the general formula of alkenes and understand that they are
unsaturated hydrocarbons with a carbon-carbon double bond which
consists of a  and a  bond
b. explain E-Z isomerism (geometric/cis-trans isomerism) in terms of
restricted rotation around a C=C double bond and the nature of the
substituents on the carbon atoms
c. demonstrate an understanding the E-Z naming system and why it is
necessary to use this when the cis- and trans- naming system breaks
down
Independent Learning (HW) – pp 124-128 and ALL questions therein
Connector:
Write the structural formulae and displayed formulae for ethene,
propene and the three isomers of butene
Crowe2008
H
ethene
C=C
C2H4
H
H
propene
C3H6
H
H
C=C-C
H
CnH2n
H
H H
H
H
H
H
C=C C C
H
H
H H H
H
H
H H
C C=C C
H
H
H
H
H
H
C
C=C
H
H
H
C
H
H
H
Geometric (cis-trans) isomerism
cis – same side
H
H
trans – opposite side
H
C=C
H3 C
CH3
C=C
CH3
H3 C
H
The carbon-carbon double bond doesn't allow any rotation about it.
That means that it is possible to have the CH3 groups on either end
of the molecule locked either on one side of the molecule or
opposite each other.
cis-/trans- isomerism explained
Not isomers, but a
single compound
The C=C double bond is rigid and so cannot rotate
Isomers, two separate
compounds
More about cis/trans isomerism
There must be two different groups on the left-hand carbon
and two different groups on the right-hand one.
trans-but-2-ene
cis-but-2-ene
But, they do not have to be the same groups.
trans-1chloropropene
cis-1chloropropene
If blue = methyl and green = chlorine, name the four compounds here.
Does 2-chloropropene exhibit cis-/trans- isomerism?
No because there needs to be two
different groups on each carbon!
E-Z Notation for geometric isomerism
For more complex molecules the cis/trans notation does not
always work, e.g.
How the E-Z system works
•The carbon atoms at each end of the double bond are
considered separately
•The atom which has the higher atomic number is given the
higher priority
•If the two groups with the higher priorities are on the same
side of the double bond, that is described as the (Z)- isomer.
(The Z comes from a German word zusammen which means together.)
(Zame Zide)
•If the two groups with the higher priorities are on opposite
sides of the double bond, then this is the (E)- isomer.
(The E comes from the German entgegen which means opposite.)
Name these isomers:
Z-1-bromo-2-chloro-1-fluoroethene
E-1-bromo-2-chloro-1-fluoroethene
Name these isomers using the E-Z notation:
E-1,2-dichloroethene
H
H
Z-1,2-dichloroethene
H
C=C
H3 C
CH3
C=C
CH3
Z-but-2-ene
H3 C
H
E-but-2-ene
Name these isomers using the E-Z notation:
E-1-chloro-2-deuteroethene
Z-1-chloro-2-deuteroethene
D = deuterium,
2
1H
In this case, since the atomic no. of H & D is the same,
mass is used to give priority.
Homework
Draw the isomers of pentene (6) and name them.
Use the E-Z notation for any geometric isomers.
For more complicated molecules you work your way along
the carbon chains, once again prioritising by atomic number
Second position H
Second position O
Second position C
So takes priority
First position is
the same
This is a Z-isomer
First position is
the same
So takes priority
Second position =O,
O is counted twice
Name these compounds using cis/trans and E-Z notation
Does trans = E and cis = Z?
Name this compounds using cis/trans and E-Z notation:
cis-2-bromobut-2ene
E-2-bromobut-2ene
Does trans = E and cis = Z? Not always!
pent-1-ene
2-methylbut-1-ene
2-methylbut-2-ene
3-methylbut-1-ene
H
H
C
H
E-pent-2-ene
Z-pent-2-ene
H