Alkenes
• Alkenes contain a C=C double bond.
• Unsaturated compounds (contain a multiple C=C bond)
• Form an homologous series General formula CnH2n
• Chemical reactions occur at the double bond.
• Note the geometry of the atoms around the double bond. The 2
carbons and 4 other atoms lie in a plane with angles of 120˚ between
the bonds. This makes the double bond more exposed to attack.
• Note Example A p168 Sayes (Molecular formulae, Lewis structures
and Structural formulae for ethene and propene.)
Naming the Alkenes
• a prefix to show the number of carbon atoms (meth-, eth-, prop-, etc)
• the suffix “-ene” to show the functional group (alkene)
• a number in front from butene onwards, to show the location along
the chain of the double bond.
e.g. 1-butene CH2=CHCH2CH3
2-butene CH3CH=CHCH3 (See Fig. 11.3)
The chain is numbered from the end nearest the double bond.
• Substituents are shown first, alphabetically, and numbered according
to their position on the parent chain. The parent chain is the longest
chain containing the double bond.
e.g. 3-ethyl-2-methyl-2-pentene CH3C(CH3)=C(C2H5)CH2CH3
If two substituents are the same, they get a number each, and “di-“ is
prefixed to the substituent.
e.g. 2, 3-dimethyl-3-hexene
3, 3-diethyl-1-hexene
Cis – Trans Isomerism
 Stereoisomers: Structural isomers with different arrangements
in space. e.g. cis-trans isomers and optical isomers)
 Cis-trans (geometric) isomers are forms of an alkene which
occur:
(i) - because carbon-carbon double bonds do not rotate
(Unlike C—C single bond, where rotation can occur)
(ii) - when each carbon on the double bond has two
different groups attached.
 Cis isomers have the same groups on the same side of the
molecule. Trans isomers have the same groups on opposite
sides of the double bond.
H
H
C
C
H
H
H
C
C
H H
C
H
H
C
H
H
H
H
cis-2-butene
MP -138.9 oC, BP 3.7 oC
H
C
H
C
H
trans-2-butene
MP -105.6 oC, BP 0.88 oC
Geometric isomers have different physical properties, but usually the
same chemical properties.
Physical Properties
Alkenes:
 are non-polar molecules and hence are insoluble in water and
other polar solvents
 are good solvents for other non-polar molecules
 show an increase in melting and boiling points as the molecular
mass (or chain length) increases
 have densities < 1 g cm-3, so float on water.
 at room temperature, ethane, propene and butane are gases.
Larger alkenes are liquids.
Chemical Properties
Combustion:
Complete or incomplete as for alkanes.
Alkenes do not burn as cleanly, and often produce a yellow sooty
flame.
Addition Reactions
Since alkenes are unsaturated, atoms can be added to the molecules.
Reactions involving alkenes are usually addition reactions. These
involve breaking the double bond to produce a single carbon-carbon
bond and two new single covalent bonds.
Hydrogenation: Write equation (structural formulae) showing
conditions:
Pt at 25˚C
C2H4
+
H2
C2H6

ethene
ethane
H
H
C
C
H
+
H
Pt 25 oC
H
H
Halogenation:
Bromination:
C2H4
+
ethene
H
Br2
bromine
(orange)

C
+
H
Br
Br
H
C
C
H
H
H
C2H4Br2
1,2-dibromoethane
(colourless)
H
C
H
H
H
Br
H
H
C
C
H
H
Br
Note reaction is immediate, and the bromine decolorises.
(Cf. alkanes slow, and require sunlight)
Chlorination
C2H4
ethene
H
Cl2

chlorine
(y-g)
+
C2H4Cl2
1,2-dichloroethane
colourless
H
C
C
H
+
Cl
Cl
H
H
Hydrohalogenation,
C2H4
+
HBr

Hydrogen bromide
H
C
H
Cl
C
C
H
H
Br
H
+
H
Br
H
CH3CH2Br
bromoethane
H
C
Cl
H
H
C
C
H
H
H
Hydration
A temperature of 600 K and a pressure of 60 atm. in the presence of a
phosphoric acid catalyst. (H3PO4)
H
H
C
H
ethane
C
+
H
H
O
H
Pt 600K H3PO4
H
H
H
C
C
H
H
O
H
ethanol
Note extension of these reactions to any alkene.
e.g. cyclohexene
+
bromine → 1,2-dibromocyclohexane
H
H
H
C
H
H
H
C
H
C
H
H
C
C
C
H
Br
+
Br
C
Br
C
C
H
H
H
H H
H
Br
C
C
C
H
H
H H
Addition of H2O and HCl to unsymmetrical alkenes
Unsymmetrical Addends
These molecules which are added to an alkene molecule split into two
different parts.
e.g. HCl, (H and Cl), HBr, H2O (H and OH) are unsymmetrical
e.g. H2, Cl2, Br2 are symmetrical
Unsymmetrical Alkenes
The carbon at one end of the double bond has different groups
attached, compared with the carbon at the other end.
e.g. 1-butene is unsymmetrical
H
H
C
H
C
H
C
H
C
H
H
H
e.g. 2-butene is symmetrical
H
H
C
H
C
H
H
C
H
C
H
H
Markovnikov’s Rule determines the main or major product of
addition reactions involving unsymmetrical addends to unsymmetrical
alkenes.
The hydrogen atom from HCl, HBr and H2O will add to the carbon
atom with the most hydrogen atoms. (The carbon atom rich in
hydrogen gets richer – or ‘The rich get richer’)
e.g. Addition of HCl to 1–butene
H
H
C
H
C
H
C
H
C
H
H
+
Cl
H
H
H
H
H
H
C
C
C
C
H
H
H
H
Cl
H
major product (90%)
H
H
H
H
H
C
C
C
C
H
H
H
Cl
minor product(10%)
Exercise:
Write equations for the addition of
a) HBr to 1-propene
b) H2O to 1-butene
Oxidation
Oxidation means forming new C – O bonds.
Alkenes are oxidized by cold dilute potassium permanganate
CH2CH2

CH2OHCH2OH
ethene
1,2-dihydroxyethane
H
H
C
H
C
H
+
H
[O]
H
H
C
C
O
O
H
H
H
The permanganate is reduced to manganese dioxide, a brown
precipitate
MnO4- (aq)
 MnO2(ppt)
Permanganate  manganese dioxide
Purple
brown precipitate
H
2 Methods of distinguishing between alkanes and alkenes
a) Add bromine water. (equations above)
- The alkene decolorises immediately.
- The alkane slowly & only on heating or exposure to U.V. light
b) Add dilute permanganate
- The alkene reduces the purple permanganate to a brown
precipitate, MnO2
- The alkane has no effect on permanganate.
Polymerisation of alkenes
When many small molecules unite, a giant molecule called a polymer
is produced. These polymers are the basis of the plastics, synthetic
textiles, and other industries.
Polythene
Polymerisation is initiated when a catalyst radical attacks the ethene
double bond, taking one of its electrons, to produce a carbon radical.
A radical is a molecule with a single unpaired electron. Radicals are
very reactive species.
RO. + CH2=CH2  RO—CH2CH2.
This leaves an odd electron on one of the carbon atoms which then
attacks another ethylene molecule in a similar fashion to propagate
(continue) the chain reaction.
RO—CH2CH2. + CH2=CH2  RO—CH2CH2—CH2CH2.
This process continues until the reaction is terminated.
Ignoring the catalyst, the reaction can be represented as,
nC2H2  (C2H2)n,
where n could be over a thousand.
Monomer
Monomer S.F
Polymer
Polymer
Name
and M.F.
Name
Use
ethene
polyethene
bottles,
H
H
(ethylene)
(polyethylene tubing,
C
C
)
bread
H
H
bags,
sheets
styrene
(ethylbenzene)
H
H
C
polystyrene
C
H
propene
(propylene)
H
H
C
C
H
H
C
H
vinyl chloride
(chloroethene)
H
H
H
C
C
H
Cl
tetrafluoro-ethene
F
F
C
F
polypropene
(polypropylene)
C
F
Polyvinylchloride
(PVC)
‘rigid
foams’
moulded
objs
electrical
insul.
drinking
cups
fibre for
carpets
and
clothing
raincoats,
shower curtain
garden
hose
Polytetrafluoroethene
(Teflon)
bearings
gaskets
non-stick
-frypans