Download Alkenes

ALKENES
The alkenes are a homologous series of unsaturated hydrocarbons which all have
the general formula CnH2n.
Alkenes contain two hydrogen atoms less than the corresponding alkane and are
therefore described as unsaturated. They contain one carbon-carbon double covalent
bond per molecule.
The first members of this homologous series are:
H
H
C
C
H
CH3
C
H
ethene
CH3
C
H
CH3
H
H
CH3
E-but-2-ene
CH2CH3
C
H
C
H
H
C
H
C
propene
CH3
C
H
C
H
Z-but-2-ene
H
but-1-ene
Naming alkenes (the E/Z system)
You may see alkenes with the words cis or trans at the start of their name. This naming
system is not on the A level syllabus. The Cahn Ingold Prelog (CIP) or E-Z naming
system, which is now preferred, works by assigning priority to each of the groups
bonded to the carbons in the C=C based on atomic mass of the bonded group. If the
two highest priority groups lie on the same side of the molecule it is given the prefix Z(from the German word zusammen meaning together). If the two highest priority groups
are on different sides of the molecule it is given the prefix E- (from the German word
entgegen meaning opposite).
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 1
E-3-methylpent-2-ene
Z-3-methylpent-2-ene
In the above example the two groups on the left hand carbon are H and CH 3. Since a C
high a higher atomic mass than H it is given the higher priority. In the case of the right
hand carbon both atoms attached are C atoms and so we move along the bonds to the
next atom. When we find aonther C atom in the case of the C 2H5 group this is given a
higher priority than the CH3 group.
The arrows indicate the highest priority groups on each C atom of the C=C.
Bonding
In, for example, ethene C2H4 both carbon atoms form three -bonds (by overlap of their
three sp2 hybrid orbitals): one bond to each of two hydrogen atoms and the third to the
second carbon atom. The remaining electron on each carbon atom is in a 2p-orbital.
The two 2p-orbitals overlap sideways to form a -bond.
H
H
2p
2p
C
C
H
H
H
H
C
H
H
H
C
-orbital
C
C
H
H
Since all six atoms lie in the same plane,
the molecule is described as planar.
120o
The bond angle is 120o
H
For other alkenes, the double bond and the four atoms attached to it are planar.
C
C
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 2
Reduction
Alkenes are reduced to alkanes by catalytic hydrogenation. This involves cis-addition
of hydrogen across the double bond. The conditions used are:
 finely divided nickel catalyst
 temperature: 150oC
On the surface of the catalyst, hydrogen molecules (H2) split into hydrogen atoms (H).
Two hydrogen atoms are then added to the same side of the double bond (cis-addition)
simultaneously.
H
H
C
+
C
H
H
H2
H
H
H
C
C
H
H
H
Vegetable oils (liquids) are esters of glycerol and unsaturated fatty acids (long-chain
carboxylic acids). To convert these oils to margarine, they are catalytically
hydrogenated (hardened) to partially saturate the double bonds. The products are fats
(solids). The greater the degree of saturation, the harder (higher m.p.) the fat will be. By
adjusting the extent of satuartion, hard or soft margarines can be produced.
Electrophilic Addition
The -orbital in alkenes is a centre of high electron density which is readily polarisable.
This means that alkenes will be reactive towards reagents which are electron deficient;
these are known as electrophiles.
ELECTROPHILES are ELECTRON PAIR ACCEPTORS. They are particles which
have centres of low electron density and which are attacked by centres of high
electron density in other molecules or ions.
Common electrophiles are: H+, NO2+, >C=O, R+, RCO+, RCOCl
The principal reaction of alkenes is electrophilic addition. This can be represented in
general terms by the equation:
C
C
+
X
Y
C
C
X
Y
The double covalent bond is replaced by two new single covalent bonds; the product is
saturated.
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 3
1. Reaction with bromine water
When an alkene is shaken with bromine water, the red-brown colour of the bromine
water is discharged to give a colourless solution. This reaction is used as a test for
unsaturation.
H
H
H
H
C
+
C
H
H
Br2
H
ethene
(unsaturated)
C
C
Br
Br
H
1,2-dibromoethane
(saturated)
Under these conditions, alkanes do not react with bromine, so the reaction can be used
to distinguish alkenes from alkanes.
Mechanism:
The Br2 molecule is polarised by close approach to the alkene -bond. The positive end
of the dipole is then attacked.
H2C=CH2 +

-
Br
Br
The carbocation which forms is then attacked by the nucleophile, Br-.
:Br-
Br
+
H2C-CH2
The final product is 1,2-dibromoethane
H2C-CH2
Br
Br
NUCLEOPHILES are ELECTRON PAIR DONORS. They are particles which have
centres of high electron density and which attack centres of low electron
density in other molecules or ions.
Common nucleophiles are: CN-, H-, Br-, OH-, NH3 , OR-, HSO4-, H2O, RNH2
2. Reaction with hydrogen bromide
Alkenes react with hydrogen bromide both in concentrated aqueous solution and in the
gas phase. The product is the corresponding bromoalkane.
H
H
C
+
C
H
HBr
H
ethene
(unsaturated)
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 4
H
H
H
C
C
H
Br
bromoethane
(saturated)
H
Mechanism:
The mechanism is similar to that of bromination. Owing to the difference in
electronegativity of hydrogen and bromine, HBr in the gas phase has a permanent
dipole. Hydrogen, the positive end of the dipole, is attacked by the -orbital on the
alkene. In aqueous solution, HBr dissociates into ions. The H+(aq) ions are electrophiles
and are similarly attacked by the -orbital on the alkene.
: Br
H2C=CH2
+
H
-
Br
H2C-CH2
+
H2C-CH2
Br
H
bromoethane
H
3. Reaction with sulphuric acid
When alkenes are passed into cold, concentrated sulphuric acid, they react to form the
corresponding alkyl hydrogensulphate. After adding water and warming, hydrolysis of
the alkyl hydrogensulphate takes place to produce an alcohol.
H
H
C
+
C
H
H
H2SO4
H
ethene
(unsaturated)
H
H
H
C
C
H
OSO3H
H
H
H
C
C
H
OSO3H
H
ethyl hydrogensulphate
(saturated)
+ H 2O
H
H
H
C
C
H
OH
H
+ H2SO4
ethanol
Since the overall result of this reaction is the addition of water to ethene, it is known as
the hydration of ethene.
: O.SO3H
Mechanism:
H2C=CH2
+
H
-
O,SO3H
+
H2C-CH2
H
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 5
O.SO3H
H2C-CH2
H
ethyl hydrogensulphate
Direct Hydration of Ethene
In industry, the hydration of ethene is carried out by the direct reaction of ethene and
steam. The conditions used are:
 temperature: 300oC
 pressure: 65 atmospheres
 catalyst: phosphoric acid (H3PO4) adsorbed on celite
H
H
C
+
C
H
H
H 2O
H
H
H
C
C
H
OH
H
Addition to Unsymmetrical Alkenes
When an unsymmetrical molecule (e.g. HBr) adds to an unsymmetrical alkene such as
propene, two addition products are possible. In practice, there is one major product, 2bromopropane.

CH3.CH=CH2
CH3.CHBr.CH3
2-bromopropane
X
+ HBr
CH3.CH2.CH2Br
1-bromopropane
The favoured product is the one which is formed via the more stable carbocation
intermediate. The order of stability of carbocations is:
R
+
C
R
>
R
+
C
H
>
H
+
C
H
R
R
R
tertiary
secondary
primary
This is due to the positive inductive effect. Alkyl groups (eg methyl group) have a
tendency to push electron density towards the carbon atom with the positive charge
thus stabilising it. The more alkyl groups there are, the greater the stabilising effect.
+
2-bromopropane is formed via the secondary carbocation:
CH3.CH.CH3
1-bromoproane is formed via the primary carbocation:
CH3.CH2.CH2
+
Since the secondary carbocation is the more stable, 2-bromopropane is formed as the
major product.
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 6
In general, the direction of addition to an unsymmetrical alkene is
given by Markovnikov’s rule:
‘The more positive end of the molecule being added
bonds to the alkene carbon atom which is bonded to the
larger number of hydrogen atoms.’
Markovnikov
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 7
POLYMERS
Polymers are usually divided into two categories:
 Addition polymers
 Condensation polymers
ADDITION POLYMERS
In addition polymerisation, a large number of molecules of a monomer combine to
produce a polymer, but no other product is formed. The polymer has the same empirical
formula as the monomer but has a larger molecular mass, which could be as high as
500,000.
Polymerisation is initiated by a radical or an ion (this depends on the catalyst system
which is used) which adds to a carbon-carbon double bond. For example:
.
X + CH2=CH2
X
.
CH2CH2
Further molecules of monomer then add sequentially to the reactive end, and the chain
grows in length.
The overall reaction for addition polymerisation can be represented as:
H
n
R
C
H
(
C
H
H
R
C
C
H
H
)n
The repeating unit is enclosed within the brackets. n is a large number, which typically
lies in the range 100 to 10,000. The end groups constitute such a small fraction of the
polymer molecule that they are usually omitted.
Under the right conditions, ethene molecules will add to each other to form a giant
molecule (a polymer) by the process of addition polymerisation.
The backbone of a polyalkene molecule is made up of singly-bonded carbon atoms.
The molecule is saturated and is chemically unreactive. These polymers are nonbiodegradable. The attractive forces between polymer chains are weak. This means
that, on heating to relatively low temperatures, they soften and become mouldable:
these are thermosoftening plastics. They can be recycled.
The properties of polymers can be altered by adding plasticisers which push the
polymer chains further apart thus weakening the intermolecular forces. This means that
chains can slide past each other more easily and the polymer becomes more flexible.
With the possible exception of p.v.c., these plastics are flammable. When heated, p.v.c.
decomposes releasing HCl; this acts as a self-extinguisher.
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 8
H
H
n
C
C
H
H
ethene
(unsaturated)
H
H
C
C
H
H
n
poly(ethene)
(saturated)
There are two types of poly(ethene): low density poly(ethene) or LDPE and high density
poly(ethene) or HDPE. The manufacturing conditions are:
LDPE
200oC
2000 atmospheres
trace of oxygen
temperature:
pressure:
the polymer has an Mr of ~50,000 and a softening temperature of 120 oC
temperature:
30oC
pressure:
2 atmospheres
catalyst:
(C2H5)3Al / TiCl4
HDPE
the polymer has an Mr of ~300,000 and a softening temperature of 130oC
Other addition polymers include:
CH3
n
H
C
H
H
C
H
propene
H
C
H
Cl
C
H
chloroethene
(pvc)
H
n
C
C
H
H
n
H
H
C
C
H
Cl
n
poly(chloroethene)
H
C
H
poly(propene)
H
n
CH3
C
H
H
H
C
C
H
phenylethene
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 9
n
poly(phenylethene)
(polystyrene)
These polymers are thermosoftening polymers and can be recycled.
Examples of addition polymers are:
Monomer
Polymer
Use
ethene, CH2=CH2
poly(ethene)
bags, buckets
propene, CH3CH=CH2
poly(propene)
ropes, containers
phenylethene, CH2=CHC6H5
poly(phenylethene)
chloroethene, CH2=CHCl
poly(chloroethene)
(PVC)
window frames,
guttering, pipes
propenenitrile, CH2=CHCN
poly(propenenitrile)
acrylic fibres
tetrafluoroethene, CF2=CF2
poly(tetrafluoroethene)
non-stick surface
‘vinyl’ records,
drainpipes
methyl 2-cyanopropenoate,CH2=C(CN)COOCH3 poly(methyl 2-cyanopropenoate)
TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 10
super glue