Topic 12.3
ALKENES AND EPOXYETHANE
Structure and bonding in alkenes
Addition reactions of alkenes
Epoxyethane
Mill Hill County High School
ALKENES
1. Structure and bonding in alkenes
Alkenes are hydrocarbons containing a carbon-carbon double bond. The atoms around
the carbon-carbon double bond adopt a planar arrangement and the bond angle is
120o.
H
H
C
C
H
H
The presence of the C=C bond gives alkenes a number of chemical properties that are
not seen in alkanes.
i)
Since the alkene contains -bonds, it is possible to break the -bond and form
-bonds with other species without forcing any atoms on the molecule to
break off. As a result alkenes (unlike alkanes) are capable of undergoing
addition reactions. Molecules which contain -bonds and which can hence
undergo addition are said to be unsaturated. Molecules which do not contain
-bonds and which hence cannot undergo addition are said to be saturated.
Alkenes are unsaturated and can hence undergo addition. Addition is the
combination of two or more molecules to form a single molecule. Addition
reactions are generally faster than substitution reactions since only weak bonds are broken, rather than stronger -bonds.
ii)
The -bond in an alkene is an area of high electron density. It can thus attract
electrophiles and undergo heterolytic fission. Heterolytic fission is the
breaking of a covalent bond which results in both electrons going to the
same atom. This is in contrast to alkanes which can only react with free
radicals and undergo homolytic fission. An electrophile is a species which
can accept a pair of electrons from a species with a high electron density.
The fact that alkenes can react by heterolytic mechanisms, however, does not mean
that the -bond will not undergo homolytic fission as well. On the contrary - since the
bond is non-polar it is very likely to undergo homolytic fission.
Alkenes can thus react in two ways:
- by free radical addition
- by electrophilic addition
The ability of alkenes to undergo addition, and their ability to react with electrophiles
as well as free radicals, means that they are much more reactive than alkanes.
2.
Electrophilic addition reactions of alkenes
In the presence of electrophiles, the C=C -bond tends to undergo electrophilic
addition.
Possible electrophiles are hydrogen halides (H-Cl, H-Br and H-I) and halogens
(Br-Br, Cl-Cl and I-I). Alkenes also undergo an electrophilic addition reaction with
H2SO4.
a)
with hydrogen halides
Eg CH2=CH2 + H-Br  CH3CH2Br
The H in the H-X bond has a positive dipole and is attacked by the pair of electrons
on the C=C bond, which undergoes heterolytic fission:
C
C
C
+
H
X
C
+
H
+
X
-
Note that the curly arrow indicates the movement of a pair of electrons.
The halide ion then attacks the carbocation to form a haloalkane:
C
C
+
H
:X
C
-
H
C
X
This reaction is fairly quick and takes place readily at room temperature.
b)
with halogens
Eg CH2=CH2 + Br2  CH2BrCH2Br
The Br-Br molecule is non-polar but in the presence of alkenes the electrons move to
one side of the molecule and it acquires a temporary dipole (in other words a dipole is
induced by the alkene). The +ve Br is then attacked by the alkene:
C
C
C
+
Br
Br
C
C
Br
C
+
Br
Br
+
+
C
: Br
Br
C
Br
A dibromoalkane is formed. The Br2 should be dissolved in water or an organic
solvent, and is decolorised during the reaction. (orange  colourless).
The reaction is fairly quick and takes place readily at room temperature.
If bromine solution is added to an alkene and the mixture shaken, it will thus
decolorise and this is a good test for an alkene.
c)
with H2SO4
eg CH2=CH2 + H2SO4  CH3CH2HSO4
Alkenes will undergo an electrophilic addition reaction with cold concentrated
sulphuric acid:
C
C
C
+
H
O
H
+
..
O-
C
C
H
O
O
O
O
O
C
S
OH
O
S
OH
O
S
OH
If the mixture is then warmed and water added, the H2SO4 group will be replaced by
an OH group and an alcohol will be formed:
C
H
C
O
S
HO
O
O
+
H
H
O
C
C
O H
+
H
O
S
H
O
H
This is known as a hydrolysis reaction. Hydrolysis means using water to break
covalent bonds.
The overall reaction is thus:
C
C
+
H
H
C
C
O H
O
H
This is a two step reaction:
Step 1: cold concentrated H2SO4
Step 2: warm and add H2O
It is a useful way of converting alkenes into alcohols in the laboratory.
O
O
3. Unsymmetrical alkenes
Unsymmetrical alkenes are those in which the two carbon atoms in the double bond
are not attached to the same groups.
Eg propene
Eg but-1-ene
H
H
C
H
H
C
C
H
CH3
Eg 2-methylpropene
H
C
C
CH2CH3
H
Eg 2-methylbut-1-ene
CH3
H
CH3
H
CH3
C
C
H
C
CH2CH3
Alkenes in which both carbon atoms are attached to the same groups are known as
symmetrical alkenes.
Eg ethene
Eg but-2-ene
H
H
C
H
CH3
C
C
H
H
C
CH3
H
If unsymmetrical alkenes react with unsymmetrical eletrophiles such as H-X or
H2SO4, there are two possible products:
Eg propene with hydrogen bromide
Route 1:
H3C
H
C
C
H
H
H
H3C
C
H
Propene
..
Br

C
CH3 H
H
H
C
C
Br
H
H
H
Br
+
2-bromopropane
H
Route 2:
H3C
H
C
CH3 H
C
H
H
C
H
C
H
H
H
..
Br
Br

Propene
CH3 H
+
H
C
C
H
Br
H
1-bromopropane
The two products are not formed in equal quantities. The likelihood of one product
being formed over the other depends on the stability of the carbocation
intermediate.
In route 1, the intermediate is a secondary carbocation, as the carbon holding the
positive charged is attached to two other carbon atoms:
H
H3C
C
H
+
C
H
H
In route 2, the intermediate is a primary carbocation, as the carbon holding the
positive charge is attached to one other carbon atom:
CH3 H
H
C
C
+
H
H
Secondary carbocations are more stable than primary carbocations. Tertiary
carbocations are even more stable than secondary cations. This is because alkyl
groups (like CH3 and C2H5) are electron releasing; that is to say they have a surplus of
electron density which they can release onto adjacent atoms. This electron density
helps spread out the positive charge on the carbocation and makes it more stable. Thus
the more alkyl groups that are attached to the carbon atom holding the positive charge,
the more stable the carbocation.
Since secondary carbocations are more stable than primary carbocations, it follows
that the product of route 1 (2-bromopropane) is a more likely product than the product
of route 2 (1-bromopropane).
Thus 2-bromopropane will be the major product and 1-bromopropane will be the
minor product.
In general, the more stable carbocation will be the one which is more highly
substituted. The more electronegative part of the electrophile will thus always attach
itself to the more highly substituted carbon atom.
This is known as Markownikoff's rule: "The more electronegative part of the
electrophile will usually attach itself to the more highly substituted carbon atom".
The major product of the addition reaction is known as the Markownikoff product.
The minor product of the addition reaction is known as the anti-Markownikoff
product.
Eg but-1-ene + HBr
H
CH3CH2
C
C
H
H
CH3CH2
H
CH3CH2
C
+
C
H
H
H
H
H
C
C
H
H
H
Br
CH3CH2
The minor product is 1-bromobutane
Br
H
H
C
CH3CH2
H
H
C
H
+
H
H
..
Br
primary carbocation
The major product is 2-bromobutane.
C
major product
secondary carbocation
CH3CH2
C
H
..
Br
Br
H
H
C
C
H
Br
H
minor product
Eg 2-methylbut-2-ene + H2SO4, then warm and dilute
H
CH3
C
C
CH3
C
+
CH3
CH3
H
O
H
O
O
C
tertiary carbocation
CH3
CH3
C
+
H
H
O
H
O
S
C
CH3
H
O
CH3
O
secondary carbocation
C
C
OH
CH3
H
CH3
C
C
minor product
The minor product is 3-methylbutan-2-ol
Symmetrical alkenes only give one product when elecrophiles are added.
Unsymmetrical alkenes only give one product if the electrophile is symmetrical
(Eg propene Br2).
Thus two products are only obtained when both the alkene and the electrophile are
unsymmetrical. In such cases the identity of the major and minor products can be
predicted by Markownikoff's rule.
Other addition reactions of alkenes
a)
addition of H2 (hydrogenation)
When alkenes are mixed with hydrogen gas over a nickel catalyst at 150oC, an
addition reaction takes place across the double bond in a reaction known as
hydrogenation:
C
C
+
H
eg CH2=CH2 + H2  CH3CH3
H
H
OH CH3
The major product is 2-methylbutan-2-ol
4.
H
major product
CH3
C
CH3
CH3
H
CH3
O
H
CH3
C
CH3
S
CH3 H
H
C
C
H
H
Although it is not useful to convert small alkenes into alkanes, it is useful to convert
very large alkenes (or polyunsaturates) such as those found in oils into very large
alkanes (or polysaturates) such as those found in margarine. Although polyunsaturates
are better for you, they are usually liquids. The polysaturates are usually solids which
makes them more useful in the kitchen. The manufacture of margarine from vegetable
oil is an example of the hydrogenation of an alkene.
b)
addition of steam (hydration)
When alkenes are treated with steam at 300 oC, a pressure of 60 atmospheres and a
phosphoric acid (H3PO4) catalyst, the H2O is added across the double bond and an
alcohol is formed in a reaction known as hydration:
C
+
C
H
H
O
C
C
OH
H
Eg CH2=CH2 + H2O  CH3CH2OH
This is a common industrial method for the production of pure ethanol.
c)
addition polymerisation
Alkenes can be made to join together in the presence of high pressure and a suitable
catalyst. This is known as addition polmerisation.
n
C
C
C
C
n
The product of this addition process is a very long hydrocarbon chain. This is known
as a polymer. Since it is a product of an addition reaction (unlike some other
polymers) it is known as an addition polymer. Since it is made from an alkene it is
known as a polyalkene.
Addition polymers can be made from any alkene:
H
n
H
H
C
C
H
H
H
C
C
H
H
Eg ethene
poly(ethene)
n
H
CH3
CH3
H
n
C
C
C
C
H
H
H
H
Eg propene
n
poly(propene)
H
C2H5
C
C
H
H
C2H5
H
n
C
C
H
H
Eg but-1-ene
n
poly(but-1-ene)
H
CH3
C
C
CH3
H
n
C
C
H
CH3
CH3 H
Eg but-2-ene
n
poly(but-2-ene)
CH3 H
CH3
n
H
C
CH3
C
C
C
H
Eg 2-methylpropene
CH3 H
n
poly(2-methylpropene)
The most favourable conditions for the polymerisation of alkenes can be deduced
from Le Chatelier's principle, if two points are noted:
- the reaction involves breaking -bonds only, and many -bonds are made. The
reaction is thus exothermic.
- The reaction involves a reduction in the total number of moles:
Since the reaction is exothermic, the best yield is obtained at low temperature. Since
the number of gas moles decreases, the best yield is obtained at high pressure.
4. Summary of addition reactions of alkenes
a) Alkene  polyalkene
Conditions: low T, high p.
Equation:
n
C
C
C
C
n
Type of reaction: addition polymerisation
b) Alkene  halogenoalkane
Reagent: HX(g)
Conditions: room T
Equation:
C
C
+
H
Br
C
C
Br
H
Mechanism: electrophilic addition
c) Alkene  dihalogenoalkane
Reagent: X2 in water or in an organic solvent
Conditions: room T
Equation:
C
+
C
Br
Br
C
C
Br
Br
Mechanism: electrophilic addition
d) Alkene  alkylhydrogensulphate
Reagent: concentrated sulphuric acid
Conditions: cold
Equation:
C
C
+
OSO3H
H
C
C
H
OSO3H
C
C
H
H
Mechanism: electrophilic addition
e) Alkene  alkane
Reagent: hydrogen
Conditions: 150 oC, Ni catalyst
Equation:
C
C
H
+
H
Type of reaction: hydrogenation
f) Alkene  alcohol
Reagent: steam
Conditions: 300 oC, 60 atm, H3PO4 catalyst
Equation:
C
C
+
H
H
Type of reaction: hydration
O
C
C
OH
H
EPOXYETHANE
1. Manufacture of epoxyethane
Ethene is a flammable hydrocarbon and burns with a smoky flame to give carbon
dioxide and water. If ethene is reacted with a limited amount of oxygen under special
conditions, however, it can be partially oxidised to form a compound called
epoxyethane:
O
H
H
H
H
+ 1/2 O O
C
C
C
C
H
H
H
H
The conditions required are 250 – 400 oC and a silver catalyst.
This reaction is exothermic and care must be taken to keep the temperature low, as
epoxyethane is explosive at higher temperatures.
2. Structure of epoxyethane
Epoxyethane is a cyclic compound with a three-membered ring. When a carbon atom
forms four single bonds, the most stable arrangement is a tetrahedral arrangement
with a bond angle of 109o. The angle in the epoxyethane ring is only 60o. This is
significantly less than the desired angle of 109o, and this results in a large repulsive
force between the bonds. This is known as ring strain:
O
C
C
C
approx 60o
approx 109o
As a result, the epoxyethane molecule is highly reactive, and most of its reactions
result in the break-up of the strained ring structure. Due to the partial positive charge
on the carbon atoms, the molecule is particularly susceptible to attack by
nucleophiles:
O
H
C
H
C
H
H
Nu:
3. Reaction of epoxyethane with water (hydrolysis)
Water is a nucleophile and will attack the epoxyethane ring to form a diol:
O
H
C
H
+
C
H
H
H
H
HO
H
H
C
C
H
H
OH
O
The conditions used are: 60 oC and a sulphuric acid catalyst, or 200 oC and 14
atmospheres.
This reaction is an example of hydrolysis. Hydrolysis means “using water to break
covalent bonds”. The diol produced in this reaction is known as ethane-1,2-diol.
Ethane-1,2-diol is a very useful compound. It has two important uses:
i)
antifreeze:
ethane-1,2-diol is an important component of antifreeze. It freezes at -12 oC and
mixes completely with water.
ii)
making polyesters:
ethan-1,2-diol is an important raw material for the manufacture of polyesters.
Polyesters are important synthetic fibres which are widely used to make clothes.