CHEM 1
Making Haloalkanes
Chloromethane is a haloalkane – it is a colourless gas. Around 2% of
chloromethane is used as a solvent in the manufacture of artificial
rubber, but around 75% is used to make silicones.
Can be manufactured by reacting alkanes with halogens. The only bonds
in alkanes are C-C and C-H bonds with relatively high mean bond
enthalpies. Alkanes do not react easily with many substances because
these strong bonds must be broken. Reactions involving alkanes have
high activation energies.
Bond Breaking
When the covalent bond between two atoms breaks, one electron can be
transferred to each atom. This is called homolytic fission.
X:Y  X˙+ Y˙
The two products are called free radicals. The dots show that each
one has an unpaired electron. The electron is available for bonding,
so free radicals are very reactive. Alkanes can react with halogens
using free radicals
A Substitution Reaction
Methane and chlorine do not react together in the dark, but they do
in UV light.
CH4 + Cl2  CH3Cl + HCl
A hydrogen atom in methane is replaced by a chlorine atom in a free
radical substitution reaction.
Reaction Mechanism
Three main steps in free radical substitution reaction between
methane and chlorine:
Initiation: Existing bonds must be broken for the reaction to
begin. The Cl-Cl bonds in chlorine are weaker than the C-H bonds in
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methane, so the energy in the UV light is more likely to break them.
This is an example of homolytic fission. The chlorine radicals are
very reactive and can react with methane molecules.
Propagation: When a free radical reacts with a molecule, a new
free radical and a new molecule are formed.
Termination: A termination reaction happens when 2 free
radicals react to form a molecule.
Further Substitution
Further substitution reactions can happen to chloromethane, producing
dichloromethane, trichloromethane and finally tetrachloromethane. The
chance of these forming can be decreased if the methane is in excess.
Removing by-products like these increases costs and reduces
efficiency of the process.
Damaging the Ozone Layer
Haloalkanes that contain chlorine and fluorine are called
chlorofluorocarbons (CFC’s).
Ozone is an allotrope of oxygen, formed in the upper atmosphere by
free radicals. UV light from the sun provides the energy needed to
break the O=O bond in oxygen molecules. The oxygen atoms formed are
two unpaired electrons and can react with other oxygen molecules to
form ozone.
The ozone layer is in the stratosphere 15-40km above the Earth’s
surface. Ozone absorbs UV light and so prevents most of it reaching
the ground. When ozone absorbs UV light it disassociates to form an
oxygen molecule and an oxygen atom. Both reactions (creating ozone
and decomposing it) happen constantly so the concentration in the
stratosphere is the same.
UV light provides the energy to break C-Cl bonds in CFC’s when they
reach the stratosphere. The radicals react with ozone and decompose
it, and then oxide radicals react with ozone again and decompose it.
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Reactions like this have decomposed enough ozone to cause a ‘hole’
in the ozone layer. This is a region where the ozone concentration is
much lower than elsewhere in the stratosphere.
Repairing the Ozone Layer
In 1985 the concentration of ozone in the stratosphere decreased far
more than expected. Research eventually ruled out natural causes and
blamed CFC’s (widely used in aerosols and refrigerators).
1978 four countries banned the non-essential use of CFC’s in aerosol
cans.
1987 twenty-four countries signed ‘Montreal Protocol on Substances
that Deplete the Ozone Layer’, introducing strict limits on
production and use of CFC’s.
Since then different substances with the same function but not
damaging to the ozone layer have been produced.
Alcohols from Alkanes
Apart from astatine at the bottom of group 7, all halogens are more
electronegative than carbon, so the carbon-halogen bond in
haloalkanes is polar. The carbon atom is electron-deficient because
the halogen atom withdraws negative charge from it. The presence of
the electron-deficient carbon atom makes the haloalkanes liable to
attack by nucleophiles.
A nucleophile is a species with a lone pair of electrons that is
available to form a co-ordinate bond. Nucleophiles are attracted to
regions of positive charge. Can be:
 Negatively charged ions such as OH- and CN Molecules with a lone pair of electrons, such as H2O and NH3
Nucleophiles like this can attack the electron-deficient carbon atom
in a haloalkane molecule. They can bring about a reaction in which
they replace the halogen atom = nucleophilic substitution reactions.
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Bromoethane reacts with warm dilute sodium hydroxide solution or dilute
potassium hydroxide solution to form ethanol:
-
CH3CH2Br(aq) + OH (aq)  CH3CH2OH(aq) + Br(aq)
The bromine atom has been replaced by OH. Similar reactions happen with
other haloalkanes. These reactions are nucleophilic substitution
-
reactions because they involve a nucleophile, in this case the OH .
The reaction mechanism is shown with the help of curly arrows,
indicating the movement of a pair of elecrons from its tail to its
head. The arrow can show movement of:
 A lone pair of electrons
 The pair of electrons in a covalent bond.
Nitriles and Amines from Haloalkanes
The rate of nucleophilic substitution depends upon the halogen in the
haloalkane. The reactivity of the haloalkanes depends in the strength
of the carbon-halogen bond. The lower its bond enthalpy the weaker it
is and more easily broken.
The carbon-halogen bond is weakest in Iodoalkanes, so these are most
reactive. Reactivity decreases going to Bromoalkanes and then
Chloroalkanes. The carbon-fluorine bond is so strong that the
Fluoroalkanes are not readily attacked by nucleophiles.
Hydrolysis of haloalkanes:
The oxygen atom in water molecule has 2 lone pairs of electrons, so
water can act as a nucleophile. It is weaker than OH- but can still
produce the same products in reactions with haloalkanes. Water breaks
the haloalkane down. The rate of hydrolysis can be measured using a
silver nitrate solution.
As water and haloalkane reacts, halide ions form. These react with
silver ions to form precipitates.
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-
The cyanide ion CN acts as a nucleophile and reacts with haloalkanes
to produce nitriles. The ion is provided by sodium cyanide NaCN, or
potassium cyanide KCN. These need to be dissolved in ethanol and
heated with the haloalkane to work.
Ammonia NH3 acts as a nucleophile as its nitrogen atom has a lone
pair of electrons. Ammonia reacts with haloalkanes to form amines,
with the amine group NH2.
Alkenes from Haloalkanes
When a haloalkane reacts with excess ammonia, the ammonia can act as
a nucleophile and a base. The hydroxide ion can also react as a
nucleophile and a base. When the hydroxide ion acts as a base, an
elimination reaction happens, producing an alkene instead of an
alcohol.
If 2-bromopropane is heated strongly with concentrated sodium or potassium
hydroxide propene is formed:
CH3CH2CH2Br + OH-  CH3CHΞCH2 + H2O + Br
 The hydroxide ion forms a co-ordinate bond with a hydrogen atom.
 The bonding pair of electrons from the carbon-hydrogen bond forms a
second covalent bond between two carbon atoms.
 The carbon-bromide bond breaks, releasing a bromide ion.
All three pairs of electrons move simultaneously and there is no
intermediate species. The hydrogen atom that is attacked is joined to a
carbon atom next to the carbon atom with the carbon-halogen bond. The
hydrogen atom is removed as a hydrogen ion, which I accepted by the
hydroxide ion to form water.
When haloalkanes react with hydroxide ions, both types of reaction
can happen together = concurrent substitution and elimination. The
reaction conditions can be adjusted to favour one type of reaction
over the other:
Type of
Reaction Conditions
Hydroxide ion
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reaction
Substitution
Elimination
Temperature
Low
High
Hydroxide
Dilute
Concentrated
Solvent
Water
Ethanol
acts as a
Nucleophile
base
The favoured type of reaction is also influenced by whether the
haloalkane is primary, secondary or tertiary haloalkane, shortened to
O
O
O
1, 2, 3.
Two products of elimination are available from longer, unsymmetrical
secondary haloalkanes