3.

3 Halogenoalkanes
H
Naming Halogenoalkanes H H H
H C H
H H H
Based on original alkane, with a prefix indicating halogen atom: H C C C H
H C C C C H
Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I.
Br H H H Cl H H
Substituents are listed alphabetically
1-bromopropane 2-chloro-2-methylbutane
Classifying halogenoalkanes
Halogenoalkanes can be classified as primary, secondary or tertiary depending
on the number of carbon atoms attached to the C-X functional group.
H
H H H H H H
H C H
H H H
H C C C H H C C C H
H C C C C H

Br H H H Br H H Cl H H

Primary halogenoalkane Secondary halogenoalkane Tertiary halogenoalkane

One carbon attached to the Two carbons attached to the Three carbons attached to the
carbon atom adjoining the carbon atom adjoining the carbon atom adjoining the
halogen halogen halogen

Reactions of Halogenoalkanes Halogenoalkanes undergo either Organic reactions are

substitution or elimination reactions classified by their
mechanisms
1. Nucleophilic substitution reactions
Substitution: swapping a halogen atom for another atom or groups of atoms

Nucleophile: electron pair donator e.g. :OH-, :NH3, CN-

:Nu represents any nucleophile – they
The Mechanism: We draw (or outline) mechanisms to always have a lone pair and act as
show in detail how a reaction proceeds electron pair donators

Nu:-
The nucleophiles
H H The carbon has a small
attack the positive H H positive charge because
carbon atom
of the electronegativity
H C C δ+ X δ- H C C Nu + X- difference between the
carbon and the halogen
H H
H H

We use curly arrows in mechanisms (with

A curly arrow will always start
two line heads) to show the movement of
from a lone pair of electrons or
two electrons
the centre of a bond

The rate of these substitution reactions depends on the strength Bond enthalpy /
of the C-X bond kJmol-1
The weaker the bond, the easier it is to break and the faster the reaction.
C-I 238
C-Br 276
The iodoalkanes are the fastest to substitute and the
fluoroalkanes are the slowest. The strength of the C-F bond is C-Cl 338
such that fluoroalkanes are very unreactive
C-F 484

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 Comparing the rate of hydrolysis reactions

Hydrolysis is defined as the splitting of a molecule ( in this Water is a poor nucleophile but it can
case a halogenoalkane) by a reaction with water react slowly with halogenoalkanes in a
substitution reaction
CH3CH2X + H2O  CH3CH2OH + X- + H+

Aqueous silver nitrate is added to a halogenoalkane. The CH3CH2I + H2O  CH3CH2OH + I- + H+

halide leaving group combines with a silver ion to form a Ag+ (aq) + I-(aq)  AgI (s) - yellow precipitate
silver halide precipitate.
The precipitate only forms when the halide ion has left the The iodoalkane forms a precipitate with
halogenoalkane and so the rate of formation of the precipitate the silver nitrate first as the C-I bond is
can be used to compare the reactivity of the different weakest and so it hydrolyses the quickest
halogenoalkanes.

The quicker the precipitate is formed, the faster the substitution AgI (s) - yellow precipitate
reaction and the more reactive the halogenoalkane. AgBr(s) – cream precipitate forms faster
The rate of these substitution reactions depends on the strength AgCl(s) – white precipitate
of the C-X bond . The weaker the bond, the easier it is to break
and the faster the reaction.

Nucleophilic substitution with aqueous hydroxide ions

Change in functional group: halogenoalkane  H H H
H H H
alcohol
Reagent: potassium (or sodium) hydroxide
H C C C Br + KOH  H C C C OH + KBr
Conditions: In aqueous solution; Heat under reflux H H H H H H
Mechanism: Nucleophilic Substitution
1-bromopropane propan-1-ol
Type of reagent: Nucleophile, OH-

H H The aqueous conditions needed

δ+ δ- is an important point. If the
-
H3C C Br H3C C OH + :Br solvent is changed to ethanol
- an elimination reaction occurs.
HO:
H H

Alternative mechanism for tertiary halogenoalkanes You don’t need to learn this but
there have been application of
Tertiary halogenoalkanes undergo nucleophilic substitution in a different way understanding questions on this

CH3 CH3 CH3 Tertiary halogenoalkanes

+ - undergo this mechanism as the
H3C C Br H3C C :OH H3C C OH tertiary carbocation is stabilised
CH3 CH3 by the electron releasing methyl
CH3
groups around it. (See alkenes
The Br first breaks topic for another example of this).
The hydroxide Also the bulky methyl groups
away from the
nucleophile then prevent the hydroxide ion from
halogenoalkane to
attacks the positive attacking the halogenoalkane in
form a carbocation
carbon the same way as the mechanism
intermediate.
above.

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Nucleophilic substitution with cyanide ions

Change in functional group: halogenoalkane  H H H H H H

nitrile
Reagent: KCN dissolved in ethanol/water mixture H C C C Br + :CN-  H C C C CN + Br-
Conditions: Heating under reflux H H H
H H H
Mechanism: Nucleophilic Substitution
1-bromopropane butanenitrile
Type of reagent: Nucleophile, :CN-

Note: the H H This reaction increases the length of

mechanism is δ+ δ- the carbon chain (which is reflected in
identical to the H3C C Br H3C C CN + :Br - the name) In the above example
above one - butanenitrile includes the C in the
NC:
H H nitrile group

Naming Nitriles

Nitrile groups have to be at the end of a chain. Start

numbering the chain from the C in the CN. Note the naming: butanenitrile and not
butannitrile.
CH3CH2CN : propanenitrile

H3C CH CH2 C N 3-methylbutanenitrile

CH3

Nucleophilic substitution with ammonia

Change in functional group: halogenoalkane 

amine H H H
H H H

Reagent: NH3 dissolved in ethanol NH2 +

H C C C Br + 2NH3  H C C C NH4Br
Conditions: Heating under pressure (in a sealed
tube) H H H
H H H

Mechanism: Nucleophilic substitution propylamine

Type of reagent: Nucleophile, :NH3

Naming amines:
In the above example
H H propylamine, the propyl shows
H
δ+ δ- the 3 C’s of the carbon chain.
CH3 CH2 C Br + :Br -
CH 3CH2 C N H Sometimes it is easier to use the
3HN: IUPAC naming for amines e.g.
H H H Propan-1-amine
:NH3

Further substitution reactions can

occur between the halogenoalkane
H and the amines formed leading to a
lower yield of the amine. Using
CH 3 CH 2 C NH 2 + NH Br
4 excess ammonia helps minimise this.
H

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2. Elimination reaction of halogenoalkanes Elimination: removal of small molecule
(often water) from the organic molecule
Elimination with alcoholic hydroxide ions
H H H H H H
Change in functional group: halogenoalkane 
alkene H C C C H + KOH  H C C C + KBr + H2O
Reagents: Potassium (or sodium) hydroxide H H Br H H
Conditions: In ethanol ; heat
1-bromopropane propene
Mechanism: Elimination
Type of reagent: Base, OH-

Note the importance of H H H H

the solvent to the type of
reaction here. CH3 C C H + Br - + H2O
CH3 C C H
Aqueous: substitution
Br H
Alcoholic: elimination -
:OH

Often a mixture of products from both elimination and substitution occurs

2-methyl -2- H
chlorobutane can give
H H C H
With unsymmetrical secondary 2-methylbut-1-ene and H H H
and tertiary halogenoalkanes H C H 2-methylbut-2-ene
H H H C C C C H
two (or sometimes three)
H C C C C H H
different structural isomers can H H
H
be formed H Cl H H
H C H
H H H

H C C C C H

H H

The structure of the halogenoalkane also has an effect on the

degree to which substitution or elimination occurs in this reaction.
Primary tends towards substitution
Tertiary tends towards elimination

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 Uses of Halogenoalkanes
Chloroalkanes and chlorofluoroalkanes can be used as solvents. Halogenoalkanes have also been
used as refrigerants, pesticides
CH3CCl3 was used as the solvent in dry cleaning.
and aerosol propellants
Many of these uses have now been stopped due to the toxicity of
halogenoalkanes and also their detrimental effect on the atmosphere.

Ozone Chemistry

The naturally occurring ozone (O3) layer in the upper Ozone in the lower atmosphere
atmosphere is beneficial as it filters out much of the sun’s is a pollutant and contributes
harmful UV radiation. towards the formation of smog.

Man-made chlorofluorocarbons (CFC’s) caused a hole to form in the ozone

layer.
Chlorine radicals are formed in the upper atmosphere when energy from
ultra-violet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to CF2Cl2  CF2Cl  + Cl
break.

The chlorine free radical atoms catalyse the

.
Cl + O3  ClO + O2
.
decomposition of ozone, due to these reactions, .
ClO + O3  2O2 + Cl
.
because they are regenerated. (They provide an
alternative route with a lower activation energy) Overall equation
2 O3  3 O2
These reactions contributed to the formation of a
hole in the ozone layer. The regenerated Cl radical means
that one Cl radical could destroy
many thousands of ozone molecules.

Legislation to ban the use of CFCs was supported by HFCs (Hydro fluoro carbons) e.g.
chemists and that they have now developed CH2FCF3 are now used for refrigerators
alternative chlorine-free compounds. and air-conditioners. These are safer
as they do not contain the C-Cl bond.

The C-F bond is stronger than the C-Cl bond and is not affected by UV.

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