Topic 12.4
HALOALKANES
Structure and bonding in haloalkanes
Nucleophilic substitution
Elimination
Mill Hill County High School
HALOALKANES
1. Structure
Halogenoalkanes are molecules containing a C-X bond, where X = F, Cl, Br or I.
C
X
The molecules are generally saturated, and so cannot undergo addition reactions.
The C-X bond is polar, and the carbon is +ve. Therefore the molecule can react with
nucleophiles.
Thus halogenoalkanes tend to undergo nucleophilic substitution reactions.
The X can combine with an adjacent H atom to form a stable HX molecule. Thus
halogenoalkanes can also undergo elimination reactions.
Although halogenolkanes are polar, there is no hydrogen bonding between them and
as a result they are not generally soluble in water.
Haloalkanes can be divided into three classes: primary, secondary and tertiary.
2. Nucleophilic substitution reactions
The three nucleophiles most commonly used in nucleophilic substitution of
halogenoalkanes are hydroxide ions, OH-, cyanide ions, CN- and ammonia, NH3.
a) with hydroxide ions
Halogenoalkanes react with hydroxide ions when boiled under reflux with aqueous
NaOH or aqueous KOH:
R-X + OH-  R-OH + XThe nucleophile (ie the hydroxide ion) attacks the +ve carbon atom from behind,
forcing the X atom to leave as the halide ion. It is a one-step mechanism:
H
CH3
HO:
H
H
C
X
CH3
C
OH
+
H
Note that the hydroxide ion is behaving as a nucleophile in this reaction.
X
-
bromoethane  ethanol
Eg
H
CH3
HO:
H
C
Br
CH3
H
C
OH
+
Br
C
OH
+
Cl
-
H
2-chloropropane  propan-2-ol
Eg
H
CH3
HO:
H
C
Cl
CH3
CH3
-
CH3
b) with cyanide ions
Cyanide ions are nucleophiles and react with halogenoalkanes by nucleophilic
substitution to give nitriles. The halogenoalkane should be boiled under reflux with
KCN in aqueous ethanol.
R-X + CN-  R-CN + XThe mechanism is exactly the same as with the hydroxide ion.
H
CH3
NC:
H
C
X
CH3
H
C
CN
:C
N
+
X
-
H
Note that the CN- ion has the following structure:
Thus the lone pair of electrons is on the carbon, not the nitrogen. It is thus the carbon
which attaches itself to the organic molecule.
Eg bromoethane  propanenitrile
H
CH3
NC:
H
H
C
Br
CH3
H
C
CN
+
Br
Eg 2-chloropropane  2-methylpropanenitrile
H
CH3
NC:
H
C
CH3
Cl
CH3
+
CN
C
Cl
-
CH3
The reaction with cyanide ions is significant because it increases the number of
carbon atoms on the chain, so it provides a way of ascending the homologous
series. It is thus very useful in organic synthesis.
c) with ammonia
If a halogenoalkane is heated with ethanolic ammonia in a sealed tube, a primary
amine is formed:
R-X + 2NH3  R-NH2 + NH4X
The mechanism is again nucleophilic substitution:
H
H
H
C
CH3
X
C
CH3
H
+
N
H
H
:NH3
H
X
+
The initial substitution step forms the intermediate R-NH3+ ion. The H is removed by
another ammonia molecule to form the amine:
H
H
C
CH3
: NH3
+
N
H
H
H
C
CH3
H
H
:
N
NH+
4
+
H
H
bromoethane  aminoethane
Eg
H
C
CH3
H
:NH3
H
H
Br
C
CH3
+
N
H
H
H
+
H
:NH3
H
N
C
CH3
H
H
Br
+
+
NH4
2-chloropropane  2-aminopropane
Eg
H
H
H
Cl
C
CH3
CH3
CH3
C
H
:NH3
+
N
H
CH3
H
CH3
:NH3
+
H
N
C
H
CH3
Cl
-
+
+
NH4
3. Elimination of hydrogen halides
If halogenoalkanes are boiled with an ethanolic solution of KOH instead of with an
aqueous solution, they will undergo elimination of an HX molecule to give an alkene:
R1R2CHR3R4CBr + OH-  C2H4 + Br- + H2O
NaOH is not used since it is only sparingly soluble in ethanol. This reaction works
best if distillation apparatus is used since the alkene product is volatile.
The hydrogen is always lost from a carbon atom adjacent to the carbon atom attached
to the halogen (all the hydrogen atoms which could be removed have been circled).
Sometimes this can result in more than one possible product:
H
H
C
C
H
H
H
C
C
Br
H
H
H
H
bromoethane
H
H
H
C
C
ethene
H
H
H
C
C
Cl
C
H
CH3
H
H
H
1-chloropropane
H
H
H
C
C
propene
H
C
C
H
CH3
H
H
H
Cl
H
2-chloropropane
propene
C
H
H
H
H
H
H
C
C
C
C
H
H
C
C
Br
H
C2H5
H
H
H
H
1-bromobutane
but-1-ene
During the above elimination reactions there is only one possible product.
H
H
H
H
H
H
C
H
C
H
H
C
C
Br
H
H
C
C
H
C2H5
2-bromobutane
but-1-ene
In this reaction, losing an H atom on the other side of the Br atom results in two
different products:
H
H
H
H
H
C
C
C
C
H
H
Br
H
2-bromobutane
H
CH3
H
H
or
C
C
C
H
CH3
CH3
cis but-2-ene or
H
C
CH3
trans but-2-ene
The mechanism of this reaction involves the hydroxide ion attacking a hydrogen atom
on the haloalkane:
HO :
R
H
R
C
C
R
R
R
X
R
C
R
C
R
Note that the hydroxide ion is behaving as a base, not a nucleophile
+ X +
H2O
Eg 1-chloropropane  propene
HO :
CH3
H
H
C
C
H
H
H
H
C
X
CH3
+ X +
C
H2O
H
4. Rates of reaction of halogenoalkanes
The rate of substitution or elimination of halogenoalkanes depends on the ease with
which the C-X bond can be broken. This depends on the strength of the C-X bond,
which in turn depends on the length of the bond.
Since the C-F bond is very short, it is very strong and difficult to break. Thus
fluoroalkanes react very slowly.
The C-Cl bond is longer and weaker than the C-F bond, and the C-X bonds become
progressively longer and weaker on descending the group. Thus the C-I bond is the
longest, weakest and easiest to break and thus iodoalkanes react the most quickly.
Thus rates of reactions decrease in the order:
Iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes
As the halogen atom becomes larger, the C-X bond becomes longer, weaker and more
difficult to break and the corresponding halogenoalkanes react more quickly.
5. Summary of reactions of haloalkanes
Haloalkane  alcohol
Reagent: NaOH(aq) or KOH(aq)
Conditions: warm under reflux
Equation: R-X + OH-  R-OH + XMechanism: nucleophilic substitution
Role of hydroxide ion: nucleophile
Haloalkane  nitrile
Reagent: KCN in aqueous ethanol
Conditions: boil under reflux
Equation: R-X + CN-  R-CN + XMechanism: nucleophilic substitution
Haloalkane  Amine
Reagent: ammonia in ethanol in a sealed tube
Conditions: heat
Equation: R-X + 2NH3  R-NH2 + NH4X
Mechanism: nucleophilic substitution
Haloalkane  alkene
Reagent: KOH in ethanol
Conditions: heat
Equation:
R
H
R
C
C
R
R
R
C
X
R
Mechanism: elimination
Role of hydroxide ion: base
R
C
+
R
X +
H2O