Rates of reaction of the halogenoalkanes
scientific knowledge and understanding:
Halogenoalkanes are classified as either primary, secondary or
tertiary. In primary halogenoalkanes the halogen atom is covalently
bonded to a carbon atom which is bonded to one other carbon, to two
other carbons in secondary and three others in tertiary.
Apparatus for a reflux
Increasing the size of the halogen atom in the halogenoalkane decreases
the bond strength, increasing the reactivity of the molecule, as the bond is
easier to break.
BOND ENTHALPY (kJ mol-1)
It can clearly be seen that the bond enthalpy (energy needed to break bond)
of the C-X bond decreases as we move down as we go from C-F to C-I.
Halogenoalkanes are volatile liquids. As they are organic, any reactions
they partake in are usually quite slow and therefore must be heated under
reflux. A reaction carried out under reflux is one in which a condenser is
positioned vertically in the neck of the reaction flask to prevent any of the solvent/reactant liquids from evaporating.
Halogenoalkanes do not mix with water, so must be dissolved in ethanol before
being treated with any aqueous solutions.
Nucleophilic substitution is a reaction in which an electron-donor atom in a
molecule is eliminated, and replaced by another which will form a stronger
covalent bond in the
Primary halogenoalkanes will undergo nucleophilic substitution with
the following mechanism (SN2):
Nu – Nucleophile X – Halogen atom
A nucleophile is an atom/molecule, which has a lone pair of electrons.
It bonds by ‘attacking’ an electron deficient carbon atom and donating
its free pair of electrons. A nucleophile will only displace a halogen
it if it forms a stronger bond with the carbon.
In this investigation I will be examining the rate of reaction of
chloroethane, bromoethane and iodoethane, by treating them with a
solution of silver nitrate in a mixture of ethanol and water, and
timing how long it takes for a silver halide precipitate to become
The faster the precipitate appears, the more reactive the
Primary halogenoalkanes will be used, as they take longer to react
than secondary or tertiary halogenoalkanes, and I want to be able to
keep up with the rate of the experiment.
The main reaction in this experiment will be the nucleophilic
substitution occurring between the halogenoalkane and water in the
R–Hal + H2O ROH + H+ + Hal-
The ethanol in the aqueous solution will dissolve the halogenoalkanes
so they are able to mix with water.
Bonds in the water molecules will then break giving OH and H ions.
The OH ions will behave as a nucleophile because they have a lone
pair of electrons, which when accepted by the carbon, will form a