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
BOND ENTHALPY (kJ mol-1)
carbon-fluorine 467
carbon-chlorine 340
carbon-bromine 280
carbon-iodine 240
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
concerned molecule.
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
visible.
The faster the precipitate appears, the more reactive the
halogenoalkane.
Primary halogenoalkanes will be used, as they take longer to react
than secondary ...
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Dropping pipettes will also be used, to minimise any risk of contact
with these chemicals when transferring them to the required vessel.
Safety goggles, lab coat and surgical gloves must be worn at all times
during the experiment to reduce the risk of any contact between
chemicals and eyes/skin.
If any chemicals, are ingested wash mouth out and give a glass or two
of water. If vapour is inhaled, supply plenty of fresh air and keep
warm. If liquid is splashed on eye, flood with gently running tap
water for ten minutes, or spilt on skin or clothes wash the affected
area thoroughly and remove contaminated clothing.
In all cases tell the supervising teacher and seek medical attention.
If any chemicals are spilt on the floor/workbench inform the
supervising teacher immediately so that the spill can be dealt with
safely.
Sources
- www.answers.com/topic/nucleophilic-substitution-reaction
- http://www.teachmetuition.co.uk/glossary.htm
- B.Ratcliff, H.Eccles, D.Johnson, J.Nicholson, and J.Raffan (2000)
Chemistry 1, Cambridge University Press
> Chapter 7, Basic concepts in organic chemistry: page 91
> Chapter 12, Halogenoalkanes: pages 137-139
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