15 Alcohols and halogenoalkanes
Practical guidance
Organic synthesis
Principles
Inorganic reactions between ionic compounds in solution are often fast. The reactions
used to make organic compounds are generally slow because they involve bond
breaking in covalent molecules. This means that the reactants in a synthesis have to be
mixed and heated together for some time.
Stages in the preparation of an organic compound
Planning
Planning an organic preparation involves choosing a suitable reaction or sequence of
reactions for making the product from available starting chemicals. Once this decision has
been made it is possible to work out appropriate reacting quantities from the equation and
to decide on the conditions for the reaction. At the same time a risk analysis helps to
ensure that there are no avoidable hazards and that the planned procedure minimises risks
(see the Practical guidance: ‘Safety in the lab’ on this website).
Carrying out the reaction
The reactants are measured out and mixed in a suitable apparatus. Cooling may be
necessary while mixing some chemicals. Then it is often necessary to heat the reaction
mixture for some time while preventing loss of chemicals with the help of a reflux
condenser.
Figure 1 
Heating in a flask with a
reflux condenser prevents
vapours escaping while the
reaction is happening.
Vapours from the boiling
reaction mixture flow back
(reflux) into the flask.
Separating the product from the reaction mixture
Chemists sometimes talk about ‘working up’ the reaction mixture to describe the steps
involved in separating the crude product. Distillation may be suitable if the main product
is a liquid. If the main product is a solid it can be separated in an impure state by filtration.
Purifying the product
Products are generally contaminated with by-products of the main reaction and with
some of the reagents used during the preparation.
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© G. Hill and A. Hunt 2008 Edexcel Chemistry for AS
15 Alcohols and halogenoalkanes
Practical guidance
A liquid product can be purified by shaking in a tap funnel with reagents that do not
mix with the product but which can extract the impurities. Then drying and distillation
gives a pure product.
Figure 2 
If the organic product is a
liquid which does not mix
with water it can be partly
purified by shaking with
aqueous reagents in a tap
funnel.
Checking the identity, purity and yield of the product
The final distillation of a liquid provides clues to the identity and purity of the
product. During the distillation the main fraction should distil off over a narrow
temperature range at the boiling temperature of the liquid.
Infrared spectroscopy is a quick and convenient way to check the identity and purity
of any organic product. The spectrum of the product can be checked against a
definitive spectrum in a database (see Topic 17).
Measuring the melting temperature of a solid helps to check whether or not it is pure
and to confirm its identity. Pure solids melt at a sharp temperature. Another useful
technique for checking the purity of a solid is thin-layer chromatography. A pure
compound produces just one spot on the developed chromatogram.
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© G. Hill and A. Hunt 2008 Edexcel Chemistry for AS