12 Arenes – aromatic hydrocarbons
Practical guidance
Preparation of an organic solid
Principles
Inorganic reactions between ionic compounds in aqueous solution are usually fast. In
contrast, the reactions involving organic compounds are generally slow because they
involve bond breaking in covalently bonded molecules. This means that, in many
cases, the reactants must be mixed and heated together for some time.
Stages in the preparation of an organic compound
Carrying out the reaction
Assuming that the amounts of the reactants have been calculated, the conditions of
the reaction are known and a risk analysis has been made, the first stage in preparing
an organic compound is to carry out the reaction.
The reactants are measured out and mixed in a suitable container. Cooling may be
necessary during the mixing of some chemicals. Then, it is often necessary to heat the
reaction mixture for some time while preventing loss of chemicals with the aid of a
reflux condenser (Figure 1).
Figure 1 ►
Heating in a flask with a
reflux condenser
prevents vapour
escaping while the
reaction takes place.
Vapours from the
boiling reaction mixture
condense and flow back
(re-flux) into the flask.
You should choose the safest available method when heating organic chemicals. This
may mean electrical heating or heating with a water bath if the chemicals are highly
flammable and a naked flame would be dangerous.
Separating the product from the reaction mixture
Chemists sometimes talk about ‘working up’ the reacted mixture to describe the steps
involved in separating the impure 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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12 Arenes – aromatic hydrocarbons
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
are used to obtain a pure product.
Impure solids can be purified by recrystallisation. (See later.)
Checking the identity, purity and yield of the product
The final distillation of a liquid provides clues to its identity and purity. During the
distillation, the main fraction should distil off over a narrow temperature range at the
boiling temperature of the liquid.
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.
Infrared spectroscopy is a quick and convenient way to check the identity and purity
of any organic product (solid or liquid). The spectrum of the product can be checked
against a definitive spectrum in a database. (A helpful website for spectra is
www.le.ac.uk/spectraschool/ )
Preparing a solid product
The preparation of methyl 3-nitrobenzoate by nitration of methyl benzoate in Topic
12 illustrates the methods which can be used to make and purify organic solids.
Mixing the reagents and carrying out the reaction
A measured amount of methyl benzoate is dissolved in concentrated sulfuric acid and
then reacted with concentrated nitric acid between 5 and 15 C. After 15 minutes, the
reaction mixture is poured onto crushed ice and crystals of methyl 3-nitrobenzoate
form.
Separating the product from the reaction mixture
Filtering under reduced pressure using a Buchner funnel separates the impure product
from the aqueous reagents (Figure 2).
Figure 2 ►
Separating an impure
solid product by
suction filtration.
Purifying the product
The first stage in purifying the impure solid is to run a little cold water through the
product on the filter paper. This washes away excess reagents.
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© G. Hill and A. Hunt 2009 Edexcel Chemistry for A2
12 Arenes – aromatic hydrocarbons
Practical guidance
Then, the solid can be recrystallised from the minimum volume of hot solvent, in this
case, ethanol (Figure 3).
Figure 3 ►
Purifying a solid
product by
recrystallisation.
Final purification, identification and determination of the yield
The purified product is dried in air at room temperature. Drying can be more
complete with the help of a desiccator. This is a sealed container with a drying agent
in its base (Figure 4).
Figure 4 ►
Drying a solid in a
desiccator.
If time is limited, the purified product can be dried using absorbent paper.
Once the pure product is dry, it should be transferred to a weighed sample tube in
order to measure the actual yield.
Measuring the melting temperature of the dry solid helps to confirm its identity and
purity (Figure 5). Pure organic solids melt sharply at a temperature characteristic of
the product.
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© G. Hill and A. Hunt 2009 Edexcel Chemistry for A2
12 Arenes – aromatic hydrocarbons
Practical guidance
Figure 5 ►
Two methods of
measuring the melting
temperature of a solid.
Assessing your practical skills
Assessing your practical skills
Preparing for practical work
You will not be expected to plan an organic preparation or any other experiment as
part of the assessment of your practical skills. You may, however, be asked to identify
possible hazards and carry out a risk assessment.
You might also be asked to write equations for reactions involved in a preparation,
estimate suitable quantities of reactants, calculate theoretical yields of products and
suggest appropriate apparatus to use.
Carrying out the preparation
You are, of course, expected to be proficient in all the practical procedures listed in
the course specification and your teacher will assess your practical competence by
watching you during practical work.
Analysing and drawing conclusions
Generally, the skills of analysis and drawing conclusions do not apply in carrying out
an organic preparation.
Evaluation
Part of your assessment will involve an evaluation of the outcomes of your
preparation – the appearance of your product, the yield and the accuracy of its melting
temperature or boiling temperature.
Appearance of the product
The appearance of your product can give clues to its quality. Pure organic solids are
often white and crystalline. Pure liquids are clear and not cloudy.
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© G. Hill and A. Hunt 2009 Edexcel Chemistry for A2
12 Arenes – aromatic hydrocarbons
Practical guidance
Calculating yields
Measuring the percentage yield allows you to assess the efficiency of your preparation.
The percentage yields in organic preparations can vary greatly. In a single-step
preparation, the yield may vary from 50% to 90% depending on the possibility of side
reactions and the method of purifying the product.
Measuring melting and boiling temperatures
Pure compounds have definite melting temperatures and boiling temperatures. The
values of these for many organic compounds are listed in the Data sheets on the
Dynamic Learning Student website. The values for many other compounds are
tabulated in the Edexcel Data booklet and in other data books.
Watching a solid melt gives clues to its purity. Pure solids melt sharply over a narrow
range of temperatures, no more than 2 C. Impure solids soften and melt over a range
of temperatures.
It is possible to estimate the boiling temperature of the main product in a liquid
during fractional distillation, but this method of purification does not give such pure
products as the recrystallisation of solids. Therefore, crystalline derivatives of liquids
are sometimes made so that these can be recrystallised and identified by their melting
temperature.
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