Practical guidance12 Arenes – aromatic hydrocarbons

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 reactionAssuming 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).

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 mixtureChemists 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

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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

Practical guidance12 Arenes – aromatic hydrocarbons

main product is a solid, it can be separated in an impure state by filtration.

Purifying the productProducts are generally contaminated with by-products of the main reaction and with some of the reagents used during the preparation.

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 productThe 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 mixtureFiltering under reduced pressure using a Buchner funnel separates the impure product from the aqueous reagents (Figure 2).

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Practical guidance12 Arenes – aromatic hydrocarbons

Purifying the productThe 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.

Then, the solid can be recrystallised from the minimum volume of hot solvent, in this case, ethanol (Figure 3).

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).

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Figure 2 ►Separating an impure solid product by suction filtration.

Figure 4 ►Drying a solid in a desiccator.

Figure 3 ►Purifying a solid product by recrystallisation.

Practical guidance12 Arenes – aromatic hydrocarbons

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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Practical guidance12 Arenes – aromatic hydrocarbons

Assessing your practical skills

Assessing your practical skills

Preparing for practical workYou 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 preparationYou 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 conclusionsGenerally, the skills of analysis and drawing conclusions do not apply in carrying out an organic preparation.

EvaluationPart of your assessment will involve an evaluation of the outcomes of your preparation – the appearance of your product,

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Figure 5 ►Two methods of measuring the melting temperature of a solid.

Practical guidance12 Arenes – aromatic hydrocarbons

the yield and the accuracy of its melting temperature or boiling temperature.

Appearance of the productThe 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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Practical guidance12 Arenes – aromatic hydrocarbons

Calculating yieldsMeasuring 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 temperaturesPure 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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