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Organic Procedures
Specification
d) Describe the techniques used in simple organic preparations such as distillation and heating under reflux
Heating under reflux - This allows reactions to occur slowly, over a long period of time, without any loss of volatile liquid. The solvent evaporates and is condensed and returns to the flask.
Many organic reactions are slow and require prolonged heating
- To achieve this without loss of liquid, reaction mixtures are heated in a flask carrying a vertical condenser.
- This is heating under reflux; the solvent is condensed and returned to the flask, so the mixture can be heated as long as desired.
- To heat the round bottomed flask, either use a water bath, an oil bath or a heated plate mantle. A Bunsen burner isn’t really suitable.
Simple distillation
To separate a volatile solvent from a mixture
- Simple distillation is used where a volatile component has to be separated from a mixture, the other components of the mixture being very much volatile or non-volatile.
- The mixture is heated.
- The fraction that boils is collected within the temperature range of the fraction. (normally 1 or 2 degrees before the boiling temperature)
- The condenser cools the fraction so it distils and is collected in the receiving flask.
Fractional distillation
To separate mixtures of volatile liquids.
- Re-crystallisation - Used to purify a solid material by removing both soluble and insoluble impurities. The choice of solvent is important. The substance must be easily soluble in the boiling solvent and much less soluble at room temperature. This ensures the smallest possible loss of material, although some loss is inevitable with this technique.
Re-crystallisation method
- Dissolve the solid in the minimum amount of boiling solvent. This ensures that the solution is saturated with respect to the main solute but not with respect to the impurities, which are present in much smaller amounts.
- Filter the hot mixture through a preheated filter funnel. This removes insoluble impurities. The hot funnel is necessary to prevent the solute crystallising and blocking the funnel. Filtration under vacuum using a Buchner funnel is often preferred, because it is fast.
- Cool the hot filtrate, either to room temperature or, if necessary, in a bath of iced water. Rapid cooling gives small crystals, slow cooling large ones. The large crystals are often less pure.
- Filter the cold mixture using a Buchner funnel.
- Wash the crystals with a small amount of cold solvent. This removes any impurity remaining on the surface of the crystals. A small amount of cold solvent is used so that the crystals aren’t washed away / don’t dissolve.
- Suck the crystals as dry as possible on the filter.
- Transfer the crystals to a desiccator to dry. Drying between filter paper is sometimes recommended, but it is a very poor method.
Melting point determination
This is used to determine the purity of the re-crystallisation solid. Place small amount of the solid in the sealed end of a capillary tube. Place in the melting point apparatus. A sharp melting point over a small range shows purity, when compared with the set-book value of a higher melting point, that indicates an impure solid.
Organic tests
- Collect 10 cm³ of the samples.
- Test the samples in the following order
- Alkenes – bromine water --> decolourises --> alkene
- Alcohols – Spatula of solid PCl5. Test fumes with damp litmus paper --> litmus red? White fumes near ammonia? --> alcohol
- Halogenoalkane – Add NaOH, ethanol as solvent. Shake and warm for 3 minutes. Cool and add nitric acid + silver nitrate. --> white = chloride; cream = bromide; yellow = iodide. --> Confirm with ammonia
- Alkane, the substance left is the alkane.
Also See
Why not read these other revision notes for Edexcel Unit 3B - Laboratory Techniques?
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