Experiment 10 Preparation of the benzoate of phenol.


Many phenols yield crystalline benzoyl derivatives with benzoyl chloride in the presence of sodium hydroxide (Schötten-Baumann method).



To the phenol (0.5 g) is added 5% sodium hydroxide (10 mL) in a well-corked boiling tube or a small conical flask.

Benzoyl chloride (2 mL, density 1.21 g cm-3) is added in small quantities at a time, and the mixture shaken vigorously with occasional cooling under the tap or in ice water.

After 15 minutes the solid benzoate separates out: the solution should be alkaline at the end of the reaction; if not alkaline, or if oily, add a solid pellet of sodium hydroxide and shake again.

Collect the benzoate, wash thoroughly with cold water, and recrystallise from ethanol (NO FLAMES!). Carry out the tests for phenols and esters (p 57).

In your lab book record and explain the differences in the IR spectra of the starting material and product.

crystalline product
A sample of product prepared in the C10J laboratory

The IR spectrum of phenyl benzoate is available.

Benzoyl chloride is LACHRYMATORY and should be handled with care under a FUME HOOD.

Phenol is not only toxic but will cause severe burns.


To show how TLC may be used to assess the purity of acompound and to determine the components present in amixture.

The term chromatography describes a technique whereby substances may be separated from one another when they are partitioned between two phases, a mobile phase and a stationary phase. Suppose a mixture of two compounds A and B are placed, for example, on a column of silica (the stationary phase) and that B is more strongly adsorbed than A. If a liquid (the mobile phase, in which both compounds are soluble) is now passed over the stationary phase both A and B will tend to be removed from the silica and be carried along in the direction of liquid flow. Since B is more strongly adsorbed than A on the silica, it is less easily removed by the liquid. If the latter is collected in fractions, it will be observed that the first fraction will contain compound A only and the latter fractions will contain compound B. The original mixture is thus separated into its individual components.

The above separation technique was first applied to the separation of coloured compounds, e.g. the separation of pigments in plant material, but it is now widely used for both coloured and non-coloured materials. The following types of chromatographic separation are routinely employed in chemical laboratories:

1.        Column
2.        Paper
3.        Thin layer
4.        Gas-solid and gas-liquid chromatography.

In thin layer chromatography, the stationary phase (e.g. silica, alumina, cellulose) is deposited as a thin layer (0.1 - 2 mm thick) on a flat supporting surface, normally a piece of glass of suitable dimensions (e.g. 5 cm x 20 cm x 0.5 cm). The adsorbent is generally held in place with a binding agent such as starch or plaster of Paris. The mixture to be separated is first dissolved in a suitable solvent then applied (by means of capillary) as a small spot on the stationary phase a short distance from one end. The plate is then placed vertically in a developing chamber containing a small amount of a suitable solvent which serves as the mobile phase. The latter should be sufficient to cover the lower edge of the plate but the liquid surface must be below the applied spot. The chamber is closed and the solvent is allowed to ascend the layer by capillary action until it is a short distance from the upper edge of the plate. The latter is then removed from the chamber and the height of the solvent front noted. If the experimental conditions are carefully selected, the components in the mixture will be resolved as separate spots. If the components are coloured compounds they may be seen directly, or if colourless, they may be made visible by exposure to iodine vapour or by viewing the plate under ultraviolet light (if the layer contains a fluorescent indicator).

The behaviour of a particular component in a specific chromatographic system is frequently described by its Rf value. This is derived by means of the equation:

Rf = (distance travelled by compound) / (distance travelled by solvent)


Dissolve a microspatula load of the phenol provided in the minimum amount of ethanol. Use a capillary to place a spot (not more than 3 mm in diameter) on the left side of the TLC plate about 0.5 cm from the bottom (Note 1). Allow the spot to dry in air. Repeat with your ester, placing the spot on the right side of the plate. You now have two spots in the same plate. Measure approximately 10 mL of 15 parts toluene to one part acetone (v/v) mixture to use as the developing solvent (the mobile phase) and transfer it to a clean, dry 250 mL beaker. The liquid level should be no higher than 0.5 cm. Place the TLC plate vertically in the beaker and cover it with a watch glass. Allow the solvent to rise within 1 cm of the top edge of the plate, keeping the beaker covered. Remove the plate from the beaker, and allow it to dry in air after marking the position of the solvent front (Note 2). Observe the plate under an ultraviolet lamp and mark with a pencil the position of any visible spots. Record the number of components present in each sample and obtain the Rf value for each component. On your worksheet is an area for you to draw the TLC plate with the spots observed. Hand in the TLC plate with your lab worksheet.

TLC plates
The TLC plate on the left is blank, the plate on the right shows the phenol band on the left and the ester on the right.
The blue in the background is from a piece of tissue paper. Why is it blue?

Note 1:        If the spot applied is too large, it becomes diffuse as it is carried along by the liquid phase and the components may not be resolved satisfactorily.

Note 2:        Discard eluant in waste bottle provided and not down the sink.

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