Experiment 12

Preparation of 4-acetoxybenzoic acid

Experimental Aims The aim of this exercise is to synthesise a phenyl ester (using acid catalysis), recrystallise the product, and determine its purity using melting point data and thin layer chromatography.

Experimental learning objectives At the end of this experiment you should be able to: (i) precipitate an organic compound from a reaction mixture by addition of water; (ii) isolate a solid by filtration; (iii) recrystallise a compound from ethanol-water; (iv) determine melting points; (v) calculate the yield of a product; (vi) perform chemical tests for phenols and esters; (vii) establish the purity of a compound by tlc; (viii) determine the relative polarity of starting material and product using tlc; (ix) calculate Rf values.

Introduction Phenols, unlike amines, cannot be acetylated satisfactorily in aqueous solution: acetylation of phenol proceeds readily with acetic anhydride in the presence of a little concentrated sulfuric acid as catalyst. In this experiment you will synthesise a compound, purify it by recrystallisation, and determine its melting point.

reaction of subst-phenol with acetic anhydride


possible mechanism of reaction of subst-phenol with acetic anhydride

Procedure Place 2 g. of dry 4-hydroxybenzoic acid and 3 g (3 mL) of acetic anhydride (Note 1) in a small conical flask, add 1 drop of concentrated sulfuric acid, and rotate the flask in order to secure thorough mixing. Warm on a water bath to about 50-60ºC, with stirring, for about 15 minutes. Allow the mixture to cool and stir occasionally. Add 30 mL of water, stir well and filter the precipitate at the pump.

Dissolve the solid in 6 mL of hot ethanol (NO FLAMES!) and pour the solution into 15 mL of warm water. If a solid separates at this point, warm the mixture until it dissolves and then allow the clear solution to cool slowly. Beautiful needle-like crystals will separate (be careful not to heat your solution too strongly because some decomposition of your product may occur).

Determine the melting point (Note 2) and calculate the yield of the product.

Perform the following tests on the starting material (appendix 1):
(i) bromine water test;
(ii) ferric chloride test.

Perform the following tests on your product (appendix 1):

(i) hydroxamic acid test;
(ii) bicarbonate test.

Note 1
: Acetic (ethanoic) anhydride is LACHRYMATORY and should be handled with care under a FUME HOOD.

Note 2: The m.p. of this compound is between 150 and 200 ºC

Use of thin layer chromatography (tlc) as an analytical technique

Objective: To show how TLC may be used to assess the purity of a compound and to determine the components present in a mixture.


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 is 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 or polyester 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 few crystals of the starting material (4-hydroxybenzoic acid) provided in the minimum amount of ethanol. Use a capillary to place a small 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 (4-acetoxybenzoic acid), placing the spot on the right side of the plate. You now have two on spots the same plate. Measure approximately 8 mL of the developing solvent (15 parts toluene to one part acetone) 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 to stand in the beaker with the top end resting on the wall of 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.

TLC setup sample TLC plate

Record the number of components present in each sample and obtain the Rf (retention factor) value for each component. Show the TLC plate to your demonstrator. Do not paste it in your lab work sheet.

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.

Buchner funnel setup
Filtering Apparatus
The IR and vibrational modes of 4-hydroxybenzoic acid and 4-acetoxybenzoic acid are shown as an example of an interpreted spectrum using a non-Java interactive JSmol display (HTML5/JavaScript)
Note: The spectrum of the 4-hydroxy species was recorded for the gas phase, not condensed phase.

Load 4-OH,    4-Ac,    Overlay them
grid off    grid on    show peak list

Jmol/JSpecView code conversion to JavaScript by Bob Hanson.

Copyright © 2011-2014 by The Department of Chemistry UWI, Jamaica, all rights reserved.

Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created Oct 2011. Links checked and/or last modified 29th October 2014.
URL http://wwwchem.uwimona.edu.jm/lab_manuals/c1901exp12.html