is dissolved in a mixture of aqueous Phenol
and ethanoic acid in a throw-away container. Further acidification of this solution initiates methanal , and a pink condensation polymerisation polymer is formed within a few minutes. After washing, the properties of this substance can be investigated. If a mould is used, the experiment can be extended to show the formation of plastic articles made in a mould by condensation polymerisation. thermosetting
This is a teacher demonstration, taking about 5 minutes for one polymerisation experiment. Extensions to show production of moulded articles will take longer. It should be performed in a fume cupboard to avoid exposure to pungent vapours from the reaction mixture.
The teacher will need:
Eye protection: goggles
Disposable nitrile gloves (for handling the polymer)
Access to a fume cupboard visible to the class
Measuring cylinder (100 cm
Beaker (100 cm
3, 250 cm 3), 1 of each
Throw-away containers with secure lids, at least 2 (Note 1)
Object of simple shape for making suitable mould (Note 2)
Bunsen burner (optional)
Quantities are for one demonstration:
Methanal solution (formalin), 37-40%, (TOXIC), 25 cm
Phenol (TOXIC, CORROSIVE), 20 g
Ethanoic acid, pure (glacial acetic acid) (CORROSIVE), 55 cm
Sulfuric acid, concentrated (CORROSIVE), 30 cm
3 (Note 4)
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
Read our standard health & safety guidance
Work in a fume cupboard, and wear goggles and disposable nitrile gloves throughout.
Methanal solution, HCHO(aq), (TOXIC) - see CLEAPSS Hazcard. Methanal solution (also known as formaldehyde solution, or formalin) should be in good condition. Because the vapour is unpleasant, the supply of this solution for this demonstration should be kept in the fume cupboard, in a stoppered bottle.
6H 5OH(s), (TOXIC, CORROSIVE) - see CLEAPSS Hazcard.
Ethanoic acid (glacial acetic acid), CH
3COOH(l), (CORROSIVE) - see CLEAPSS Hazcard.
Concentrated sulfuric acid, H
2SO 4(l), (CORROSIVE) - see CLEAPSS Hazcard.
1 The container used should be transparent, disposable, and preferably should have a secure lid. Used 100 g coffee jars with screw tops are ideal, but any similar size glass jar will do. Disposal: after the demonstration, dispose of the screw top container with the polymer inside as solid waste.
2 A mould for casting copies of an object can be made by pressing the object into plasticine. The object selected should have a simple external shape. Enough plasticine should be provided to press the shape into, forming a sufficiently substantial mould to retain its shape when handled.
3 Before the lesson, weigh out 20 g of phenol into small beaker, from which it may be easily tipped into the disposable container.
4 Before the lesson, add 30 cm 3 of concentrated sulfuric acid to 30 cm 3 of water slowly in a beaker, with constant stirring. Cool the resulting diluted acid to room temperature.
a Pour 25 cm 3 of aqueous methanal (formalin) into the disposable container and add 55 cm 3of glacial ethanoic acid.
b Add 20 g of phenol to the solution and stir to dissolve.
c Add 60 cm 3 of the diluted sulfuric acid to the mixture with continuous stirring. The mixture will turn pale yellow, then suddenly pink and opaque.
d Stir steadily. Within a minute or so a pink solid will gather around the stirring rod. A lot of heat is evolved.
e Pour off the remaining milky liquid and rinse the pink polymer several times with water.
f Show that the polymer is hard by poking the material with a spatula.
g Before passing the material round the class, place the screw top securely on the jar as the product will be contaminated by unreacted starting materials. It may be necessary to break off the stirring rod.
h Make a plasticine mould from the simple shape and line it with aluminium foil.
i Make another phenol-methanal solution as above, but, immediately after adding the sulfuric acid, pour some of the solution into the mould and allow it to polymerise.
j Hold a sample of the washed polymer with tongs and heat in a Bunsen flame. It will char but will not melt showing that it is thermosetting.
For an alternative procedure, see CLEAPSS Hazcard.
The polymer produced is known commercially as Bakelite, the first genuine synthetic polymer (as opposed to modified natural polymers such as Celluloid - modified cellulose). It was developed by Leo Bækeland in the early 1900’s, and it is still used to make articles such as electrical fittings.
The reaction is a condensation polymerisation in which water is eliminated as a hydrogen atom from the benzene ring of each of two neighbouring phenol molecules combines with the oxygen atom from a methanal molecule. The remaining –CH
2– group from the methanal molecule then forms a bridge between two neighbouring phenol molecules. This process, repeated many thousands of times, forms chains of phenol and methanal molecules linked in this way.
Sometimes a second hydrogen atom from the ring of a phenol molecule will also react with a methanal molecule, producing a branch in the chain, and chains may become cross-linked to each other in the same way. Notice that the hydrogen atom of the phenolic O-H group is not directly involved, although it exerts a strong influence on the course of the reaction. Eventually a random 3-dimensional network of cross-linked chains is formed, giving a rigid structure and thus a hard, inflexible material.
The product has considerable crosslinks, including some −CH
2OCH 2− linkages:
Because the tangle of cross-linked chains is almost impossible to separate, the material does not melt on heating although it will eventually break down at high temperature, giving off small molecules such as steam, and leaving a charred mass which is largely carbon.
Health & Safety checked, 2016
This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.
© Nuffield Foundation and the Royal Society of Chemistry
Page last updated January 2017