The properties of alcohols

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Lesson organisation

This experiment can be done completely by advanced students if the use of sodium is closely supervised. With intermediate students, the sodium reaction and possibly the acid dichromate reaction should be done by the teacher. The experiments will take about 45 minutes.

Apparatus and chemicals

Eye protection: goggles

Each group of students will need:

Test tubes, 2
Boiling tubes, 2
Beakers (100 cm³), 2 (see note 2)
Tin lid
Wooden splint
Heat resistant mat
Bunsen burner
Boiling tube holder
Dropping pipette (for water)
Universal indicator paper (full range, pH 1-14)

Forceps for sodium
Filter paper for sodium

Ethanol (Highly flammable) or Industrial denatured alcohol, IDA (Highly flammable, Harmful)
Propan-1-ol (Irritant, Highly flammable)
Potassium dichromate(VI) solution, 0.1 mol dm^–3 (Toxic at this concentration)
Sulfuric acid, 1 mol dm^–3 (Irritant at this concentration)
Sodium (Highly flammable, Corrosive), small pieces (see note 1)

Students will also need acess to

Universal indicator pH colour chart (full range, pH 1-14)
Anti-bumping granules

Technical notes

Ethanol (Highly flammable) or Industrial denatured alcohol, IDA (Highly flammable, Harmful) Refer to CLEAPSS Hazcard 40A
Propan-1-ol (Irritant, Highly flammable) Refer to CLEAPSS Hazcard 84A
Potassium dichromate(VI) solution (Toxic at concentration used) Refer to CLEAPSS Hazcard 78 and CLEAPSS Recipe Card 53
Sulfuric acid (Irritant at concentration used) Refer to CLEAPSS Hazcard 98A and CLEAPSS Recipe Card 69
Sodium (Highly flammable, Corrosive) Refer to CLEAPSS Hazcard 88

1 Prepare the small pieces of sodium in advance of the lesson. Using forceps, remove a large piece of sodium (Highly flammable, Corrosive) from the oil, and place on a tile. Ensure that conditions are dry. Using a scalpel or sharp knife, cut some of the sodium into a few small pieces no larger than 2 x 2 x 2 mm. Place these small pieces in a separate bottle of oil. Return the larger piece to its bottle.

Dispose of any small pieces of unused sodium by dissolving them in ethanol until all trace has disappeared and the fizzing has stopped. Then pour the solution down the sink with plenty of water.

The use of sodium by students must be supervised by the teacher.

2 The beakers must be absolutely dry.

Procedure

HEALTH & SAFETY: Wear goggles throughout.

Carry out each of these tests firstly with ethanol and then propan-1-ol:

a Place a few drops of the alcohol in a test-tube and add an equal number of drops of water. Do the liquids mix fully?

b Place a drop of the alcohol on a piece of full-range indicator paper. Note the pH.

c Place a few drops of alcohol on a tin lid on a heat resistant mat. Ignite the alcohol with a lit splint and observe the flame.

d Using forceps, take two small pieces of sodium and place them on a piece of filter paper. Dab the pieces of sodium with the filter paper to remove any excess oil.

e Place about 0.5 cm depth of each of the alcohols in a separate, dry, 100 cm³ beakers. To each, add a small piece of sodium (using forceps) and observe the reaction.

f Put 5 cm³ (about 2 cm depth) of dilute sulfuric acid in a boiling tube. Add five drops of potassium dichromate(VI) solution. Now add two drops of alcohol and a few anti-bumping granules and heat the mixture until it just boils. Is there any sign of a reaction? Is there any change of smell that could come from a new compound?

Teaching notes

Both alcohols are fully miscible with water. This is because the –OH groups hydrogen bond with the water. Higher alcohols are less soluble since the hydrocarbon chain starts to break an appreciable number of hydrogen bonds in water.

The pH of both alcohols will show as neutral. (Note that, if indicator solution is used, ethanol at least will give an acid colour. This is because ethanol is the solvent used to prepare the indicator solution, and diluting the dyes puts the mixture out of balance.) The RO^– anion is very unstable in aqueous solution, so virtually none of the reaction ROH + H₂O ↔ RO^– + H₃O⁺ occurs.

Both alcohols will burn with a fairly clean, blue flame.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
C₃H₇OH + 4½O₂ → 3CO₂ + 4H₂O

Both alcohols will fizz with sodium to form hydrogen.
C₂H₅OH + Na → C₂H₅ONa (sodium ethoxide) + ½H₂
C₃H₇OH + Na → C₃H₇ONa (sodium propoxide) + ½H₂

Both alcohols are oxidised to aldehydes, which have a sour but fruity smell.
C₂H₅OH + [O] → CH₃CHO (ethanal) + H₂O
C₃H₇OH + [O] → CH₃CH₂CHO (propanal) + H₂O

These experiments show that alcohols react similarly in all these reactions. They make clear the concept of functional group in organic chemistry. The –OH functional group behaves in the same way whether it is attached to C₂H₅ or C₃H₇. Further oxidation turns primary alcohols into acids, while secondary alcohols are only oxidised to ketones under these conditions. However, tertiary alcohols are not oxidised under these conditions but can be oxidised by stronger oxidising agents, resulting in C-C bond breaking.

Health & Safety checked, February 2008