The properties of alcohols

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 reaction with acidified dichromate should be demonstrated by the teacher. The experiments will take about 45 minutes.

Equipment

Apparatus

• Eye protection: goggles
• Test tubes x2
• Boiling tubes x2
• Beakers (100 cm³) x2 (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

Chemicals

• Ethanol (HIGHLY FLAMMABLE) or Industrial denatured alcohol, IDA (HIGHLY FLAMMABLE, HARMFUL)
• Propan-1-ol (IRRITANT, HIGHLY FLAMMABLE)
• Sodium or potassium  dichromate(VI) solution, 0.1 M (TOXIC)
• Sulfuric acid, 1 M (IRRITANT) (note 3)
• Sodium (HIGHLY FLAMMABLE, CORROSIVE), small pieces (note 1)

Equipment notes

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.
2. Dispose of any small pieces of unused sodium by dissolving them in propan-2-ol until all trace has disappeared and the fizzing has stopped. Then pour the solution down the sink with plenty of water. See CLEAPSS Hazcard HC084a for more detailed disposal information. The teacher must supervise the use of sodium by students. The beakers must be absolutely dry.
3. Remove all sodium from the bench (including bottles and spills) before issuing sulfuric acid. Alternatively, a technician should prepare acidified dichromate solution (correctly labelled) in advance to avoid the need for 1 M sulfuric acid solution to be used by students.

Health, safety and technical notes

• Read our standard health and safety guidance
• Wear goggles throughout. 
• Ethanol (HIGHLY FLAMMABLE) or Industrial denatured alcohol, IDA (HIGHLY FLAMMABLE, HARMFUL), C₂H₅OH(l) – see CLEAPSS Hazcard HC040a.
• Propan-1-ol, C₃H₇OH(l), (IRRITANT, HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC084a. 
• Sodium or potassium dichromate(VI) solution, Na₂Cr₂O₇(aq), (TOXIC) – see CLEAPSS Hazcard HC078c and CLEAPSS Recipe Book.
• Dilute sulfuric acid, H₂SO₄(aq), (IRRITANT) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB070. 
• Sodium, Na(s), (HIGHLY FLAMMABLE, CORROSIVE) – see CLEAPSS Hazcard HC088. 

Procedure

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

1. 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?
2. Place a drop of the alcohol on a piece of full-range indicator paper. Note the pH.
3. 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.
4. 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.
5. 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.
6. 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? Make sure all sodium is removed from the bench (including bottles and spills) before issuing the sulfuric acid, or prepare the acidified dichromate solution (correctly labelled) in advance to avoid the need for 1 M sulfuric acid solution to be used by students.

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.