The properties of alcohols

The experiments will take about 45 minutes.

Learning objectives

1. Make and record experimental observations.
2. Use your results to explain the properties of the alcohols.
3. Understand that members of the same homologous series behave in a similar way.

Scaffolding

Depending on your class, you may consider delivering the sodium reaction and the reaction with acidified dichromate as a teacher demonstration. Learners must be closely supervised if handling sodium. 

Integrated instructions are provided in the lesson slides as an alternative to the full written method. Integrated instructions are designed to reduce cognitive load, read more about improving practical work with integrated instructions in this article.

There is also a challenge question in the lesson slides for learners who would like to extend the learning further.

Technician notes

Read our standard health and safety guidance and carry out a risk assessment before running any live practical.

Safety and hazards

• Wear splash-proof goggles throughout. 
• Learners must be supervised in the use of sodium.
• It is important that all containers of flammable substances are kept stoppered and away from naked flames. Consider using a gas lighter instead of a Bunsen burner to light the alcohol in step 2 of the experiment. In case this is not possible, try reducing the number of Bunsen burners by having clear designated areas for learners to light a splint away from flammable substances. Alternatively, individual stations for different steps of this practical can be set up around the laboratory.
• Chemicals supplied:
  • Ethanol (HIGHLY FLAMMABLE) or Industrial denatured alcohol, IDA (HIGHLY FLAMMABLE, HARMFUL), C₂ H₅ OH(l)
    Refer to CLEAPSS Hazcard HC040a, SSERC or contact your local safety advisory body.
  • Propan-1-ol, C₃ H₇ OH(l), (IRRITANT, HIGHLY FLAMMABLE).
    Refer to CLEAPSS Hazcard HC084a, SSERC or contact your local safety advisory body.. 
  • Acidified potassium dichromate(VI) solution, K₂ Cr₂ O₇ (aq), (TOXIC, CORROSIVE)
    Refer to CLEAPSS Hazcard HC078c  and HC098a, SSERC or contact your local safety advisory body.
  • Sodium, Na(s), (HIGHLY FLAMMABLE, CORROSIVE)
    REfer to CLEAPSS Hazcard HC088, SSERC or contact your local safety advisory body.

Preparation and disposal

Full preparation and disposal instructions can be found in the teacher and technician 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.

Method

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 can lid on a heat resistant mat. Make sure that all the bottles of alcohols are closed and kept far away from the can lid. 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. Place 3 drops of acidified potassium dichromate(VI) solution into two wells of the spotting tile and add 2 drops of ethanol to the first well and 2 drops of propna-1-ol to the second well. Observe the colour change(s) in each well over 5–10 minutes. Do you notice a change of smell that could come from a new compound?

Discussion questions

Follow up questions can be found at the end of the lesson slides. Answers to the questions can be found in the lesson slides and in the teacher and technician notes.

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.
C₂H₅ OH + 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.