Three isomeric alcohols

Learners can explore alcohols with the C₄H₁₀O, in this explorative practical.

Equipment

Materials per group 

• Samples of butan-2-ol, 2-methylpropan-2-ol, and 2-methylpropan-1-ol, 2 cm³, labelled as different unknowns.
• Iodine solution (10% I2 in KI(aq)) [dissolve 10 g iodine and 20 g of potassium iodide in deionised water and make up to 100 cm³] 
• Sodium hydroxide solution,  2 mol dm^–3
• Dilute sulfruric acid, 1 mol dm^–3 
• Potassium dichromate(VI) solution, 0.1 mol dm^–3 [dissolve 2.9 g of potassium dichromate(VI) in deionised water and make up to 100 cm³].

These reagents are used to perform the iodoform test and to carry out the oxidation of the alcohols.

Equipment per group 

• Test tubes
• Test tube rack
• Boiling tubes
• Beaker, 250 cm³ 
• Bunsen burner
• Tripod
• Gauze and bench mat
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here. 
• Keep away from sources of ignition.
• Wear eye protection. 
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete. 
• Students must get their methods checked before they start any practical work. The use of fume cupboards is encouraged.
• Any dichromate that has not gone green should be reduced (probably easiest to use one of the alcohols that works) then test tube contents can be washed to waste. 
• Butanols are flammable, eye and respiratory irritants, harmful if swallowed and can cause skin damage. See CLEAPSS Hazcard HC084b
• For more information on iodine solutions, see CLEAPSS Hazcard HC054
• Sodium hydroxide is corrosive to skin and eyes, see CLEAPSS Hazcard HC091a
• Dilute sulfuric acid in an irritant to skin and eyes, see CLEAPSS Hazcard HC098a
• Potassium dichromate solution can be a carcinogen, mutagen, reproductive toxin, skin/respiratory sensitiser, is a skin irritant, harmful if inhaled and hazardous to the aquatic environment. Always wear gloves and goggles. See CLEAPSS Hazcard HC078c.

Commentary

This is an exercise in traditional organic chemistry. A structured approach would be to ask the students to work through the following questions and activities: 

1. Write out the structural formulae of the three possible alcohols. 
2. Classify the alcohols as primary, secondary or tertiary.
3. Examine the structures and predict which compounds will undergo reactions such as the iodoform reaction or oxidation reactions. There are, of course, other possible reactions. In the case of the two mentioned above, you could then ask the students which isomer undergoes: (i) both the oxidation and the iodoform reaction; (ii) the oxidation, but not the iodoform reaction; and (iii) neither reaction.
4. Carry out the reactions, entering your observations in the following table.
5. From your observations in give the identity of A, B and C.

Procedure

The following procedures were found to work particularly well in trialling. 

Iodomethane reaction 

Place six drops of an alcohol in a test-tube. Add 1 cm³ of iodine in potassium iodide solution followed by sodium hydroxide solution drop by drop until the brown colour of the iodine just disappears (about 2 cm³). The test is positive if a yellow precipitate (triiodomethane) is produced. 

Oxidation reaction 

Place dilute sulphuric acid in a boiling tube to a depth of 1 cm. Add three drops of potassium dichromate(VI) solution and then five drops of alcohol. Warm the mixture gently and note if the orange colour of the dichromate(VI) is replaced by the green colour. If this occurs, then the alcohol has been oxidised.

Extension

This problem is capable of extension in a variety of ways. During trialling, some institutions used different functional group isomers – e.g. C₃H₆O₂ ethyl methanoate, methyl ethanoate, propanoic acid; or the three isomers of C₄H₉Cl.

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.