Alcohols
10.2 Functional group chemistry (6.5 hours)
2. Alcohols (estimated 1.5 hours)
Pause for thought
The chemistry covered in this part of sub-topic 10.2 is really very simple. It can be summarized by saying that alcohols burn in oxygen to give carbon dioxide and water, primary alcohols are oxidised first to aldehydes then carboxylic acids, secondary alcohols are oxidized to ketones and tertiary alcohols cannot be oxidized whilst still retaining the carbons chain. All alcohols can react with carboxylic acids to form esters. This condensation reaction is also known as esterification. Be careful to stress that although the class of compounds is correctly called alcohols the IB calls the -OH functional group the hydroxyl group, although probably it is more correct to call it the hydroxy group so that, for example, the systematic (IUPAC) name of lactic acid, CH3CH(OH)COOH is 2-hydroxypropanoic acid, not 2-hydroxylpropanoic acid.
In fact alcohols, and ethanol in particular, provide a useful respite from the purely organic chemistry approach. Ethanol has many attributes similar to water – or put another way the simplest 'alcohol' of all is water, H-O-H. It could be worth considering using ethanol (and the other alcohols) to re-enforce the chemistry of some of the other core topics.
For example:
Topic 1. (1.1) Deduction of chemical equations when reactants and products are specified.
Student should be able to deduce a general equation for the combustion of any alcohol CxHyO.
CxHyO + (x + y/4 -½)O2 → xCO2 + y/2H2O
Topic 3. (3.2) Group trends should include the treatment of the reactions of alkali metals with water.
You could ask students to deduce what might happen if a very small piece of sodium metal is placed in ethanol rather than water. From the equation with water they should be able to work out that hydrogen gas will be evolved.
Na(s) + H-O-H(l) → Na+(aq) + OH–(aq) + ½H2(g)
Na(s) + R-O-H(l) → Na+(aq) + OR–(aq) + ½H2(g)
The reaction of alcohols with sodium is not actually on the IB programme but it does help to re-enforce the understanding of the reaction of sodium with water.
Topic 4. (4.3) Prediction of molecular polarity from bond polarity and molecular geometry.
By substituting just one of the hydrogen atoms in water with a –C2H5 group students can easily see that ethanol is polar. They should also be able to deduce that it is not as polar as water as there is only one hydrogen atom with δ+, not two . An interesting demonstration is to put a beaker of water (about 10 cm3) in a household microwave oven for ten seconds or so and record the temperature rise. Ask students what will happen if you repeat it with 10 cm3 of ethanol. They will probably predict the temperature will not rise as much as it is less polar. In fact it will rapidly boil as its specific heat capacity is lower.
Topic 5. (5.1) A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.
Combusting alcohols using a spirit lamp under a calorimeter and then determining the enthalpy change is a classic experiment.
Topic 6. (6.1) Explanation of the effects of temperature on rate of reaction and (6.1.) Investigation of rates of reaction experimentally and evaluation of the results.
The obvious link with Topic 6 is that heat is required to bring about the oxidation of primary and secondary alcohols with an acidified solution of potassium dichromate(VI). The heat, of course increases the energy of the reacting particles so that more will have the necessary activation energy for the reaction to proceed. However if you got them to deduce what will happen when sodium metal is reacted with ethanol (see Topic 1 above) then you could ask them to design or investigate an experiment to measure the rate when different alcohols are used (e.g. CH3OH, C2H5OH, C3H7OH and C4H9OH) to see what happens to the reaction rate when the carbon chain length increases.
Topic 7. (7.1) Deduction of the equilibrium constant expression (Kc) from an equation for a homogeneous reaction.
One of the classic reactions used to illustrate homogeneous equilibrium is the esterification reaction between ethanol and ethanoic acid. You could even bring in a bit of Topic 2 here and ask them how they could determine whether it is the oxygen atom from the alcohol or the carboxylic acid that ends up in the water. The answer of course is to use isotopic labelling with 18O.
Topic 8. (8.1.) Deduction of the conjugate acid or conjugate base in a chemical reaction.
Students know that hydroxide ions are a strong base. This is because hydroxide ions are the conjugate base of water which is a very weak acid (Kw = 1.00 x 10-14). Ethanol is an even weaker acid (Ka = approximately 10-16) so they should be able to deduce that the ethoxide ion, C2H5O–, is an even stronger base than the hydroxide ion, OH–.
Topic 9. (9.1) Deduction of redox reactions using half-equations in acidic or neutral solutions.
Often when writing the alcohol oxidation equations in Topic 10 [O] is used to represent the oxygen from the oxidizing agent. But under topic 9 students could be asked to write the full equation and even in Topic 10 this is still true as it states under 'Applications and skills', "Writing equations for the oxidation reactions of primary and secondary alcohols(using acidified potassium dichromate(VI) or potassium manganate(VII) as oxidizing agents).". So give them the half-equations for the reduction of dichromate(VI) ions and manganate(VII) ions in acidic solution and ask them to deduce the balanced redox equations for the oxidation of ethanol to ethanal and then to ethanoic acid. It is all good practice.
Topic 11. (11.1) Uncertainties and errors in measurement and results.
If you have got this far, you deserve an alcoholic drink. It is bound to increase the uncertainty and error in your measurements.
Learning outcomesAfter studying this sub-topic students should be able to: Understand:
Apply their knowledge to:
| Clarification notesNo specific guidance is given for alcohols. International-mindednessThe misuse of alcohol is a serious problem in many countries and can have a detrimental impact both on their economies and on their social structures. |
Teaching tipsSimple though it sounds, impress upon your students the need to take the oxygen atom in the alcohol into account when they balance the equations for the complete combustion of alcohols. Too often in IB exams they forget to do that. It is worth getting students to burn some alcohols on a watch glass although they probably will already have burned ethanol in a spirit lamp for Topic 5 (or at least set fire to brandy on a Christmas pudding!). The oxidation with acidified dichromate(VI) is also worth doing practically. Either in a test-tube or as the full-scale distillation and reflux to obtain both ethanal and ethanoic acid when ethanol is oxidized. Even with just the test-tube reaction they should observe the colour change from orange to green and get an apple-like smell as the ethanal is formed. Although [O] is often just used when writing organic oxidation equations, ensure that they can deduce (not just simply remember) the full equation. After all they have already come across balancing redox equations in Topic 9 and this is a good place to put it into practice. Rather than just stating that tertiary alcohols are not readily oxidized whilst still maintaining the carbon chain, it is good to get the students to work out why not. You can bring in a bit of social responsibility and say that the oxidation of ethanol with acidified dichromate ions forms the basis of the simple 'blow in the bag' breathalyzer. It also occurs naturally with wine when it is left open to the air and it explains 'wine vinegar'. It is fun to get them to make a couple of esters practically on a small scale. As well as making ethyl ethanoate get them to make methyl salicylate as the smell of 'oil of wintergreen' is very distinctive. | Study guide
Pages 87 & 88 QuestionsFor ten 'quiz' multiple choice questions with the answers explained see MC test: Alcohols. For short-answer questions on alcohols which can be set as an assignment for a test, homework or given for self study together with model answers see Alcohols questions. Vocabulary listprimary
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Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
Other resources
1. Combustion of alcohols. A 'different' demonstration showing four different alcohols (methanol, ethanol, propan-1-ol and butan-1-ol) burning in a 'whoosh' bottle.
2. Oxidation of alcohols by Richard Thornley. A good summary (although some care needed when he calls C=O an aldehyde!).
3. Because the internal assessment focuses so much on data analysis etc some IB students are never taught many good chemistry practical techniques. Distillation can be used to separate the ethanal formed during the oxidation of ethanol. This video takes students through various ways of distilling products using Quickfit apparatus.