1.2 The alcohols

The alcohols

An alcohol is any organic compound where there is a hydroxyl functional group (-OH) bound to a carbon atom. The general formula for a simple alcohol is C ${}_n$ H ${}_{2n+1}$ OH.

The simplest and most commonly used alcohols are methanol and ethanol ( [link] ).

The alcohols have a number of different uses:

• methylated spirits (surgical spirits) is a form of ethanol where methanol has been added
• ethanol is used in alcoholic drinks
• ethanol is used as an industrial solvent
• methanol and ethanol can both be used as a fuel and they burn more cleanly than gasoline or diesel (refer to Grade 11 for more information on biofuels as an alternative energy resource.)
• ethanol is used as a solvent in medical drugs, perfumes and vegetable essences
• ethanol is an antiseptic

Naming the alcohols

The rules used to name the alcohols are similar to those already discussed for the other compounds, except that the suffix of the name will be different because the compound is an alcohol.

Give the IUPAC name for the following organic compound

1. Identify the functional group

   The compound has an -OH (hydroxyl) functional group and is therefore an alcohol. The compound will have the suffix -ol.

2. Find the longest carbon chain

   There are three carbons in the longest chain. The prefix for this compound will be 'prop'. Since there are only single bonds between the carbon atoms, the suffix becomes 'propan' (similar to the alkane 'propane').

3. Number the carbons in the carbon chain

   In this case, it doesn't matter whether you start numbering from the left or right. The hydroxyl group will still be attached to the middle carbon atom, numbered '2'.

4. Look for any branched groups, name them and give their position on the carbon chain.

   There are no branched groups in this compound, but you still need to indicate the position of the hydroxyl group on the second carbon. The suffix will be -2-ol because the hydroxyl group is attached to the second carbon.

5. Combine the elements of the compound's name into a single word in the order of branched groups; prefix; name ending according to the functional group.

   The compound's name is propan-2-ol.

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Give the IUPAC name for the following compound:

1. Identify the functional group

   The compound has an -OH (hydroxyl) functional group and is therefore an alcohol. There are two hydroxyl groups in the compound, so the suffix will be -diol.

2. Find the longest carbon chain

   There are four carbons in the longest chain. The prefix for this compound will be 'butan'.

3. Number the carbons in the carbon chain

   The carbons will be numbered from left to right so that the two hydroxyl groups are attached to carbon atoms with the lowest numbers.

4. Look for any branched groups, name them and give their position on the carbon chain.

   There are no branched groups in this compound, but you still need to indicate the position of the hydroxyl groups on the first and second carbon atoms. The suffix will therefore become 1,2-diol.

5. Combine the elements of the compound's name into a single word in the order of branched groups; prefix; name ending according to the functional group.

   The compound's name is butan-1,2-diol.

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Naming the alcohols

1. Give the structural formula of each of the following organic compounds:
   1. pentan-3-ol
   2. butan-2,3-diol
   3. 2-methyl-propanol
2. Give the IUPAC name for each of the following:
   1. CH ${}_3$ CH ${}_2$ CH(OH)CH ${}_3$

   2. 

Physical and chemical properties of the alcohols

The hydroxyl group affects the solubility of the alcohols. The hydroxyl group generally makes the alcohol molecule polar and therefore more likely to be soluble in water. However, the carbon chain resists solubility, so there are two opposing trends in the alcohols. Alcohols with shorter carbon chains are usually more soluble than those with longer carbon chains.

Alcohols tend to have higher boiling points than the hydrocarbons because of the strong hydrogen bond between hydrogen and oxygen atoms.

Alcohols can show either acidic or basic properties because of the hydroxyl group. They also undergo oxidation reactions to form aldehydes, ketones and carboxylic acids.

Case study : the uses of the alcohols

Read the extract below and then answer the questions that follow:

The alcohols are a very important group of organic compounds, and they have a variety of uses. Our most common use of the word 'alcohol' is with reference to alcoholic drinks. The alcohol in drinks is in fact ethanol . But ethanol has many more uses apart from alcoholic drinks! When ethanol burns in air, it produces carbon dioxide, water and energy and can therefore be used as a fuel on its own, or in mixtures with petrol. Because ethanol can be produced through fermentation, this is a useful way for countries without an oil industry to reduce imports of petrol. Ethanol is also used as a solvent in many perfumes and cosmetics.

Methanol can also be used as a fuel, or as a petrol additive to improve combustion. Most methanol is used as an industrial feedstock, in other words it is used to make other things such as methanal (formaldehyde), ethanoic acid and methyl esters. In most cases, these are then turned into other products.

Propan-2-ol is another important alcohol, which is used in a variety of applications as a solvent.

Questions

1. Give the structural formula for propan-2-ol.
2. Write a balanced chemical equation for the combustion reaction of ethanol.
3. Explain why the alcohols are good solvents.