Many people think of alcoholic drinks when the term alcohol is used. However, alcoholic drinks mainly contain only one type of alcohol – ethanol. Alcohols are actually a family of organic compounds, all of which contain a –OH functional group. Some alcohols are very important as fuels and solvents. Alcohols have similar chemical properties to each other and undergo similar chemical reactions.
The names of alcohols end with ‘-ol’ – eg ethanol.
The first three alcohols in the homologous series are methanol, ethanol and propanol. These alcohols are highly flammable, making them useful as fuels. They are also used as solvents in marker pens, medicines, and cosmetics (such as deodorants and perfumes).
Ethanol is the alcohol found in alcoholic drinks such as wine and beer. Ethanol is usually mixed with petrol for use as a fuel
- Methanol; 1 carbon atom, CH3OH
- Ethanol; 2 carbon atoms, CH3CH2OH
- Propanol; 3 carbon atoms, CH3CH2CH2OH
Properties of methanol, ethanol, propanol
The alcohols methanol, ethanol and propanol all have the following properties:
They are colourless liquids that dissolve in water to form a neutral solution (pH7).
They react with sodium to produce hydrogen and a salt. For example:
ethanol + sodium → hydrogen + sodium ethoxide
This reaction is similar but less vigorous to the reaction of water with sodium. This is due to the similarity in structure between water and the –OH group in alcohols.
They burn in the air, releasing energy and producing carbon dioxide and water.
You should be able to write balanced equations for the combustion reactions of alcohols. For example:
methanol + oxygen → carbon dioxide + water
2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l)
Carboxylic acids are a group of important organic chemicals. Vinegar contains ethanoic acid, which is a carboxylic acid. All carboxylic acids have a –COOH functional group, and have similar reactions as a result. They are weak acids because this functional group is only partly ionised in solution.
Carboxylic acids 2
Carboxylic acids 3
- Methanoic acid; 1 carbon atom, HCOOH
- Ethanoic acid; 2 carbon atoms, CH3COOH
- Propanoic acid, 3 carbon atoms, CH3CH2COOH
Carboxylic acids 4
Ethanoic acid from ethanol
The addition of chemical oxidising agents - such as acidified potassium dichromate.
The action of microbes in aerobic conditions (in the presence of oxygen). This happens on a small scale when a bottle of wine is left open and exposed to air. On a commercial scale, it is achieved in a fermenter using acetic acid bacteria.
Properties of carboxylic acids
Carboxylic acids have the following properties:
They dissolve in water to produce acidic solutions (pH less than 7).
They react with carbonates to produce carbon dioxide and a salt and water. For example:
calcium carbonate + ethanoic acid → calcium ethanoate + water + carbon dioxide
They all react with alcohols, in the presence of an acid catalyst, to form esters. For example:
ethanol + ethanoic acid → ethyl ethanoate + water
Esters are organic compounds formed by the reaction of an alcohol with a carboxylic acid. They have the functional group –COO–.
they are volatile - they are liquids that become vapours easily
they have distinctive ‘fruity’ smells
These properties make esters very useful as food flavourings, and as perfumes in cosmetics. Some esters are obtained from natural sources, such as fruits. Others are manufactured.
Making ethyl ethanoate
Ethyl ethanoate is the ester made from ethanol and ethanoic acid. Sulfuric acid is added to act as a catalyst in the reaction.
ethanol + ethanoic acid ethyl ethanoate + water
CH3CH2OH(aq) + CH3COOH(aq) CH3CH2OOCCH3(aq) + H2O(l)
The distinctive smell of ethyl ethanoate (which is like modelling glue) can be detected as the reaction proceeds. Excess ethanoic acid in the reaction mixture is neutralised with sodium hydrogencarbonate, then a few drops of the mixture added to water so that the smell can be detected more effectively.
The first part of an ester’s name comes from the alcohol - it ends with the letters 'yl'. The second part of its name comes from the carboxylic acid - it ends with the letters 'oate'.
Here are three examples:
Name of alcohol Name of carboxylic acid Name of ester Ethanol Propanoic acid Ethyl propanoate Butanol Methanoic acid Butyl methanoate Pentanol Ethanoic acid Pentyl ethanoate
Apart from ethyl ethanoate, you are not expected to name esters. However, you are expected to be able to recognise esters from their names or structural formulae. Remember to look for the –COO– group.