Alcohols, Carboxylic Acids and Esters
This page runs through three types of compound: alcohols, carboxylic acids and esters. I know these sound exotic but chances are you can find them easily around your house. Alcohols can be found in cleaning products and alcoholic drinks, carboxylic acids in citrus fruits and esters in perfumes.
Alcohols
Alcohols are a homologous series of compounds containing the ‘-OH’ functional group, which is nothing more than an atom of oxygen bound to an atom of hydrogen. Alcohols are named in the same way we learnt to name alkanes and alkenes. The structures of the first four alcohols in the homologous series are shown below:
Oxidation of ethanol
Ethanol can be oxidised by:
- Burning in air or oxygen (complete combustion)
- Reaction with oxygen in the air to form ethanoic acid (microbial oxidation)
- Heating with potassium dichromate (VI) in dilute sulfuric acid to form ethanoic acid
Burning in air or oxygen: when alcohols are burnt in air which contains a plentiful supply of oxygen, they will undergo complete combustion to produce water and carbon dioxide. Remember that oxidation is simply the gaining of oxygen or the loss of electrons.
Reaction with oxygen to form ethanoic acid: some microorganisms feed on and get energy from alcohols by reacting the alcohols with oxygen. This results in the formation of ethanoic acid which is a carboxylic acid. This reaction is why wine goes bad if it is left out in the open - the ethanol reacts with oxygen in the air, producing ethanoic acid which gives the wine a vinegary taste.
Heating with potassium dichromate (VI) to form ethanoic acid: when you mix ethanol with potassium dichromate (in the presence of dilute sulfuric acid which acts as a catalyst), the dichromate acts as an oxidising agent. It oxidises the ethanol, causing it to gain another atom of oxygen, forming ethanoic acid.
Manufacturing ethanol
Ethanol is manufactured by two different methods - the method used depends on whether you’re in a country which has a greater supply of oil or sugar.
Reacting ethene (from crude oil) with steam
Fermentation of sugar
Reacting ethene with steam: ethene is an alkene (it contains a double carbon bond) which is extracted from fractional distillation of crude oil. It can be reacted with water vapour (steam) to form ethanol. This process requires a phosphoric acid catalyst, temperatures of around 300oC and 60-70 atmospheres of pressure.
Advantages: cheap process and ethanol is the only product (100% atom economy)
Distadvantage: crude oil is a finite resource
Fermentation of sugar: sugars such as glucose can be broken down into ethanol and carbon dioxide during fermentation by yeast. Yeast contain an enzyme called zymase which catalyses the reaction. This process requires a temperature of around 30oC (any higher and the enzyme will denature) and anaerobic conditions.
Advantages: only uses renewable materials - sugar cane can be regrown very easily. Sugar cane is readily available, esecially in some developing countries with a warmer climate.
Disadvantage: ethanol is not the only product so it needs to be purified
Carboxylic acids
Carboxylic acids are a homologous series of molecules containing the ‘-COOH’ functional group. This functional group is always found at the end of the molecule and consists of a carbon atom forming a double bond to an oxygen atom, whilst also forming a single bond to the alcohol functional group. The first four carboxylic acids in the homologous series are shown below:
Get a bit of ethanoic acid on there.. and some sodium chloride while you’re at it.
The carboxylic acids are weak acids. You’ll be already familiar with the second one in the series without even realising - ethanoic acid dissolved in water is vinegar so there’s a good chance you’ve eaten a fair amount of ethanoic acid splashed onto your fish and chips.
The ethanoic acid is usually made by the oxidation of ethanol using microorganisms such as yeast, in the reaction we learnt about at the top of this page.
Reacting carboxylic acids with metals and carbonates
Just like other acids, carboxylic acids react with metals to form a salt and hydrogen. The salt will be named first with the metal and then the carboxylic acid part, adding the -oate suffix on the end. For example:
Carboxylic acids also react with metal carbonates to form a salt, water and carbon dioxide, for example:
The ester ethyl propanoate smells like pineapple
Esters
So far we’ve learnt about alcohols and carboxylic acids - if we react these molecules together you’ll form an ester. Esters are a homologous series which contain the functional group ‘-COO’ and can be identified by their distinctive fruity smell. They also evaporate easily (we say they are volatile) which makes them a useful group of compounds to use in perfumes and food flavourings.
Let’s look at the reaction between ethanol and ethanoic acid to produce the ester, ethyl ethanoate. This reaction requires an acid catalyst, which is usually concentrated sulfuric acid.
You can see how the -OH from the carboxylic acid joins together with a hydrogen from the alcohol to form water. The removal of water creates the -COO ester bond, joining the carboxylic acid and alcohol molecules together to form the ester.
Naming esters
The first part of the name comes from the alcohol and the second part from the carboxylic acid. For example, the ester butyl propanoate would have been formed by reacting butanol and propanoic acid together.