Alkenes and Alcohols

Alkenes and alcohols are two types of homologous series - families of molecules which share the same general formula and react in the same way. Please note that this topic is only included on the AQA triple science specification.

Structure and formulae of alkenes

The first four members of the alkene homologous series are called ethene, propene, butene and pentene.

Reactions of alkenes

Alkenes contain the C=C functional group, which influences the kind of reactions that alkenes can take part in.

Alkenes undergo combustion reactions when reacted with oxygen. Just like alkanes, alkenes burn to form carbon dioxide and water. However, alkenes are much more likely to undergo incomplete combustion to form carbon monoxide, or just carbon (soot), which is seen as a smoky flame.

Alkenes also undergo addition reactions with hydrogen, water and the halogens. These molecules are added to the carbons of the double carbon bond, forming a molecule with just single bonds. This is an addition reaction, which means the two molecules are added together to form a single product.

Alkenes react with hydrogen to form alkanes. This reaction is known as hydrogenation and is used in the manufacture of margarine. It requires the presence of a nickel catalyst.

Alkenes react with water to form alcohols. This is a hydration reaction and is used in the manufacture of ethanol. It requires the presence of a phosphoric acid catalyst.

Alkenes react with halogens (such as bromine) to form halogenoalkanes. It takes place in the absence of a catalyst at room temperature and pressure. The reaction between alkenes and bromine causes a colour change from orange to colourless and is used as a test for alkenes.

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 as alkanes and alkenes. The structures of the first four alcohols in the homologous series are known as methanol, ethanol, propanol and butanol.

Reaction of alcohols with sodium

Alcohols react with sodium to form a salt and hydrogen gas. For example, when a small piece of sodium is added to a solution of ethanol, bubbles of hydrogen gas can be seen and a colourless solution of sodium ethoxide (the salt) is formed.

Burning alcohols in air (combustion)

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.

Solubility of alcohols in water

When an alcohol, such as ethanol, is added to water it will dissolve to form a solution. Shorter alcohol molecules are the most soluble and become more insoluble as the length of the carbon chain increases. Longer alcohols, such as butanol, may not mix with water very well and will form two distinct layers.

Reaction of alcohols with an oxidising agent

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.

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 methanoic acid, ethanoic acid, propanoic acid and butanoic acid.

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. Weak acids are acids which only partially ionise. This means that when they are dissolved in water, they do not release hydrogen ions very readily. This gives carboxylic acids a much higher pH (around 3-4) compared to stronger acids like hydrochloric acid (pH approx. 1-2).

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:

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.

Next Page: Polymers