Organic Reactions

Organic chemistry is the study of compounds that contain carbon. In GCSE Chemistry, you’ll learn about some key families of organic compounds, how they react, and how we use them in everyday life and industry. This section covers alkenes, alcohols, carboxylic acids, and esters – and how these different groups are connected through a series of important reactions.

Alkenes and Their Reactions

Alkenes are unsaturated hydrocarbons – this means they have at least one double bond (C=C) between carbon atoms. The general formula for alkenes is CₙH₂ₙ. Common examples include ethene (C₂H₄) and propene (C₃H₆).

• The first four alkenes are ethene (two carbon atoms), propene (three carbon atoms), butene (four carbon atoms), pentene (five carbon atoms).
• Alkenes combust completely in a large amount of oxygen to produce water and carbon dioxide.
• Alkenes react via addition reactions. The double bond opens up to create a single bond. Alkenes can react with Halogens this way.
• Mix alkenes with steam and then pass it over a catalyst to form alcohols.

Because of their double bond, alkenes are more reactive than alkanes and take part in addition reactions, where the double bond opens up and new atoms are added. Important reactions of alkenes include:

• Hydrogenation – adding hydrogen to make an alkane.
• Reaction with halogens – e.g. bromine water goes from orange to colourless.
• Hydration – reacting with steam to produce alcohols, such as ethanol.

Alcohols and Ethanol

Alcohols are organic compounds that contain an –OH functional group. They are widely used as fuels, solvents, and in alcoholic drinks. The first four alcohols are methanol, ethanol, propanol, and butanol.

Ethanol is a particularly important alcohol. It can be produced in two ways:

1. Hydration of ethene (a chemical method using steam and a catalyst)
2. Fermentation of sugars using yeast in anaerobic conditions (a slower, renewable method used in industry and brewing)

The properties of alcohols:

• Contain an -OH functional group.
• They are flammable.
• Their solutions have a neutral pH.
• They react with sodium.
• They can be oxidised.
• Methanol and ethanol are used as solvents.
• Ethanol is used as fuel in spirit burners, it's clean and non-smelly.
• Ethanol is found in alcoholic drinks, it's made by fermentation.

Carboxylic Acids

Carboxylic acids contain the –COOH functional group. They are weak acids that partially ionise in water and react with bases, carbonates, and metals — producing salts, water, and carbon dioxide. Examples include:

• Methanoic acid
• Ethanoic acid (commonly known as vinegar)

They have many uses in food preservation and as intermediates in chemical reactions.

Esters and Esterification

Esters are formed when a carboxylic acid reacts with an alcohol, in the presence of an acid catalyst such as sulfuric acid. This reaction is called esterification. Esters have distinctive sweet smells and are often used in perfumes, flavourings, and solvents.

Example:
Ethanol + ethanoic acid → ethyl ethanoate + water

Esters are useful, versatile compounds that demonstrate how functional groups combine in organic reactions.

Also see Polymers, Crude Oil & Fuels, Chemical Analysis

Links for Learning

Bitesize: Alkanes
Pass my exams: Alkenes & Alkenes Homologous Series
Bitesize: Alcohols
Pass my exams: Alcohols
Bitesize: Carboxylic acids and esters
Pass my exams: Carboxylic Acids