AQA GCSE FOUNDATION Chemistry: Alkenes and Unsaturated Hydrocarbons

AQA GCSE Foundation Chemistry: Alkenes and Unsaturated Hydrocarbons

Introduction

Alkenes are a type of hydrocarbon that contain a carbon-carbon double bond. This makes them unsaturated, meaning they can react to add more atoms to their structure. This section will explore the properties and reactions of alkenes, particularly focusing on the addition reaction with bromine water.

What are Alkenes?

• Hydrocarbons: Compounds made up of only hydrogen and carbon atoms.
• Alkenes: Hydrocarbons containing at least one carbon-carbon double bond.
• General formula: CnH2n (where 'n' represents the number of carbon atoms)

Examples of alkenes:

• Ethene (C2H4): The simplest alkene, with one double bond.
• Propene (C3H6): Has one double bond and three carbon atoms.
• Butene (C4H8): Has one double bond and four carbon atoms.

The Double Bond

• The double bond in alkenes is responsible for their reactivity.
• It is formed by sharing two pairs of electrons between two carbon atoms.
• This creates a region of higher electron density, making the double bond susceptible to attack by electrophiles.

Reactions of Alkenes

Addition Reactions:

• Alkenes readily undergo addition reactions where atoms from another molecule are added across the double bond.
• The double bond breaks, and a single bond forms with each of the added atoms.

1. Reaction with Bromine Water:

• Observation: Bromine water is an orange-brown solution. When added to an alkene, the orange-brown colour disappears as the bromine reacts.
• Mechanism: The double bond in the alkene reacts with the bromine molecule (Br2), breaking the double bond and forming two new single bonds, one to each bromine atom.
• Equation:

C<sub>2</sub>H<sub>4</sub> + Br<sub>2</sub> ? C<sub>2</sub>H<sub>4</sub>Br<sub>2</sub>
(Ethene) + (Bromine) ? (1,2-dibromoethane)

• Test for unsaturation: This reaction is used to test for the presence of a double bond in a molecule. If bromine water decolourises, it indicates the presence of an alkene.

2. Reaction with Hydrogen:

• Observation: In the presence of a nickel catalyst and heat, hydrogen gas reacts with an alkene to form an alkane.
• Mechanism: The double bond breaks, and a hydrogen atom adds to each carbon atom that was part of the double bond.
• Equation:

C<sub>2</sub>H<sub>4</sub> + H<sub>2</sub> ? C<sub>2</sub>H<sub>6</sub>
(Ethene) + (Hydrogen) ? (Ethane)

3. Reaction with Hydrogen Halides:

• Observation: Hydrogen halides (like HCl) add across the double bond of an alkene.
• Mechanism: The double bond breaks, and a hydrogen atom and a halogen atom add to the carbon atoms that were part of the double bond.
• Equation:

C<sub>2</sub>H<sub>4</sub> + HCl ? C<sub>2</sub>H<sub>5</sub>Cl
(Ethene) + (Hydrogen chloride) ? (Chloroethane)

Summary

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. They are reactive and readily undergo addition reactions with bromine water, hydrogen, and hydrogen halides. This reactivity makes them useful in various industrial applications.