Alkane Reactions

Understanding the reactions involving saturated hydrocarbons, such as alkanes, is essential for mastering chemical processes and performing well in exams. This note provides definitions, key concepts, and reaction mechanisms.

Definition and Examples

• Linear Alkanes:
  • Methane (CH₄): The simplest alkane with one carbon atom.
  • Ethane (C₂H₆): Two carbon atoms in the chain.
  • Propane (C₃H₈): Composed of three carbon atoms.
  • Butane (C₄H₁₀): Includes four carbon atoms.

• Cyclic Alkanes:
  • Cyclopentane: Due to its cyclic structure, it has fewer hydrogen atoms.

Characteristics of Alkanes

• Non-Polar Nature:

  • Alkanes are non-polar due to an even distribution of electrons.

• Chemical Stability:

  • They prefer substitution reactions over addition reactions due to their stable covalent bonds.

Halogenation Overview

Halogenation refers to the substitution of hydrogen atoms in alkanes with halogen atoms like chlorine or bromine.

• Reaction Conditions:
  • UV Light is necessary to provide the energy required to initiate the reaction by breaking halogen bonds to form radicals.

Free Radical Substitution Mechanism

Free radical substitution describes the halogenation process in saturated hydrocarbons, such as alkanes.

Mechanism Steps

Initiation

• Photodissociation: UV light is used to break halogen molecules into reactive radicals.

Propagation

• Chain Reaction:
  • Reaction 1: An alkane interacts with a halogen radical, forming a methyl radical and hydrogen halide.
  • Reaction 2: A methyl radical reacts with another halogen molecule, continuing the chain.

Termination

• Recombination: Radicals combine to form stable molecules, concluding the reaction chain.

Specific Reaction Examples

• Methane Chlorination: $\text{CH}_4 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{HCl}$
• Ethane Bromination: $\text{C}_2\text{H}_6 + \text{Br}_2 \rightarrow \text{C}_2\text{H}_5\text{Br} + \text{HBr}$

Writing and Balancing Equations

• Follow Steps:
  • Identify reactants and products.
  • Draft the skeleton equation.
  • Balance by ensuring equal numbers of atoms on each side.

Worked Example

For the reaction between propane and chlorine:

1. Identify the reactants: propane (C₃H₈) and chlorine (Cl₂)
2. Identify the main product: chlorinated propane (C₃H₇Cl)
3. Identify the byproduct: hydrogen chloride (HCl)
4. Write the balanced equation: $\text{C}_3\text{H}_8 + \text{Cl}_2 \rightarrow \text{C}_3\text{H}_7\text{Cl} + \text{HCl}$

Safety and Practical Considerations

• Conduct these reactions in well-ventilated laboratories and wear protective gear.
• Ensure UV light is used only to initiate reactions.

Molecular Modelling

Molecular Modelling: A technique for visualising chemical reactions and structures.

• Utilise physical kits or digital tools for visualisation.

Sample Problems with Solutions

1. Problem: Write a balanced equation for the chlorination of methane. Solution: CH₄ + Cl₂ → CH₃Cl + HCl

2. Problem: Balance the equation for the bromination of propane. Solution: C₃H₈ + Br₂ → C₃H₇Br + HBr

3. Problem: What products form when ethane reacts with chlorine in the presence of UV light? Solution: C₂H₆ + Cl₂ → C₂H₅Cl + HCl

Common Mistakes

• Misunderstanding stoichiometric coefficients.
• Neglecting side reactions.