Alkanes - Reactions of Alkanes with Halogens

Question 1

What is a free radical?

Answer 1

• Species with an unpaired electron (represented by a dot).
• Unpaired electron makes them very reactive.

Question 2

Describe the steps in a free radical reaction?

Answer 2

• Initiation (forming of free radicals).
• Propagation (repeated reactions that regenerate more free radicals).
• Termination (when 2 free radicals combine to form a stable product).

Question 3

What is the general equation for the reaction between an alkane & a halogen?

Answer 3

• Alkane + halogen → halogenoalkane + hydrogen halide
• e.g. C₆H₁₄ + Br₂ → C₆H₁₃Br + HBr

Question 4

State + explain the conditions required for alkane molecules to undergo reactions with halogens.

Answer 4

• Alkanes react with halogens when exposed to UV light.
• The energy from the UV light breaks the halogen-halogen bond to initiate the reaction.

Question 5

What type of reaction occurs when alkanes react with halogens?

Answer 5

Free radical substitution.

Question 6

What is the equation for the initiation step in the reaction between methane & chlorine?

Answer 6

Cl₂ → Cl⋅ + Cl⋅
2 free radicals are produced.

Question 7

How do the relative amounts of chlorine & methane present determine the identity & ratio of products produced in the reaction chlorine & methane.

Answer 7

• If methane is in excess, more chloromethane (CH₃Cl) will be produced.
• If chlorine is in excess, futher substitutions of H with Cl may take place which will form dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃), and tetrachloromethane (CCl₄).

Question 8

State + explain the limitations of radical substitution reactions in organic synthesis.

Answer 8

Leads to a mixture of products being formed as a result of:
- Further substitutions at the same position in the carbon chain.
- Substitutions at different points in the carbon chain.

Question 9

Give 3 equations for the possible termination steps in the reaction between methane & chlorine.

Answer 9

Cl⋅ +Cl⋅ → Cl₂
CH₃⋅ + CH₃⋅ → C₂H₆
Cl⋅ + CH₃⋅ → CH₃Cl
2 free radicals combine to produce a single molecule.

Question 10

Give 2 equations for the possible propagations steps in the reaction between methane & chlorine.

Answer 10

CH₄ + Cl⋅ → CH₃⋅ + HCl
CH₃⋅ + Cl₂ → CH₃Cl + Cl⋅
A free radical reacts to produce another free radical.