Alkanes and Free-radical Substitution

Detailed Explanations

A. Why alkanes are relatively unreactive

Alkanes mainly contain strong, non-polar σ bonds. Because there is no strongly polar bond (no permanent δ + centre) and no π bond, therefore they do not react readily with typical nucleophiles/electrophiles under mild conditions.

B. Conditions and what they imply

• Conditions: Cl₂ or Br₂ with UV light.
• UV implies homolytic fission: a bond splits evenly to form two radicals.

C. Workflow: writing a free-radical substitution mechanism (exam method)

1. Write the initiation step: X₂ → [UV] 2X..
2. Write 2 propagation steps that show (i) H atom removal and (ii) halogen addition.
3. Write one termination step (any two radicals combine).
4. Check: every radical has a dot, and atoms are conserved in each step.

Mini example (chlorination of methane):

• Initiation: Cl₂ → [UV] 2Cl.
• Propagation: Cl. + CH₄ → HCl + CH₃. then CH₃. + Cl₂ → CH₃Cl + Cl.

D. Mechanism stages (example: chlorination)

1) Initiation

Homolytic fission produces radicals: Cl₂ → [UV] 2Cl.

2) Propagation

Radicals are used and re-formed (this is why it’s a chain reaction): Cl. + CH₄ → HCl + CH₃. CH₃. + Cl₂ → CH₃Cl + Cl.

3) Termination

Two radicals collide to form a non-radical product (chain stops): Cl. + Cl. → Cl₂ CH₃. + Cl. → CH₃Cl CH₃. + CH₃. → C₂H₆

E. Why mixtures form (and why bromine is “more selective”)

• If an alkane has more than one type of hydrogen (e.g. primary vs secondary), substitution can occur at multiple positions → isomer mixture.
• Multiple substitutions can occur (e.g. CH₃Cl then CH₂Cl₂ etc.), especially for methane.
• Bromination is slower but tends to give a higher proportion of the “more stable radical” product (often more selective than chlorination).

F. Combustion, pollutants and the atmosphere

Complete combustion in excess oxygen forms carbon dioxide and water. Limited oxygen can produce carbon monoxide and soot:

CH₄ + 2O₂ → CO₂ + 2H₂O 2CH₄ + 3O₂ → 2CO + 4H₂O

In vehicle engines, high temperatures allow nitrogen and oxygen from air to form nitrogen oxides. Sulfur impurities in fuel form sulfur dioxide, and unburnt hydrocarbons can escape. Their effects differ: carbon monoxide is toxic, nitrogen oxides contribute to photochemical smog and acid deposition, sulfur dioxide contributes to acid deposition, particulates harm health, and carbon dioxide strengthens the greenhouse effect.

Catalytic converters reduce emissions of carbon monoxide, nitrogen oxides and unburnt hydrocarbons, but they do not remove the carbon dioxide formed by burning fuel. A complete environmental answer names the pollutant, its source and its effect rather than calling every emission “greenhouse gas”.