Reactions Of Alkanes

Reactions of alkanes, including combustion and halogenation

Reactions of Alkanes: Combustion and Radical Halogenation

Alkanes are saturated hydrocarbons that are relatively inert, yet two reactions matter most in introductory organic chemistry: combustion and free-radical halogenation (Cl₂ or Br₂). Both can be summarized with simple equations, but halogenation requires understanding radical chain mechanisms and selectivity.


Combustion of Alkanes

Complete combustion converts an alkane to carbon dioxide and water:

CxHy + (x + y/4) O₂ ⟶ x CO₂ + (y/2) H₂O

Key points:

  • Highly exothermic; requires a spark or sufficient heat to initiate.
  • Adequate oxygen gives a clean burn (CO₂ + H₂O).
  • Oxygen-poor conditions yield CO, soot, and partially oxidized byproducts (aldehydes, ketones).
  • Longer/more reduced hydrocarbons release more energy per mole; combustion is a redox process (carbon oxidized, O₂ reduced).

Radical Halogenation (Cl₂ / Br₂)

Radical halogenation replaces a hydrogen with a halogen via a free-radical chain:

R-H + X₂ ⟶ R-X + H-X  (X = Cl, Br)

Reaction stoichiometry for radical halogenation: alkane + Cl₂/Br₂ → alkyl halide + HX

Chain mechanism

Initiation

X₂ ⟶[hν or Δ] 2 X·

Homolytic cleavage of Br₂ into bromine radicals

Propagation

R-H + X· ⟶ R· + H-X

R· + X₂ ⟶ R-X + X·

Propagation step showing hydrogen abstraction and halogen capture

Termination
Radicals recombine, e.g.:

X· + X· ⟶ X₂, R· + X· ⟶ R-X, R· + R· ⟶ R-R

Termination step with radical recombination

Reactivity and Selectivity

  • Relative C–H reactivity: tertiary > secondary > primary.
  • Chlorination: fast, less selective (≈5 : 3.5 : 1 ratio for tertiary:secondary:primary), so mixtures of mono-, di-, polyhalogenated products form.
  • Bromination: slower, more selective; strongly favors tertiary C–H and gives one major product when conditions are tight.
  • Fluorination: too exothermic/violent for standard lab halogenation.
  • Iodination: endothermic and generally not synthetically useful under these conditions.

Downstream Functionalization

  • The alkyl halide product can undergo nucleophilic substitution (SN1/SN2) or elimination (E1/E2), opening paths to alcohols, ethers, alkenes, etc. Radical halogenation is thus a gateway to further diversification.
  • Cracking/reforming (industrial) also converts long alkanes into shorter alkanes/alkenes using heat and catalysts.

Summary

  • Combustion: balance C, H, O → CO₂ + H₂O; the stoichiometric recipe ensures a clean burn under excess oxygen.
  • Free-radical halogenation: understand initiation/propagation/termination, remember tertiary > secondary > primary, and know that bromination is more selective than chlorination.
  • Focus on substitution at sp³ C–H bonds; this is not an addition reaction.