Alkanes: properties, reactions, mechanisms

Alkanes – Properties, Production, Reactions & Mechanisms (Cambridge AS & A‑Level Chemistry 9701)

1. General Features

• Alkanes are the simplest saturated hydrocarbons. General formula: $C$_nH_2n+2 (n ≥ 1).
• All C–C and C–H bonds are single σ‑bonds; the carbon atoms are sp³ hybridised.
• Consequences:
  • Non‑polar molecules – only London dispersion forces act between them.
  • Relatively low reactivity compared with alkenes, alkynes and aromatics.
  • Colourless gases or liquids at 25 °C; major components of natural gas and petroleum.

2. Production of Alkanes

2.1 Catalytic Hydrogenation of Alkenes

Alkenes are converted into the corresponding alkanes by adding H₂ across the C=C bond.

2.2 Cracking of Long‑Chain Alkanes

Cracking deliberately breaks C–C bonds in heavy n‑alkanes to give shorter‑chain alkanes (and alkenes). Two industrial variants are used.

Both routes are exploited to produce LPG (propane, butane) and to generate feed‑stock alkenes for polymer manufacture.

3. Physical & Structural Properties

3.1 General Trends

• Inter‑molecular forces: Only London dispersion forces → low boiling points for small alkanes.
• Boiling point, melting point & density increase steadily with molecular size (more electrons → stronger dispersion forces).
• Solubility: Insoluble in water; soluble in non‑polar organic solvents (benzene, ether, carbon tetrachloride).
• Isomerism: Only structural (chain) isomerism. No geometric or optical isomerism because every carbon is tetra‑substituted with single bonds.

3.2 Representative Physical Data

4. Main Reactions of Alkanes

4.1 Combustion

1. Complete combustion (excess O₂)

   $C$_nH_2n+2 + \left(n+\tfrac12\right) O₂ \rightarrow n CO₂ + (n+1) H₂O \quad \Delta H < 0

   Produces carbon dioxide, water and a large amount of heat – the basis of fuel‑energy applications.

2. Incomplete combustion (limited O₂)

   $C$_nH_2n+2 + \tfrac{3n+1}{2} O₂ \rightarrow n CO + (n+1) H₂O \; (+\; \text{soot})

   Gives toxic carbon monoxide and carbon (soot), both hazardous.

4.2 Free‑Radical Halogenation (Substitution)

General equation (X = Cl or Br):

$C$_nH_2n+2 + X₂ \xrightarrow{hu} C_nH_2n+1X + HX

The reaction proceeds by a radical chain mechanism (see Section 5). Chlorination is fast and non‑selective; bromination is slower and more selective.

4.3 Industrial Cracking (Re‑visited)

• Deliberate production of LPG (propane, butane) and light alkenes (ethylene, propene).
• Thermal cracking favours formation of alkenes; catalytic cracking, because of acid sites, gives a higher proportion of saturated products.

4.4 Catalytic Reforming

Heavy n‑alkanes are converted into branched alkanes and cycloalkanes using platinum‑group metal catalysts (Pt, Pt‑Re, Pt‑Os) at 500 °C and 1–2 atm H₂. The process raises the octane rating of gasoline and supplies cycloalkanes for petrochemical synthesis.

5. Mechanism of Free‑Radical Halogenation

5.1 Chain‑Reaction Overview

1. Initiation – homolytic cleavage of X₂ under UV light.
2. Propagation – alternating hydrogen abstraction and radical substitution.
3. Termination – combination of two radicals, removing them from the chain.

5.2 Detailed Steps

5.2.1 Initiation

$X₂\; \backslash xrightarrow\{hu\}\; 2\backslash ,X^\backslash bullet$

5.2.2 Propagation

1. Hydrogen abstraction
   $C$_nH_2n+2 + X^\bullet \rightarrow C_nH_2n+1^\bullet + HX
2. Radical substitution
   $C$_nH_2n+1^\bullet + X₂ \rightarrow C_nH_2n+1X + X^\bullet

5.2.3 Termination

Any two radicals may combine:

5.3 Selectivity – Influence of Radical Stability

Relative stability of carbon‑centred radicals governs the rate of hydrogen abstraction:

tert‑> sec‑> prim‑> methyl

Consequences:

• Hydrogen atoms on tertiary carbons are abstracted fastest; primary hydrogens are the slowest.
• Product distribution mirrors radical stability. Example – chlorination of isobutane (2‑methylpropane) gives predominantly tert‑butyl chloride because the tertiary radical is most stable.
• For bromination, the higher activation energy accentuates this selectivity, often giving > 90 % of the most substituted product.

5.4 Energy Profile (illustrative)

6. Summary of Key Points

• Alkanes are saturated, non‑polar hydrocarbons with formula $C$_nH_2n+2.
• Physical properties (boiling point, density, solubility) depend solely on molecular size and dispersion forces.
• Industrial production:
  • Hydrogenation of alkenes (catalysts: Pt, Ni **or** Pd; 150–300 °C, 1–5 atm H₂).
  • Thermal cracking (≥ 500 °C, no catalyst) and catalytic cracking (≈ 450 °C, solid acid catalysts such as zeolites).
• Key reactions:
  • Complete and incomplete combustion – source of energy and a safety hazard.
  • Free‑radical halogenation – radical chain mechanism; product ratios predicted from radical stability.
  • Cracking – industrial source of LPG and light alkenes.
  • Catalytic reforming – improves octane rating and provides cycloalkanes.
• Understanding the radical chain (initiation → propagation → termination) enables accurate prediction of both the rate and the selectivity of halogenation reactions.