Alkanes — Structure, Properties, and Industrial Processing

Alkanes are the simplest organic molecules and, commercially, the most important. Crude oil is a complex mixture of alkanes that provides most of the world's fuel, lubricants, and raw materials for plastics. Understanding alkane structure and the industrial processes that separate and transform them — fractional distillation and cracking — is essential at A-Level. This lesson covers OCR A-Level Chemistry A (H432) specification 4.1.2 (a)–(d).

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1. Definition and General Formula

Key Definition — Alkane: A saturated hydrocarbon containing only single C–C and C–H bonds, with general formula CₙH₂ₙ₊₂ for acyclic molecules.

Key features:

• Saturated: every carbon is bonded to the maximum number of other atoms (four).
• Hydrocarbon: composed of carbon and hydrogen only.
• Sigma (σ) bonds only: all bonds are formed by direct end-to-end overlap of atomic orbitals.
• Non-polar: the electronegativity difference between C (2.55) and H (2.20) is small, so C–H bonds are effectively non-polar.

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2. Bonding and Shape Around Each Carbon

In an alkane, each carbon atom is sp³ hybridised. The four sp³ hybrid orbitals point to the corners of a tetrahedron, giving a bond angle of 109.5°.

Key Definition — Sigma (σ) bond: A covalent bond formed by the direct (head-on) overlap of atomic orbitals, lying along the internuclear axis. There is free rotation around σ bonds.

           H
           |
           C — H   (each angle 109.5°)
         / | \
        H  H  H

Each C–C and C–H bond is a σ bond. Because σ bonds allow free rotation, alkanes can adopt different conformations — they are not rigid.

2.1 Why Alkanes are Unreactive

• C–C and C–H bonds are strong (bond enthalpies of 347 and 413 kJ mol⁻¹ respectively).
• C–H bonds are non-polar, offering no electrostatic attraction for nucleophiles or electrophiles.
• There are no lone pairs or π-systems.

Alkanes therefore do not react with most common reagents: they are inert to acids, bases, nucleophiles, oxidising agents at room temperature, and reducing agents. Their principal reactions are combustion and free radical substitution with halogens in UV light (covered in Lesson 6).

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3. Physical Properties of Alkanes

3.1 Boiling Point Trend

As chain length increases, boiling point rises steadily because:

• Larger molecules contain more electrons.
• More electrons give stronger London (dispersion) forces.
• More energy is required to overcome these intermolecular forces.

Alkane	n	B.p. / °C
Methane	1	−162
Ethane	2	−89
Propane	3	−42
Butane	4	−0.5
Pentane	5	36
Hexane	6	69
Octane	8	126
Decane	10	174

3.2 Branching Lowers Boiling Point

Branched alkanes have a more compact, spherical shape. This reduces surface contact between molecules, weakening London forces, and thus lowering boiling point compared with the straight-chain isomer.

Isomer	B.p. / °C
Pentane	36
2-methylbutane	28
2,2-dimethylpropane	10

3.3 Solubility

Alkanes are insoluble in water because they cannot form hydrogen bonds with water molecules. Mixing an alkane with water would require breaking hydrogen bonds between water molecules (endothermic) without any compensating attraction — energetically unfavourable. Alkanes do dissolve in non-polar solvents ("like dissolves like").

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4. Crude Oil and Fractional Distillation

Crude oil is a complex mixture of hundreds of hydrocarbons — mostly alkanes but also cycloalkanes and some aromatics. To be useful, it must be separated into fractions with similar boiling ranges.

4.1 The Fractionating Column

Crude oil is vaporised by heating to about 350 °C at the base of a tall fractionating column. The column has a temperature gradient — hot at the bottom (≈350 °C), cool at the top (≈40 °C).

• Vapours rise through the column.
• As a vapour rises and cools, each fraction condenses when it reaches a plate at a temperature below its boiling point.
• Lighter fractions (shorter chains) rise higher before condensing; heavier fractions (longer chains) condense near the bottom.
• Residues (bitumen) run off the bottom unvaporised.