Alkanes — Structure, Properties, and Industrial Processing

Spec Mapping — OCR H432 Module 4.1.2 — Hydrocarbons (alkanes and alkenes), covering the definition of an alkane as a saturated hydrocarbon, sp³ hybridisation and tetrahedral geometry, sigma-bond character and free rotation, physical-property trends in the alkane homologous series, fractional distillation of crude oil, and thermal vs catalytic cracking (refer to the official OCR H432 specification document for exact wording).

Alkanes are the simplest organic molecules and, commercially, the most important. Crude oil is a complex mixture of hundreds of alkanes (with cycloalkanes and aromatics interspersed) that provides most of the world's transport fuel, lubricants, and feedstock molecules for the polymer industry. Understanding alkane structure — the sp³ hybridised carbon, the 109.5° tetrahedral bond angle, the freely rotating sigma framework — is essential before discussing reactivity (in Lesson 6, free-radical substitution and combustion). Understanding the industrial processes that separate and transform alkanes — fractional distillation followed by cracking — is the central A-Level treatment of how a single barrel of crude becomes a usable range of refined products. This lesson is also a useful entry point to the broader "industrial chemistry" thread that runs through OCR H432, including Haber, contact process, and the Solvay process.

Key Definition: an alkane is a saturated acyclic hydrocarbon containing only single C–C and C–H bonds with general formula $C_nH_{2n+2}$ . Every carbon is sp³ hybridised with tetrahedral geometry and a bond angle of 109.5°; every bond is a sigma (σ) bond allowing free rotation; alkanes are non-polar, with weak London dispersion forces as the only intermolecular attraction.

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.

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.

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. Free rotation also means a single skeletal formula represents an ensemble of rotamers; only at low temperature in solution does one rotamer dominate over the others.

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).

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").

4. Crude Oil and Fractional Distillation

Crude oil is a complex mixture of hundreds of hydrocarbons — mostly alkanes but also cycloalkanes and some aromatics, plus trace sulphur-containing organics and metal residues. The composition varies geographically: North Sea Brent crude is relatively "sweet" (low sulphur), Saudi Arabian Arabian Light is medium, Venezuelan Orinoco crude is heavy and high-sulphur. Different crudes need different downstream processing. To be useful, raw crude 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.

Modern industrial columns operate continuously with side-stream draws at each plate, returning some condensate to the column as reflux to sharpen the separation. The same engineering principle (countercurrent distillation with reflux) appears across the chemical industries — air separation, ammonia synthesis gas processing, and biorefining all use the same column architecture at different scales and temperatures.

4.1.1 Physical principle — Raoult's law and ideal mixtures

For an idealised binary mixture of two alkanes A and B, the partial pressure of each component above the liquid is proportional to its mole fraction: $P_A = x_A P_A^*$ (Raoult's law), where $P_A^*$ is the pure-component vapour pressure. The total vapour above the mixture is therefore enriched in the more volatile component — and condensing that vapour gives a liquid enriched in the lighter component. Repeating the vapour-condense-revaporise step on successive plates produces successively purer fractions. Crude oil is a many-component mixture rather than binary, but the same plate-by-plate enrichment principle applies, sorting hundreds of components into a few dozen useful fractions.

graph TD
  A[Crude oil in at ~350°C] --> B["Refinery gases<br/>top ~40°C, C1-C4"]
  A --> C["Gasoline/petrol<br/>~70°C, C5-C10"]
  A --> D["Naphtha<br/>~120°C, C7-C12"]
  A --> E["Kerosene<br/>~180°C, C10-C16"]
  A --> F["Diesel<br/>~260°C, C14-C20"]
  A --> G["Fuel oil<br/>~340°C, C20-C50"]
  A --> H["Bitumen residue<br/>bottom >C50"]

4.2 Uses of Each Fraction

Fraction	Carbons	Boiling range / °C	Use
Refinery gases	1–4	<40	LPG, bottled gas, feedstock
Gasoline (petrol)	5–10	40–120	Car fuel
Naphtha	7–12	120–180	Petrochemical feedstock
Kerosene (paraffin)	10–16	180–260	Jet fuel, heating
Diesel / gas oil	14–20	260–340	Diesel engines, heating
Fuel oil	20–50	>340	Ships, power stations
Bitumen	>50	residue	Road surfacing, roofing

5. Cracking

Demand for short-chain alkanes and alkenes (petrol, ethene for plastics) exceeds the proportion naturally present in crude oil. Cracking is the process of breaking long-chain alkanes into shorter, more useful molecules.

Key Definition — Cracking: The breaking of long-chain alkanes into a mixture of shorter-chain alkanes and alkenes.

Cracking is endothermic overall, because C–C bonds are being broken.

5.1 Thermal Cracking

Conditions: very high temperature (700–1200 K, typically around 1000 K) and high pressure (up to 70 atm).
Products: a high proportion of alkenes (especially ethene and propene, used to make polymers).
Mechanism: homolytic fission of C–C bonds generating carbon radicals; β-scission of those radicals generates alkenes via concerted single-bond breaking and C=C formation; eventual termination by radical recombination.

Typical example:

C₁₀H₂₂ → C₈H₁₈ (octane) + C₂H₄ (ethene)

C₁₀H₂₂ → C₆H₁₄ + C₂H₄ + C₂H₄

Thermal cracking gives a product distribution that is statistical — there is no single dominant product but a Boltzmann-weighted mixture biased toward the kinetically accessible short-chain alkenes. Industrial operators tune temperature and residence time to favour the alkenes they want (high temperature, short residence: maximises ethene; lower temperature, longer residence: shifts toward propene and butenes).

5.2 Catalytic Cracking

Conditions: high temperature (~720 K), slight pressure, zeolite catalyst (aluminosilicate).
Products: a high proportion of branched and cyclic alkanes, and aromatic hydrocarbons — ideal for high-octane petrol. Motor fuel with branched structures burns more smoothly (higher octane rating; less pre-ignition knocking in spark-ignition engines).
Mechanism: carbocation intermediates form on Brønsted acid sites of the zeolite framework; rearrange to more stable (more branched) cations before β-scission delivers the products. The catalyst is regenerated periodically by burning off the coke deposit in a separate regenerator vessel.

The zeolite catalyst is an aluminosilicate mineral with a precisely defined three-dimensional pore network of roughly 0.5–1.0 nm. Petrochemical molecules are comparable in size, so the catalyst exhibits shape selectivity: only molecules that fit the pore can enter and react. This is the molecular-scale equivalent of size-exclusion chromatography and is the key engineering reason catalytic cracking gives so different a product distribution from thermal cracking, despite both ultimately breaking C–C bonds.

5.3 Why Cracking is Economically Important

Balances supply and demand: the cracking process converts the naturally abundant long-chain fractions into the scarce short-chain fuels and feedstocks.
Produces alkenes, which are raw materials for polymers (polyethene, polypropene).
Produces branched alkanes which give higher-octane petrol with reduced knocking.
Allows refineries to optimise revenue by tailoring the cracking severity to the current market demand for ethene (polymer feedstock) vs petrol (transport fuel). The refinery's "crack spread" — the price of the cracked products minus the price of the input crude — is a routinely traded financial instrument that quantifies this economic logic.
Reduces reliance on long-chain heavy fuel oil, which has limited use today as bunker fuel for shipping but is steadily losing market share to natural gas and LNG.

graph LR
  A[Long-chain alkane] --> B{Cracking}
  B --> C["Thermal<br/>1000K, 70 atm"]
  B --> D["Catalytic<br/>720K, zeolite"]
  C --> E["Alkenes for polymers<br/>+ shorter alkanes"]
  D --> F["Branched alkanes<br/>+ aromatics for petrol"]

6. Balancing Cracking Equations

Cracking equations must balance in both atoms and bonds. The molecular formula of the products must sum to that of the starting material.

6.1 Worked Example

Decane (C₁₀H₂₂) can be cracked to give one molecule of heptane and one molecule of propene:

C₁₀H₂₂ → C₇H₁₆ + C₃H₆

Lesson 5: Alkanes — Structure, Properties, and Industrial Processing