Alkanes

Alkanes and Cycloalkanes — General Formulae

Alkanes are saturated hydrocarbons containing only single C–C and C–H bonds, with the general formula CₙH₂ₙ₊₂.

Saturated means every carbon forms four single bonds with no C=C double bonds present, so no further atoms can be added without breaking a bond. This makes alkanes relatively unreactive towards most reagents at room temperature.

Cycloalkanes contain a closed ring of carbon atoms joined by single bonds. Because forming the ring removes two hydrogens compared with the open-chain alkane, the general formula is CₙH₂ₙ.

Both families are hydrocarbons because they contain only carbon and hydrogen. They differ from alkenes, which have C=C double bonds and the same formula CₙH₂ₙ as cycloalkanes but are unsaturated.

Structural Isomerism in Organic Chemistry

Structural isomers are compounds with the same molecular formula but different structural formulae arising from a different arrangement of atoms.

The atoms can be arranged differently in three ways: by changing the carbon chain length and branching (chain isomers), by changing the position of a functional group on the chain (position isomers), or by having a different functional group altogether (functional group isomers).

In alkanes, only chain isomerism is possible because there is no functional group whose position could change. As the number of carbons increases, the number of possible chain isomers grows rapidly.

When drawing isomers, identify the longest possible carbon chain first, then move branches to different positions on shorter chains. Each unique skeleton is a separate isomer.

Examiner InsightTwo structures that look different but can be converted into one another simply by rotating around a single bond are the same molecule, not separate isomers.

Exam TipAlways check the longest chain and branch positions before claiming a new isomer.

Naming and Drawing Isomers up to C₆

IUPAC nomenclature identifies the longest carbon chain as the parent alkane and names branches as alkyl substituents with locants.

The naming steps are:

• Identify the longest continuous carbon chain — this gives the parent alkane name (e.g. pentane for five carbons).
• Number the chain from the end nearest a branch to give the lowest possible locants.
• Name each branch as an alkyl group (–CH₃ methyl, –C₂H₅ ethyl) with its locant.
• Use prefixes di-, tri-, tetra- for repeated identical branches and list multiple different branches in alphabetical order.

For cycloalkanes, the prefix cyclo- precedes the parent name (cyclopropane, cyclobutane, cyclopentane, cyclohexane). Substituents on rings are numbered to give the lowest set of locants.

The number of chain isomers grows: methane (1), ethane (1), propane (1), butane (2), pentane (3), hexane (5).

Crude Oil — Fractional Distillation, Cracking and Reforming

Crude oil is a mixture of hydrocarbons separated by fractional distillation based on differences in boiling point.

In the column, longer-chain hydrocarbons with stronger London forces have higher boiling points and condense at the hot bottom of the column. Shorter chains have weaker London forces and rise higher before condensing.

Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes by heating with a catalyst. This converts low-demand fractions into higher-value petrol-range hydrocarbons and alkenes for the polymer industry.

A typical thermal cracking equation:

Reforming converts straight-chain alkanes into branched alkanes or cyclic and aromatic compounds using a platinum catalyst, increasing the octane rating for cleaner-burning petrol.

A typical reforming equation:

MisconceptionCracking does not produce only alkanes; it always produces a mixture of shorter alkanes and alkenes because hydrogen is conserved.

Exam TipBalance the equation with one alkene per cracking step unless told otherwise.

Pollutants from Alkane Combustion

Combustion of alkane fuels in engines releases pollutants including carbon monoxide, oxides of nitrogen, oxides of sulfur, carbon particulates and unburned hydrocarbons.

The pollutants and their causes are:

Carbon monoxide is toxic because it binds to haemoglobin more strongly than oxygen, so oxygen cannot be transported around the body. Even small concentrations cause headaches, drowsiness and potentially death.

Acid rain forms when SO₂ and NOₓ dissolve in atmospheric water, producing sulfuric and nitric acids. The resulting precipitation lowers soil and lake pH, damages forests, corrodes limestone buildings and harms aquatic life.

MisconceptionCO is dangerous because it is colourless and odourless, not because it smells; victims do not know they are being poisoned.

Exam TipState that CO binds irreversibly to haemoglobin, blocking oxygen transport.

Alternative Fuels and Climate Change

Burning alkane fuels releases carbon dioxide, a greenhouse gas linked to climate change, motivating the development of more sustainable alternatives.

CO₂ absorbs infrared radiation emitted from Earth’s surface and re-emits some of it back, increasing average atmospheric temperatures. Rising temperatures cause ice melt, sea-level rise, and changing weather patterns.

Sustainability concerns also drive change because crude oil is a finite resource that cannot be replaced on a human timescale. Alternative fuels reduce reliance on fossil fuels.

Alternative fuels include bioethanol from fermentation of sugar crops and hydrogen produced by electrolysis of water. These can be used in modified internal combustion engines or in hydrogen fuel cells.

Carbon Neutrality of Different Fuels

A carbon-neutral fuel is one whose combustion releases no net CO₂ into the atmosphere over its full life cycle.

The judgement compares the CO₂ released during combustion with the CO₂ absorbed during fuel production. A truly neutral fuel achieves a zero net balance.

The combustion equations are:

Examiner InsightBioethanol is often called carbon-neutral, but in exam answers always state that the energy used in farming, harvesting and distillation usually involves fossil fuels, so it is not truly neutral.

Exam TipName a specific source of CO₂ that breaks the neutrality balance.

Reactions of Alkanes — Combustion and Halogens

Alkanes undergo two key reactions: complete and incomplete combustion with oxygen, and free radical substitution with halogens in ultraviolet light.

Complete combustion in plentiful oxygen produces CO₂ and H₂O:

Incomplete combustion in insufficient oxygen produces CO and H₂O, or in very limited oxygen, C (soot) and H₂O:

Alkanes react with halogens (Cl₂ or Br₂) only in the presence of ultraviolet (UV) light. Without UV light no reaction occurs because the halogen molecule cannot dissociate into reactive species.

A typical halogenation:

The reaction produces a halogenoalkane and a hydrogen halide. Without controls, multiple substitutions occur producing CH₂Cl₂, CHCl₃ and CCl₄.

Free Radical Substitution Mechanism

The reaction of an alkane with a halogen proceeds by free radical substitution in three stages: initiation, propagation and termination.

A free radical is a species with an unpaired electron, represented by a single dot beside the atom (e.g. Cl•, CH₃•). Free radicals are highly reactive because they seek to pair the unpaired electron.

In curly half-arrow notation, a half-arrow (single-headed) represents the movement of one electron, in contrast to a full curly arrow which represents a pair of electrons.

The three stages, using methane and chlorine:

Initiation — UV light supplies energy to break the Cl–Cl bond by homolytic fission, producing two chlorine radicals:

Propagation — radicals react with stable molecules to produce new radicals, sustaining a chain reaction:

Termination — two radicals combine, removing both unpaired electrons and ending the chain:

A major limitation in synthesis is that further substitution occurs: CH₃Cl reacts with more Cl• to give CH₂Cl₂, CHCl₃ and CCl₄. The product is therefore always a mixture, and isolating one specific halogenoalkane in good yield is difficult.

This problem worsens with longer alkanes because substitution can occur on any C–H position, producing a mixture of structural isomers as well.

MnemonicI Pinch People Twice (Initiation, Propagation, Propagation, Termination)