What you'll learn

• Why alkanes are useful fuels, but can produce harmful combustion products.
• How the bonding in alkanes and alkenes explains their very different reactions.
• How to draw and explain radical substitution and electrophilic addition mechanisms.
• How E–Z isomerism, hydrogenation and addition polymerisation arise from the C=C bond.

Hydrocarbons: the basics

Crude oil and natural gas contain many hydrocarbons. They are separated and processed by the petroleum industry to provide fuels and chemical feedstocks, including alkenes used to make polymers.

For open-chain compounds, alkanes have general formula $C_n{H}_{2n+2}$. Alkenes with one C=C double bond have general formula $C_n{H}_{2n}$.

Alkanes as fuels

Complete and incomplete combustion

Combustion is reaction with oxygen. In complete combustion, an alkane burns in plentiful oxygen to form carbon dioxide and water.

For methane:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

In limited oxygen, incomplete combustion can produce carbon monoxide, carbon, and water. Carbon monoxide is especially dangerous because it is a toxic gas that binds strongly to haemoglobin in the blood.

Benefits and drawbacks of fossil fuels

A fossil fuel is a carbon-containing fuel formed over millions of years from ancient biological material. Alkanes are valuable fossil fuels because they have high energy density, are easy to transport, and existing engines and power stations are designed around them.

The drawbacks are significant:

• CO₂ is a greenhouse gas, so large-scale fossil fuel use contributes to climate change.
• Sulfur impurities can form SO₂, and high-temperature engines form nitrogen oxides, NO and NO₂. These are acidic gases and contribute to acid rain.
• Incomplete combustion produces carbon monoxide and particulates.
• Fossil fuels are finite resources.

Why alkanes are relatively unreactive

A covalent bond is a shared pair of electrons. In alkanes, the C–C and C–H bonds are all $\sigma$-bonds.

C–C and C–H $\sigma$-bonds are quite strong and are not very polar, so alkanes do not usually react with acids, bases or electrophiles. Their important reaction, apart from combustion, is radical substitution with halogens.

Radical substitution: photochlorination

A radical is a species with an unpaired electron, shown using a dot, such as Cl•. Homolytic fission is bond breaking where each atom takes one electron from the bond, forming radicals.

In photochlorination, chlorine reacts with an alkane under ultraviolet light. With methane, one substitution product is chloromethane:

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

The radical-substitution chain mechanism is summarised below.

The three stages

Initiation creates radicals:

Cl₂(g) → 2Cl•(g)

Condition: ultraviolet light.

Propagation uses radicals and regenerates radicals:

Cl•(g) + CH₄(g) → HCl(g) + •CH₃(g)

•CH₃(g) + Cl₂(g) → CH₃Cl(g) + Cl•(g)

Termination removes radicals by combining them:

Cl• + Cl• → Cl₂

•CH₃ + Cl• → CH₃Cl

•CH₃ + •CH₃ → C₂H₆

Alkenes: the reactive C=C bond

Alkenes are more reactive than alkanes because the C=C double bond is a region of high electron density. A C=C double bond consists of one $\sigma$-bond and one $\pi$-bond.

The $\pi$-bond is exposed and electron-rich, so it attracts electrophiles. It also prevents free rotation about the C=C bond.

E–Z isomerism

Stereoisomers have the same structural formula but a different arrangement of atoms in space. E–Z isomerism occurs because rotation about a C=C double bond is restricted.

Priority is usually decided by atomic number: the atom with the higher atomic number has higher priority.

Electrophilic addition

An electrophile is an electron-pair acceptor. Alkenes undergo electrophilic addition, where an electrophile attacks the electron-rich C=C bond and atoms add across the double bond.

A carbocation is an organic ion containing a positively charged carbon atom.

The key electron-pair movements for bromine addition and HBr addition are shown below.

Addition of bromine to ethene

Overall reaction:

CH₂=CH₂(g) + Br₂(aq) → CH₂BrCH₂Br(l)

The mechanism is:

1. The C=C $\pi$-bond induces a dipole in Br₂, making the nearer bromine Brδ+.
2. A curly arrow goes from the alkene $\pi$-bond to Brδ+.
3. A curly arrow goes from the Br–Br bond to the other bromine atom, forming Br⁻.
4. Br⁻ attacks the carbocation to form 1,2-dibromoethane.

Tests for alkenes

Bromine water is orange or brown. With an alkene, it is decolourised because bromine adds across the C=C bond. An alkane gives no rapid change in the dark.

Potassium manganate(VII), KMnO₄, is purple. Cold, dilute potassium manganate(VII) is decolourised by alkenes because the C=C bond is oxidised. In alkaline conditions, a brown MnO₂ precipitate may be seen.

Addition of HBr to propene: orientation

When HBr adds to propene, two products are possible, but one is major.

CH₃CH=CH₂ + HBr → mainly CH₃CHBrCH₃

The major product is 2-bromopropane.

This happens because the reaction goes through the more stable carbocation. A secondary carbocation is more stable than a primary carbocation because alkyl groups donate electron density towards the positive carbon.

Catalytic hydrogenation

Hydrogenation is addition of hydrogen across a C=C double bond to form an alkane.

CH₂=CH₂(g) + H₂(g) → CH₃CH₃(g)

Conditions: hydrogen gas with a nickel catalyst at about 150 °C.

This reaction is important because it converts unsaturated compounds into saturated ones. Industrially, related hydrogenation reactions are used in processing vegetable oils and in making useful saturated feedstocks.

Addition polymerisation

Addition polymerisation happens when many alkene monomers join together. The C=C double bond opens up and new C–C single bonds form between monomers. No small molecule is eliminated.

For propene:

$$n{CH}_2={CHCH}_3\to {\left[{-CH}_2-CH({CH}_3)-\right]}_n$$

Polymers from alkenes and substituted alkenes are economically important because they are cheap, light, strong, waterproof and chemically resistant. Examples include poly(ethene) for bags and bottles, poly(propene) for packaging and fibres, and PVC for pipes and insulation.

The drawbacks are also important: many polymers are made from fossil-fuel feedstocks, are non-biodegradable, and can form persistent waste or microplastics. Recycling, reuse and careful disposal are therefore part of the chemistry story, not an afterthought.