Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They are the simplest class of organic compounds and form the backbone of many others. The primary source of hydrocarbons is crude oil, which is separated into various fractions by fractional distillation. This topic focuses on two fundamental families of hydrocarbons: alkanes and alkenes.

### Alkanes: Saturated Hydrocarbons

Alkanes are saturated hydrocarbons, meaning they contain only single covalent bonds between carbon atoms. Their general formula is CnH2n+2. The carbon atoms in alkanes are sp3 hybridized, resulting in a tetrahedral geometry with bond angles of approximately 109.5° around each carbon atom. These C-C and C-H single bonds are strong sigma (σ) bonds, which are covalent bonds formed by the direct, head-on overlap of orbitals.

Due to the strength and non-polar nature of their σ-bonds, alkanes are relatively unreactive. They do not react with acids, alkalis, or common oxidising agents. Their principal reactions are combustion and free-radical substitution.

Free-Radical Substitution

This is the characteristic reaction of alkanes, typically initiated by ultraviolet (UV) light. The reaction of methane (CH₄) with chlorine (Cl₂) to form chloromethane (CH₃Cl) is a classic example.

The mechanism occurs in three stages:

`Cl₂ --(UV light)--> 2Cl•`

* A chlorine radical attacks a methane molecule, abstracting a hydrogen atom to form hydrogen chloride and a methyl radical.

`Cl• + CH₄ --> •CH₃ + HCl`

* The newly formed methyl radical is also very reactive and attacks another chlorine molecule to form the product, chloromethane, and regenerates a chlorine radical.

`•CH₃ + Cl₂ --> CH₃Cl + Cl•`

This new chlorine radical can then attack another methane molecule, continuing the chain.

`Cl• + Cl• --> Cl₂`

`•CH₃ + •CH₃ --> C₂H₆`

`•CH₃ + Cl• --> CH₃Cl`

### Alkenes: Unsaturated Hydrocarbons

Alkenes are unsaturated hydrocarbons, characterised by the presence of at least one carbon-carbon double bond (C=C). Their general formula for one double bond is CnH2n.

The C=C double bond consists of two different types of bonds:

* One sigma (σ) bond: Formed by the direct overlap of sp2 hybrid orbitals between the carbon atoms.

* One pi (π) bond: Formed by the sideways overlap of the unhybridized p-orbitals, with electron density concentrated above and below the plane of the sigma bond.

The atoms involved in the double bond have a trigonal planar geometry with bond angles of 120°. The π-bond is weaker than the σ-bond and its exposed electron cloud makes it a site of high electron density. This makes alkenes significantly more reactive than alkanes.

Electrophilic Addition

The characteristic reaction of alkenes is electrophilic addition. An electrophile (an electron-pair acceptor) is attracted to the electron-rich C=C bond. The π-bond breaks, and the electrophile adds across the double bond, forming a new saturated compound.

Let's examine the mechanism for the reaction of ethene (CH₂=CH₂) with hydrogen bromide (HBr):

`CH₂=CH₂ + H-Br --> +CH₂-CH₃ + Br⁻`

`+CH₂-CH₃ + Br⁻ --> CH₂Br-CH₃`

This same mechanism applies to the addition of other molecules like Cl₂, H₂ (with a nickel catalyst), and H₂O (steam with an acid catalyst). The reaction with aqueous bromine (Br₂(aq)) is a key test for unsaturation; the orange-brown colour of bromine water is decolourised when added to an alkene as the bromine is added across the double bond.