Structure, Bonding & Reactivity (AQA A-Level Chemistry): Revision Notes

3.4.1 Structure, Bonding & Reactivity

Alkenes: Structure, Bonding, and Reactivity

Alkenes are an important class of hydrocarbons known for their characteristic carbon-carbon double bonds ($C=C$). These bonds make alkenes unsaturated, meaning they do not contain the maximum number of hydrogen atoms that a hydrocarbon can hold.

General Formula of Alkenes

The general formula for alkenes is $CₙH₂ₙ$, where $n$ represents the number of carbon atoms.

This formula applies to straight-chain and branched alkenes but excludes cyclic alkenes, which follow a different formula.

Bonding in Alkenes

Alkenes contain at least one carbon-carbon double bond ($C=C$).

This double bond consists of two parts:

• A sigma ($σ$) bond, formed by the overlap of two sp² hybrid orbitals between the carbon atoms.
• A pi ($π$) bond, formed by the side-to-side overlap of unhybridised p-orbitals above and below the plane of the sigma bond.

Characteristics of the Carbon-Carbon Double Bond

• The $C=C$ bond in alkenes creates a region of high electron density, making the bond more reactive compared to single $C-C$ bonds.
• Bonding around the $C=C$ bond is planar, with the atoms directly attached to the carbons of the double bond arranged in a trigonal planar geometry.
• The bond angle around each carbon in the double bond is approximately 120°.

Restricted Rotation in Alkenes

Unlike single bonds, the $C=C$ double bond exhibits restricted rotation due to the presence of the pi bond.

• This means the atoms attached to the carbons cannot rotate freely around the double bond. This restricted rotation is the reason why geometrical isomerism (cis/trans or E/Z isomerism) is observed in alkenes, particularly when the carbon atoms involved in the double bond have different groups attached.

Reactivity of Alkenes

The pi bond in the $C=C$ double bond is weaker than the sigma bond, making it more susceptible to reactions.

Alkenes undergo a range of addition reactions where the pi bond is broken, and new atoms or groups are added to the carbon atoms.

Typical reactions include:

• Electrophilic addition, where the high electron density of the double bond attracts electrophiles (electron-deficient species).
• Hydrogenation, where hydrogen is added across the double bond to form an alkane.
• Halogenation, where halogens (e.g., $Br₂$, $Cl₂$) add to the double bond.

Nomenclature of Alkenes

The ene suffix is used for compounds with one double bond (e.g., ethene, propene).

• When there is more than one double bond, the names end in diene (two double bonds) or triene (three double bonds).