3.3.4.1 - Structure, Bonding and Reactivity (Alkenes)

What makes an alkene an alkene

Alkenes are hydrocarbons, so they contain carbon and hydrogen only. What makes them a separate homologous series is that they contain at least one carbon-carbon double bond, written as $\ce{C=C}$.

That definition is worth learning carefully. "Unsaturated" does not just mean "reactive" or "contains a bond". It means the molecule could, in principle, add more atoms because the double bond is present. For example, ethane is $\ce{C2H6}$, but ethene is $\ce{C2H4}$. Ethene has two fewer hydrogen atoms because two of the carbon atoms are joined by a double bond instead of a single bond.

An acyclic alkene with one double bond has general formula $\ce{C_nH_{2n}}$. A cycloalkane has the same general formula but only single bonds. Thus $\ce{C4H8}$ could be but-1-ene or cyclobutane: the molecular formula alone does not establish unsaturation. An acyclic alkane with four carbons would be $\ce{C4H10}$.

The diagram below compares a saturated alkane with an unsaturated alkene and highlights the $\ce{C=C}$ bond that makes the difference.

[DIAGRAM: asset_name: 3.4.1 - Structure, Bonding and Reactivity (Alkenes) - Diagram 1; asset_slug: 3.4.1 - Structure, Bonding and Reactivity (Alkenes) - Diagram 1; recommended_method: retained_png; description: Displayed formulae of ethane and ethene shown side by side in a balanced comparison. Draw ethane as CH3-CH3 with all six hydrogen atoms shown and label it saturated. Draw ethene as CH2=CH2 with all four hydrogen atoms shown and label it unsaturated. Lightly emphasize the C=C double bond in ethene so the structural difference is immediately visible.]

For this topic, the key structural idea is simple: when you see $\ce{C=C}$ in a hydrocarbon, you should immediately think "alkene", "unsaturated", and "more reactive than an alkane".

That structural feature is not just a naming detail. It is the reason alkenes have a distinctive bonding picture and a distinctive pattern of reactions.

What the double bond contains

A carbon-carbon double bond is still covalent, but it is not a single shared region of electrons. It contains two different parts: one sigma bond and one pi bond.

The sigma bond is formed by end-on overlap of orbitals along the line between the two carbon nuclei. The pi bond is formed by sideways overlap of p orbitals. That sideways overlap puts electron density above and below the plane of the sigma bond.

This is why the $\ce{C=C}$ bond is described as a centre of high electron density. The four bonding electrons associated with the double bond are not spread evenly through the whole molecule. A particularly electron-rich region sits around the double bond, especially in the pi bond.