4dAlkenes

Alkenes differ from alkanes by one feature, and everything else follows from it.

The functional group

The carbon-carbon double bond, C=C.

Be precise: it is a carbon-carbon double bond. Saying "a double bond" is ambiguous, since C=O appears in carboxylic acids and esters and behaves quite differently.

The general formula

CₙH₂ₙ

The hydrogen count is exactly twice the carbon count.

There is no alkene with one carbon, because a C=C bond needs two carbons to exist.

Telling an alkane from an alkene by formula

Double the carbons and compare with the hydrogens:

• Equal → alkene (CₙH₂ₙ)
• Two more → alkane (CₙH₂ₙ₊₂)

This works for the two homologous series in this course. The formula alone is not absolute proof — a cycloalkane also fits CₙH₂ₙ — so where certainty matters, the bromine water test or the structure decides it.

So C₄H₈ is butene and C₄H₁₀ is butane.

Unsaturated

Unsaturated means the molecule contains a carbon-carbon double bond, so more atoms can be added to it.

Both halves matter. The double bond is present, and that is what allows addition.

Why an alkene has two fewer hydrogens

Ethene, C₂H₄, and ethane, C₂H₆, both have two carbons. Every carbon still forms four bonds in each — but in ethene, two of those bonds go to the same neighbouring carbon.

That uses up two bonding positions that would otherwise hold hydrogens, so two fewer hydrogens fit.

Why alkenes are more reactive

One bond of the double bond can open, freeing a bonding position on each carbon so atoms can add across it.

Alkanes have no such site, which is why they must react by substitution instead. The double bond is the whole difference in reactivity between the two families.

Structural formulae

The double bond is written with an = between the two carbons it joins:

Writing C₂H₄ gives the molecular formula, which does not show the functional group. If a question asks for the structural formula, the = must appear.

Addition with bromine

ethene + bromine → dibromoethane

C₂H₄ + Br₂ → C₂H₄Br₂

The double bond opens to a single bond and both bromine atoms join the molecule. There is one product and nothing left over — which is what makes it an addition reaction.

The product is a dibromoalkane, with two bromine atoms. For propene it is C₃H₆Br₂, dibromopropane: add the two formulae together and you have it.

The bromine water test

This is the standard test for unsaturation and appears constantly.

1. Add orange bromine water to the sample and shake.
2. An alkene decolourises it — orange to colourless.
3. An alkane leaves it orange — no change.

Say colourless, not clear. The orange solution was already clear in the sense of being transparent; what changes is the colour.

What the test actually shows

For a hydrocarbon, decolourising bromine water shows a carbon-carbon double bond.

Worth knowing the limitation: other substances such as sulfur dioxide also decolourise bromine water by a different reaction. So the conclusion holds when the substance is known to be a hydrocarbon.

Degrees of unsaturation

The test can also compare how much unsaturation is present — but only if equal amounts of each sample and the same volume of bromine water are used. Without that control it is the qualitative test described above and nothing more.

Under controlled conditions, margarine decolourises bromine water more than butter does, which shows margarine contains more carbon-carbon double bonds — it is the more unsaturated fat.

Isomerism in alkenes

Butene, C₄H₈, has more than one structure. The double bond can sit between the first and second carbons or between the second and third, and the chain can also branch.

So alkene isomerism comes from the position of the double bond as well as from chain branching.