Alkenes

Cracking (u4:c23) does more than top up the petrol supply — it also splits large alkane molecules into smaller ones that carry a very different bonding feature: a carbon-carbon double bond. That one structural difference defines a whole new family of hydrocarbons, the alkenes, and it makes them far more reactive than the alkanes you have just studied (u4:c24). Where an alkane chain is chemically quiet, an alkene's double bond is a ready-made handle that other molecules can grab — and the entire chemistry of this chapter hangs off that handle.

The double bond has a formal name in organic chemistry: it is the functional-group of the family, written >C=C< to show the two free valences pointing outwards from the pair of carbon atoms. Every member of the series carries exactly this group, and the group decides how the members react. The alkenes are therefore a homologous-series in the full five-part sense you met in the introduction to organic chemistry (u4:c22): one functional group, similar chemical properties, a trend in physical properties, one general formula, and consecutive members stepping up by a –CH₂– unit.

The first three members of the family are laid out below — read the columns the same way you read the alkane table in the previous lesson.

Each molecule can also be drawn as a displayed-formula, with every bond shown as an explicit line. For ethene that is two carbon atoms joined by a double line, each carbon also bonded to two hydrogen atoms by single lines — a flat H₂C=CH₂ arrangement. Propene adds a –CH₃ group on the second carbon, and but-1-ene lengthens the chain by one more carbon with the double bond still between the first two. Drawing these three, and naming the straight-chain members, is precisely the skill specification statement 4.26 trains, and Sections 2 and 4 of this lesson drill it.

Notice one consequence hiding in the table: every alkene has the same empirical formula, CH₂, no matter how big the molecule gets. That is a direct fingerprint of the double bond — it will be promoted to the family's general formula in Section 4. Another consequence sits at the very start of the series: there is no one-carbon alkene, because a C=C group needs two carbon atoms to exist. The smallest possible alkene is ethene, full stop.