A-level Chemistry/OCR/Chains, Energy and Resources/Basic Concepts and Hydrocarbons/Alkenes

Alkenes

The alkenes are unsaturated chains of carbon & hydrogen with one or more double or triple carbon carbon bonds. The simplest alkene is ethene $(C_{2}H_{4})$ the general formula for simple alkenes is $C_{n}H_{2n}$. Notably, simple alkenes all end in the "ene" suffix, as opposed to the alkanes' "ane" suffix. Another thing to note about alkenes is in their naming. The alkene functional group is the double bond, and thus when naming the compound the double bond defines whether it is, for example, pent-1-ene or pent-2-ene.

$CH_{2}=CHCH_{2}CH_{2}CH_{3}$ is pent-1-ene.

$CH_{3}CH=CHCH_{2}CH_{3}$ is pent-2-ene.

Straight chain alkenes have marginally lower boiling points than their alkane counterparts & like the alkanes (and pretty much all homologous series), all simple alkenes have the same general formula, have similar chemical properties & show a trend in physical properties.

Bonding in Alkenes

The alkenes have a carbon-carbon double bond, this means that they have one σ (sigma) bond, and also one pi bond. The bonding in alkanes only consists of σ (sigma) bonds which means they're able to be twisted around. However the pi bond along with the sigma bond in alkenes prevent free rotation. This gives rise to cis-trans isomerism, in which the larger groups on the alkene molecule are either on one side together (cis, Z) or on opposite sides to each other (trans, E).

The pi bond occurs due to the side by side overlap of adjacent p orbitals. It is weak, and can be broken easily.

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