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.

$$ CH_2 = CH CH_2 CH_2 CH_3 $$ is pent-1-ene.

$$ CH_3 CH = CH CH_2 CH_3 $$ 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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