Nomenclature and Structure of the Double Bond in Alkenes

Alkenes are named by the same longest-chain rule as alkanes, except that the parent chain must

include both carbons of the double bond, the ending changes from '-ane' to '-ene', and the chain

is numbered from whichever end gives the double bond the lower of the two possible locants; that

locant is cited immediately before the suffix, e.g. but-1-ene (double bond between C1 and C2) as

against but-2-ene (double bond between C2 and C3) -- two genuinely different compounds, not the

same one written two ways.

Structure of the double bond. Take the simplest alkene, ethene ($\text{CH}_2{=}\text{CH}_2$)

as the reference case. Each carbon undergoes $sp^2$ hybridisation, mixing one $s$ and two $p$

orbitals to give three equivalent $sp^2$ hybrid orbitals lying in a single plane at $120^\circ$ to

one another, plus one unhybridised $p$ orbital left perpendicular to that plane. Two of a carbon's

three $sp^2$ orbitals form sigma bonds to its two hydrogens, and the third forms a sigma bond,

head-on, to the $sp^2$ orbital of the other carbon; since both carbons and all four hydrogens sit

in the plane defined by the $sp^2$ orbitals, the whole $\text{H}_2\text{C}{=}\text{CH}_2$

skeleton is planar, with $\text{H--C--H}$ and $\text{H--C--C}$ bond angles close to the ideal

$120°$ (in practice about $117°$ and $121.5°$, distorted slightly by the difference between the

double and single bonds).

The two carbons' unhybridised $p$ orbitals, both perpendicular to the molecular plane and parallel

to each other, overlap sideways -- above and below the plane -- to form a pi bond, a second bond

that supplements the sigma bond already joining the two carbons. The $\text{C}{=}\text{C}$

double bond is therefore one sigma bond (from head-on $sp^2$–$sp^2$ overlap) plus one pi bond

(from sideways $p$–$p$ overlap): shorter ($\approx 134$ pm, against $154$ pm for a

$\text{C--C}$ single bond) and stronger overall, but the pi component specifically is weaker than

a sigma bond -- it is this comparatively weak, exposed pi electron cloud, sitting above and below

the plane of the molecule, that is attacked first by electrophiles in essentially every …