This section is the structural foundation for all of alkene reactivity. The C=C double bond is one sigma + one pi bond; the pi bond both blocks rotation (creating cis/trans and E/Z isomerism) and serves as the electron-rich, nucleophilic site that drives addition reactions in later tracks. Each alkene carbon is sp² hybridized, trigonal planar, ~120°.

The central quantitative idea is alkene stability, which increases with substitution: tetrasubstituted > trisubstituted > disubstituted > monosubstituted. More alkyl groups on the double bond stabilize it through hyperconjugation and the fact that sp²–sp³ bonds are stronger. This "more substituted = more stable" rule is what Zaitsev's rule later invokes to predict the major product of elimination reactions, and it is measured experimentally by heats of hydrogenation (less stable alkenes release more heat when reduced to the same alkane).

Two more stability points: for disubstituted alkenes, trans (E) is generally more stable than cis (Z) because it minimizes steric strain between the bulky groups; and conjugated double bonds (alternating with single bonds) are more stable than isolated ones, foreshadowing the dienes track.

Finally this section should firmly install E/Z assignment as the rigorous replacement for cis/trans (which fails when a carbon bears two different non-H groups). Apply Cahn–Ingold–Prelog priorities to the two groups on each double-bond carbon: higher priority on the same side = Z (zusammen), on opposite sides = E (entgegen). Degree of unsaturation should also be flagged here: each C=C (or ring) removes two hydrogens from the saturated formula (CₙH₂ₙ₊₂), letting students compute degrees of unsaturation to predict double bonds and rings from a molecular formula.

"Double the Trouble"