alkene

Meaning: An alkene has a C=C made of one σ bond and one π bond. Each alkene carbon is sp² hybridized and trigonal planar, with angles near 120°. The π electrons lie above and below the plane, are loosely held, and act as a nucleophile toward electrophiles. Rotation about C=C is restricted, so cis/trans (E/Z) isomers exist when each alkene carbon bears two different groups. More substituted alkenes are generally more stable.

Equation: CnH2n for an acyclic alkene with one double bond, where n is the number of carbon atoms (a pure number, no unit).

On the MCAT: Expect electrophilic addition. HX adds by Markovnikov's rule through the more stable carbocation. H₂ with a metal catalyst adds to the same face (syn), while Br₂ adds to opposite faces (anti) through a bromonium ion. Alkenes also form by E1 or E2 elimination, where the more substituted product (Zaitsev) usually predominates. Questions ask you to name a product, predict regiochemistry or stereochemistry, or count degrees of unsaturation.

Common confusion: A double bond is stronger than a single bond, but its π component is weaker than the σ bond. That weakness is why alkenes react by addition, which breaks the π bond and keeps the σ bond. Benzene, despite its drawn double bonds, resists addition because aromatic stability is lost.