Alkene Structure And Nomenclature

Alkene Structure and Nomenclature

Alkene Structure and Nomenclature

Alkenes are unsaturated hydrocarbons with at least one C=C double bond (general formula CnH2n). Each alkene carbon is sp², giving ~120° bond angles and a planar, rotation-restricted double bond that drives reactivity and stereochemistry.

What Defines an Alkene

  • Acyclic alkenes follow CnH2n; rings adjust hydrogen count.
  • The double bond = one σ + one π bond; the π bond is electron rich → alkenes are nucleophilic and undergo electrophilic addition.
  • No free rotation about C=C → geometric isomerism (cis/trans or E/Z) when each sp² carbon bears two different groups.

IUPAC Naming Rules

  • Parent chain includes the double bond; suffix -ene.
  • Lowest locant for the double bond; locant placed before “-ene” (e.g., hex-2-ene).
  • Multiple C=C: “-diene”, “-triene”, etc., with all locants (e.g., penta-1,4-diene).
  • Substituents named/numbered like alkanes; number from the end nearest the C=C.
  • Rings: “cyclo” + locant 1,2 for the C=C; number substituents for lowest set.
  • Geometric tags when applicable: cis/trans or E/Z (CIP priority).

Examples (acyclic and cyclic)

penta-1,4-diene
IUPAC: penta-1,4-diene

cyclohexa-1,4-diene
IUPAC: cyclohexa-1,4-diene

but-2-ene
IUPAC: but-2-ene

2-methylprop-1-ene
IUPAC: 2-methylprop-1-ene

cyclopent-1-ene
IUPAC: cyclopent-1-ene

Summary

  • Alkenes are sp², π-rich, and conform to CnH2n (acyclic).
  • Name by longest chain containing C=C, lowest locant for the double bond, “-ene” suffix, and E/Z or cis/trans when needed.
  • Planarity and restricted rotation set up geometric isomerism and guide reactivity (electrophilic additions).