Carbon-carbon double bonds are the defining feature of alkenes. These bonds consist of a sigma bond and a pi bond, creating a planar structure with restricted rotation. This unique geometry gives rise to cis-trans isomerism, where different spatial arrangements of substituents lead to distinct molecules.

Understanding alkene structure and isomerism is crucial for predicting reactivity and properties. We'll explore how to identify, draw, and name alkene isomers using cis-trans and E/Z systems. This knowledge forms the foundation for understanding more complex organic reactions and mechanisms.

Structure and Isomerism of Alkenes

Structure of carbon-carbon double bonds

• Carbon-carbon double bond composed of one sigma ($\sigma$) bond and one pi ($\pi$) bond
  • $\sigma$ bond formed by direct overlap of sp2 hybrid orbitals along the internuclear axis
  • $\pi$ bond formed by sideways overlap of unhybridized p orbitals above and below the molecular plane
• Electron density of the $\pi$ bond concentrated above and below the plane of the molecule
  • Restricts rotation around the double bond axis (restricted rotation)
  • Rotation would require breaking the $\pi$ bond, an energetically unfavorable process (ethene)
• Each carbon of the double bond has a planar, trigonal geometry with bond angles of approximately 120°
  • Remaining sp2 hybrid orbitals used to form $\sigma$ bonds to other atoms or groups (methyl, ethyl)
• Double bond planarity is a key feature of alkene structure

Conditions for cis-trans isomerism

• Alkene must have two different substituents attached to each carbon of the double bond
  • Substituents can be atoms or groups other than hydrogen (chlorine, bromine)
• Isomers are stereoisomers with the same connectivity but different spatial arrangement (geometric isomerism)
  • Cis isomer has both higher priority substituents on the same side of the double bond plane
  • Trans isomer has higher priority substituents on opposite sides of the double bond plane
• Cis-trans isomers cannot interconvert without breaking the $\pi$ bond of the double bond
  • Configurational isomers, not conformational isomers that can rotate freely (butane)
• Alkenes with two identical substituents on one or both carbons do not exhibit cis-trans isomerism
  • Meso compounds with a plane of symmetry bisecting the double bond (2-butene)

Drawing and naming alkene isomers

1. Determine the higher priority substituent on each carbon using Cahn-Ingold-Prelog rules

   • Higher atomic number takes precedence (bromine > chlorine)
   • For substituents with the same first atom, compare subsequent atoms until a difference is found

2. Draw the isomers with higher priority substituents on the same side (cis) or opposite sides (trans)

   • Represent the double bond with a solid and dashed wedge to depict the spatial arrangement

3. Name the isomers using the IUPAC nomenclature system

   • Choose the longest continuous carbon chain containing the double bond as the base name (butene)
   • Number the chain to give the double bond the lowest possible number (1-butene, not 3-butene)
   • Indicate the position of the double bond with the lower numbered carbon (1-butene, not 2-butene)
   • Assign cis or trans as a prefix based on the arrangement of higher priority substituents
   • Include any additional substituents as prefixes with their corresponding position numbers (4-chloro-2-pentene)

Stereochemistry and E/Z System

• Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules
• The E/Z system is an alternative nomenclature for describing geometric isomers
  • E (entgegen) corresponds to higher priority groups on opposite sides
  • Z (zusammen) corresponds to higher priority groups on the same side
• The E/Z system is particularly useful for more complex alkenes where cis-trans terminology may be ambiguous