Organic Chemistry - Introduction to Alkenes and Carbocations

Alkenes

Definition and Structure

• Alkenes contain a carbon-carbon double bond.
• They differ from alkanes, which have only single bonds, and from aldehydes, which have carbon-oxygen double bonds.

Nomenclature

• Identify the longest carbon chain containing the double bond.
• Use the appropriate prefix based on the number of carbons (e.g., "but" for four carbons).
• Indicate the presence of double bonds with "-ene".
• Number the chain to give the double bond the lowest possible number.
• Example: "1-butene" vs. "2-butene".

Isomerism

• Cis-Trans Isomerism:
  • Cis: The substituents are on the same side of the double bond.
  • Trans: Substituents are on opposite sides.
• E-Z Isomerism for tri- or tetra-substituted alkenes:
  • E: High-priority groups on opposite sides.
  • Z: High-priority groups on the same side.

Carbocations

Definition

• A carbocation is a carbon atom with a positive charge and an empty p-orbital.
• Structure: SP2 hybridized, planar, with 120° bond angles.

Stability

• Stability increases with more alkyl groups attached:
  • Tertiary (three alkyl groups) > Secondary (two alkyl groups) > Primary (one alkyl group).

Formation

• Formed during reactions involving alkenes, such as electrophilic addition.
• Alkenes act as nucleophiles and can form carbocations by attacking electrophiles like hydrogen ions.

Stabilization Mechanisms

• Inductive Effect: Electron donation from adjacent sigma bonds (carbon-carbon bonds).
• Hyperconjugation: Involves delocalization of electrons from C-H bonds into an empty p-orbital.
  • Requires neighboring carbon to be SP3 hybridized.

Application

• Understanding carbocations and their stability is essential for predicting reaction mechanisms involving alkenes.

Summary

• Alkenes and carbocations play a crucial role in organic chemistry, particularly in understanding reaction mechanisms.
• Familiarize with nomenclature, isomerism, and stability principles for effective chemical analysis.

Further Study

• Review this material, practice nomenclature, and apply concepts in problem sets.
• Consider consulting additional resources or attending office hours for further clarification.