Alkenes and Alkynes: Properties and Synthesis

Introduction to Alkenes and Alkynes

Alkenes and alkynes are unsaturated hydrocarbons characterized by carbon-carbon double and triple bonds, respectively. Their chemical properties and synthesis are central topics in organic chemistry, especially regarding elimination reactions of alkyl halides.

Elimination Reactions of Alkyl Halides

Mechanisms of Elimination: E2 and E1

• E2 Mechanism: A bimolecular elimination where the base removes a proton from the β-carbon as the leaving group departs from the α-carbon, forming a double bond in a single concerted step.

• E1 Mechanism: A unimolecular elimination involving two steps: formation of a carbocation intermediate after the leaving group departs, followed by deprotonation to form the alkene.

• Key Points:

  • E2 reactions are favored by strong bases and occur with secondary or tertiary alkyl halides.

  • Bulky bases (e.g., potassium tert-butoxide) favor elimination at less hindered positions, leading to less substituted alkenes.

  • Small bases (e.g., ethoxide) favor formation of the most substituted (Zaitsev) alkene.

• Example:

Stereochemistry of E2 Reactions

• For E2 elimination, the hydrogen and leaving group must be antiperiplanar (in the same plane but opposite sides).

• In cyclohexane systems, elimination occurs only when the leaving group and hydrogen are both axial.

• Example: Additional info: In cyclohexane, flipping the chair conformation may be necessary to achieve the correct geometry for elimination.

(E)-(Z) System for Designating Alkene Diastereomers

Ingold-Prelog Convention

• Assign priorities to groups attached to each carbon of the double bond.

• If the highest priority groups are on the same side, the alkene is Z (zusammen, together).

• If on opposite sides, the alkene is E (entgegen, opposite).

• Example: (Z)-2-Butene: (cis-2-butene)

Relative Stabilities of Alkenes

Cis vs. Trans Alkenes

• Cis alkenes are generally less stable than trans alkenes due to steric hindrance (crowding of substituents).

• Heat of Hydrogenation: The stability of alkenes can be measured by the exothermic heat released during hydrogenation. Lower heat of hydrogenation indicates greater stability.

• Example: (cis-2-butene) > (trans-2-butene)

Overall Relative Stability of Alkenes

• Alkene stability increases with the number of alkyl groups attached to the double bond (degree of substitution).

• Order of Stability: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted > Unsubstituted

• Example Table:

  Alkene Type	Number of Alkyl Groups	Relative Stability
  Tetrasubstituted	4	Highest
  Trisubstituted	3	High
  Disubstituted	2	Moderate
  Monosubstituted	1	Low
  Unsubstituted	0	Lowest

Synthesis of Alkenes via Elimination Reactions

Zaitsev's Rule

• When elimination can yield more than one alkene, the most highly substituted alkene is the major product (Zaitsev product).

• This rule applies when a small base is used; bulky bases may favor the less substituted (Hofmann) product.

• Example: (major, Zaitsev) (major, Hofmann)

Bulky vs. Small Bases in Elimination

• Small bases (e.g., ethoxide) favor elimination at the most substituted position.

• Bulky bases (e.g., tert-butoxide) favor elimination at less hindered, more accessible positions.

• Example:

Summary Table: Elimination Reaction Outcomes

Conclusion

Understanding the mechanisms and outcomes of elimination reactions is essential for predicting and controlling the synthesis of alkenes and alkynes. The choice of base, substrate structure, and stereochemistry all play critical roles in determining the major product and its stability.

Additional info: These notes cover foundational concepts for college-level organic chemistry, including mechanistic details, stereochemistry, and practical synthetic strategies.