Alkenes and Alkynes I: Properties and Synthesis – Elimination Reactions of Alkyl Halides

Chapter 7: Alkenes and Alkynes I – Properties and Synthesis

Elimination Reactions of Alkyl Halides

This chapter introduces the properties and synthesis of alkenes and alkynes, focusing on elimination reactions of alkyl halides. Key concepts include the stereochemistry and stability of alkenes, mechanisms of elimination reactions, and the application of Zaitsev's Rule.

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

Cahn-Ingold-Prelog Convention

The Cahn-Ingold-Prelog (CIP) convention is used to assign priorities to substituents on each carbon of a double bond, allowing for the unambiguous designation of alkene stereochemistry.

• (E)-Isomer: The groups of highest priority on each carbon are on opposite sides of the double bond (from the German 'entgegen').

• (Z)-Isomer: The groups of highest priority are on the same side of the double bond (from the German 'zusammen').

• Priority is determined by atomic number: higher atomic number = higher priority.

Example: (Z)-2-Bromo-1-chloro-1-fluoroethene has Br and F (highest priorities) on the same side; (E)-1-Bromo-1,2-dichloroethene has Br and Cl on opposite sides.

Relative Stabilities of Alkenes

Cis vs. Trans Alkenes

Alkene stability is influenced by the arrangement of substituents around the double bond.

• Trans (E) alkenes are generally more stable than cis (Z) alkenes due to reduced steric hindrance between bulky groups.

• Steric hindrance in cis alkenes raises their energy, making them less stable.

Heat of Hydrogenation

The heat of hydrogenation is the enthalpy change when an alkene is hydrogenated to an alkane. It is used to compare alkene stabilities.

• More stable alkenes have lower heats of hydrogenation (less exothermic).

• To compare stabilities, the same alkane product must be obtained from each alkene.

Example: The heat of hydrogenation for trans-2-butene is less negative than for cis-2-butene, indicating greater stability.

Substitution and Alkene Stability

Alkene stability increases with the number of alkyl substituents attached to the double-bonded carbons.

• Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted > Unsubstituted

Synthesis of Alkenes via Elimination Reactions

Dehydrohalogenation

Dehydrohalogenation is the removal of a hydrogen atom and a halogen atom from adjacent carbons, forming a double bond (alkene).

• E2 mechanism is most useful for alkene synthesis.

• E1 reactions can be problematic due to carbocation rearrangements.

• E2 reactions are favored by:

  • Secondary or tertiary alkyl halides

  • Strong bases (e.g., sodium ethoxide, potassium tert-butoxide)

• Bulky bases (e.g., potassium tert-butoxide) are used for E2 reactions of primary alkyl halides to avoid substitution.

General E2 Reaction Equation: $\ce{R-CH_2-CH_2-X + B^- \rightarrow R-CH=CH_2 + B-H + X^-}$

Zaitsev’s Rule: Formation of the Most Substituted Alkene

Regioselectivity in Elimination

When multiple elimination products are possible, the most substituted (most stable) alkene is usually favored. This is known as Zaitsev's Rule.

• Small bases (e.g., ethoxide) favor the more substituted alkene.

• Bulky bases (e.g., tert-butoxide) may favor the less substituted alkene due to steric hindrance.

Example: Dehydrohalogenation of 2-bromo-2-methylbutane with ethoxide yields mainly 2-methyl-2-butene (more substituted), while tert-butoxide yields more 2-methyl-1-butene (less substituted).

Transition State and Kinetic Control

• The transition state in E2 elimination has partial double bond character.

• More substituted (alkene-like) transition states are lower in energy (lower $\Delta G^\ddagger$).

• Product distribution can be under kinetic or thermodynamic control, depending on reaction conditions.

Formation of the Least Substituted Alkene Using a Bulky Base

Bulky bases such as potassium tert-butoxide preferentially remove less hindered (primary) hydrogens, leading to the formation of the less substituted alkene (Hofmann product).

• This is due to steric hindrance preventing access to more substituted (secondary or tertiary) hydrogens.

Example: Elimination of 2-bromo-2-methylbutane with tert-butoxide yields mainly 2-methyl-1-butene (less substituted).

The Stereochemistry of E2 Reactions: Orientation of Groups in the Transition State

Anti and Syn Elimination

For an E2 reaction to occur, the hydrogen and leaving group must be coplanar (in the same plane).

• Anti-coplanar (anti-periplanar) arrangement is preferred: the hydrogen and leaving group are on opposite sides of the molecule, leading to a staggered transition state.

• Syn-coplanar elimination is less common and occurs only in rigid systems.

Conformational Effects

• In cyclohexane systems, elimination occurs only when the leaving group and the hydrogen are both axial (anti-coplanar).

• Newman projections can be used to visualize the required geometry for E2 elimination.

Example: In trans-1-chloro-2-methylcyclohexane, elimination occurs only when both the chlorine and the β-hydrogen are axial.

Alkene Substitution	Relative Stability	Heat of Hydrogenation (kJ/mol)
Tetrasubstituted	Most stable	Least negative
Trisubstituted	More stable	Less negative
Disubstituted (trans)	Stable	Intermediate
Disubstituted (cis)	Less stable	More negative
Monosubstituted	Least stable	Most negative

Additional info: The notes above are based on standard organic chemistry concepts and the provided slides. Some explanations and examples have been expanded for clarity and completeness.