Alkenes: Structure and Synthesis – Key Concepts and Mechanisms

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CHAPTER 11: Alkenes – Structure and Synthesis

Introduction to Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Their structure, nomenclature, and reactivity are central topics in organic chemistry, with implications for synthesis and stereochemistry.

Alkene Structure: Characterized by a double bond (C=C), which consists of one sigma (σ) and one pi (π) bond.
Examples: Simple linear alkenes and cyclic alkenes such as 3-methylcyclohexene.
Double Bond Position: Alkenes are classified as terminal (double bond at the end of the chain) or internal (double bond within the chain).

Stereoisomerism in Alkenes

Alkenes exhibit stereoisomerism due to restricted rotation around the double bond, leading to cis-trans (E/Z) isomers.

Cis (Z) Isomer: Substituents on the same side of the double bond.
Trans (E) Isomer: Substituents on opposite sides of the double bond.
E/Z Naming: Assign priorities at each sp2 carbon using Cahn-Ingold-Prelog rules; E (entgegen) for opposite sides, Z (zusammen) for same side.
Example: 2-butene exists as both cis-2-butene and trans-2-butene.

Structure of the Double Bond

The double bond in alkenes consists of a strong sigma bond and a weaker pi bond, which influences chemical reactivity and physical properties.

Sigma (σ) Bond: Formed by head-on overlap of sp2 hybrid orbitals.
Pi (π) Bond: Formed by side-on overlap of unhybridized 2p orbitals, creating an electron-rich π cloud above and below the plane of the molecule.
Bond Strength: The π bond is relatively weak compared to the σ bond.
Energy Diagram: σ orbital is lower in energy than π orbital; antibonding orbitals (σ*, π*) are higher in energy.

Strength and Isomerization of the π Bond

The π bond in alkenes is susceptible to breakage, which is necessary for cis-trans isomerization.

Cis-Trans Isomerization: Requires breaking the π bond, allowing rotation around the C–C axis.
Activation Energy: (typical C–C single bond strength ≈ 90 kcal mol−1).
Orbital Alignment: In cis and trans isomers, 2p orbitals are parallel; in the transition state, they are perpendicular.

Relative Stability of Alkenes

Alkene stability can be compared by measuring the heat of hydrogenation (ΔH) for different isomers.

Heat of Hydrogenation: The enthalpy change when an alkene is converted to an alkane by addition of H2 in the presence of a catalyst.
Stability Trend: Lower (less negative) ΔH indicates greater stability.

Alkene Isomer	ΔH (kcal/mol)
1-Butene (terminal)	-30.3
cis-2-Butene (internal, cis)	-28.6
trans-2-Butene (internal, trans)	-27.6

Relative Stability: Internal > terminal; trans > cis.

Factors Affecting Alkene Stability

Alkene stability is influenced by hyperconjugation and steric hindrance.

Hyperconjugation: Delocalization of electrons from adjacent C–H bonds into the π system increases stability.
Steric Hindrance: Bulky groups on the same side (cis) cause strain, making cis isomers less stable than trans.
General Order of Stability:

Alkene Type	Relative Stability
CH2=CH2 (ethene)	Least stable
RCH=CH2 (terminal)
RCH=CHR (cis)
RCH=CHR (trans)
Trisubstituted
Tetrasubstituted	Most stable

Synthesis of Alkenes: Elimination Reactions

Alkenes are commonly synthesized via elimination reactions, particularly E2 mechanisms, which show regioselectivity based on the base used.

E2 Elimination: A strong base removes a proton from a β-carbon, leading to double bond formation and loss of a leaving group.
Regioselectivity: The position of the double bond in the product depends on the base and substrate structure.
Saytzev's Rule: Non-bulky base favors formation of the more substituted (internal) alkene (thermodynamically controlled).
Hofmann Rule: Bulky base favors formation of the less substituted (terminal) alkene (kinetically controlled).

Saytzev's Rule: The Transition State

Saytzev's Rule is explained by the lower-energy transition state leading to the more stable, highly substituted alkene.

Transition State: The pathway with the lowest activation energy is favored, resulting in the most stable product.
Example: Elimination of 2-bromo-2-methylbutane with NaOCH3 yields the more substituted alkene.

Hofmann Rule

When a bulky base is used, steric hindrance prevents formation of the more substituted alkene, favoring the less substituted product.

Kinetically Controlled: The reaction proceeds via the pathway with the lowest steric hindrance, not necessarily the most stable product.
Example: Elimination of 2-bromo-2-methylbutane with KOtBu yields the terminal alkene.

Stereospecificity of the E2 Reaction

The E2 elimination is stereospecific, meaning the stereochemistry of the starting material determines the stereochemistry of the product.

Anti-Periplanar Geometry: The hydrogen and leaving group must be anti-periplanar for E2 elimination to occur.
Product Stereochemistry: One diastereomer of the substrate gives only one stereoisomer of the alkene.
Example: R,R diastereomer gives only E-alkene (trans); R,S diastereomer gives only Z-alkene (cis).

Summary Table: Alkene Formation by E2 Elimination

Base Type	Major Product	Control
Non-bulky (e.g., NaOCH3)	Internal (more substituted) alkene	Thermodynamic
Bulky (e.g., KOtBu)	Terminal (less substituted) alkene	Kinetic

Additional info: The notes also reference Newman projections for visualizing anti-periplanar geometry in E2 reactions, which is essential for understanding stereospecificity in alkene formation.