Chapter 5: Alkenes – Structure, Reactivity, and Thermodynamics

Alkenes: Structure and Nomenclature

Introduction to Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. Their general formula is CnH2n for acyclic alkenes and CnH2n-2 for cycloalkenes. Alkenes have fewer hydrogens than the corresponding alkanes, making them unsaturated.

• Saturated hydrocarbons: Maximum number of C-H bonds (e.g., ethane CH3CH3).

• Unsaturated hydrocarbons: Fewer than the maximum number of hydrogens (e.g., ethene CH2=CH2).

Example: Ethene (H2C=CH2) is the simplest alkene.

Key Vocabulary

• Vinyl Carbon: Carbon directly attached to the double bond.

• Allylic Carbon: Carbon adjacent to the double bond.

• Allyl Group: The smallest possible group containing an allylic carbon.

Mechanism and Reactivity of Alkenes

Mechanism of Reaction

Alkene reactions often proceed via mechanisms where reactants are converted into products through the movement of electrons. Curved arrows are used to show the flow of electrons.

• Electrophile: Electron-deficient atom or molecule, seeks electrons.

• Nucleophile: Electron-rich atom or molecule, donates electrons.

• Transition State: The highest energy state during a reaction step.

Thermodynamics of Alkene Reactions

Exergonic and Endergonic Reactions

Thermodynamics describes the energy changes during chemical reactions.

• Exergonic reaction: Releases more energy than it consumes.

• Endergonic reaction: Consumes more energy than it releases.

• Enthalpy ($\Delta H$): Heat given off or consumed during a reaction.

• Entropy ($\Delta S$): Measure of freedom of motion in a system.

• Gibbs Free Energy ($\Delta G$): $\Delta G = \text{free energy of products} - \text{free energy of reactants}$

Example: Hydrogenation of alkenes is typically exergonic and exothermic.

Hydrogenation and Alkene Stability

Hydrogenation of Alkenes

Hydrogenation is the addition of hydrogen to a double bond, often using a metal catalyst. This process converts alkenes to alkanes.

• Catalytic Hydrogenation: Requires a catalyst (e.g., Pd/C, Pt).

• Reduction Reaction: Addition of hydrogen increases the number of C-H bonds.

• Heat of Hydrogenation: Heat released during hydrogenation, usually a negative value ($\Delta H$).

Example: Hydrogenation of 2-methyl-2-butene releases $-26.9$ kcal/mol.

Determining Relative Stabilities of Alkenes

The stability of alkenes can be compared by measuring their heats of hydrogenation. More stable alkenes have lower (less negative) heats of hydrogenation.

Alkene	Heat of Hydrogenation ($\Delta H$ kcal/mol)
2-methyl-2-butene	-26.9
2-methyl-1-butene	-28.5
3-methyl-1-butene	-30.3

Additional info: The product of each hydrogenation is 2-methylbutane.

Cis and Trans Isomer Stability

Alkenes can exist as cis or trans isomers, which differ in the arrangement of substituents around the double bond.

• Trans isomers: More stable, less negative heat of hydrogenation.

• Cis isomers: Less stable, more negative heat of hydrogenation.

• Steric Strain: Electron clouds in cis isomers interact, causing instability.

Isomer	Heat of Hydrogenation ($\Delta H$ kcal/mol)
trans-2-butene	-27.6
cis-2-butene	-28.6

Kinetics: How Fast is the Product Formed?

Reaction Rate and Factors Affecting It

The rate of a chemical reaction depends on several factors:

• Number of collisions between reactant molecules.

• Fraction of collisions with sufficient energy to overcome the activation barrier.

• Fraction of collisions with proper orientation.

Increasing concentration and temperature generally increases reaction rate.

Reaction Coordinate Diagram

The reaction coordinate diagram shows the energy changes during a reaction. The highest point is the transition state, which determines the rate-limiting step.

Kinetics and Free Energy

• $\Delta G^\ddagger = \text{free energy of the transition state} - \text{free energy of the reactants}$

• If $\Delta G$ is negative, the product is thermodynamically stable.

• If $\Delta G$ is positive, the product is thermodynamically unstable.

Catalysis

Catalysts and Enzymes

A catalyst increases the rate of reaction by providing a new pathway with a lower activation energy. Enzymes are biological catalysts, usually proteins, that speed up reactions by binding substrates at their active site.

• Active Site: Region of enzyme where substrate binds and reaction occurs.

• Molecular Recognition: Ability of one molecule to recognize another via interaction.

Summary Table: Key Concepts in Alkene Chemistry

Concept	Definition/Explanation
Alkene	Unsaturated hydrocarbon with C=C double bond
Hydrogenation	Addition of H2 to double bond, forms alkane
Exergonic Reaction	Releases more energy than it consumes
Endergonic Reaction	Consumes more energy than it releases
Transition State	Highest energy point in reaction pathway
Catalyst	Substance that increases reaction rate, not consumed
Enzyme	Protein catalyst in biological systems

Additional info: The notes also cover the effect of substituents on alkene stability, the role of steric strain, and the importance of molecular recognition in enzyme catalysis.