kapitel 5

Alkenes: Structure, Nomenclature, and Reactivity

Introduction to Alkenes

• Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C).

• They are unsaturated hydrocarbons, meaning they contain fewer hydrogen atoms than saturated hydrocarbons (alkanes).

• General molecular formula: CₙH₂ₙ for acyclic alkenes; CₙH₂ₙ₋₂ for cyclic alkenes.

• Degree of unsaturation indicates the number of rings and double bonds present in a molecule.

Learning Objectives

• Topics included in the chapter: temperature scale naming, trans fats in food, color in carrots and flamingos, insect population control via compounds, and molecular recognition in biological systems.

Importance of Alkenes in Biology

• Ethene (H₂C=CH₂), the simplest alkene, serves as a plant hormone influencing growth and maturity processes.

• Alkenes contribute to the odors and flavors in plants, e.g., citronellol from rose and geranium oils, and limonene from citrus oils.

5.1 Molecular Formulas and Degree of Unsaturation

Pheromones

• Insects use pheromones, often alkenes, for communication (e.g., sex attractants).

• Example of pheromone traps used to control pests such as gypsy moths.

General Molecular Formulas

• Noncyclic alkane: CₙH₂ₙ₊₂

• Cyclic alkane: CₙH₂ₙ

• Noncyclic alkene: CₙH₂ₙ

• Cyclic alkene: CₙH₂ₙ₋₂

• Degree of unsaturation counts double bonds or rings; e.g., C₈H₁₄ has 2 degrees of unsaturation.

Saturated vs. Unsaturated Hydrocarbons

• Alkanes: saturated hydrocarbons (maximum H atoms)

• Alkenes: unsaturated (fewer H from double bonds)

5.2 Nomenclature of Alkenes

Systematic Naming Rules

• Use suffix "-ene" for alkenes; replace "-ane" from corresponding alkanes.

• Examples: Ethene (common name: ethylene), Propene (common name: propylene).

• Number longest carbon chain that has the double bond for proper suffix placement.

• For alkenes with double bonds, number the chain to minimize the bond's position number.

cis and trans Isomer Naming

• Stereoisomers use cis (Z) or trans (E) prefixes based on substituent positions.

• Di- or Tri- indicate multiple double bonds in names (e.g., 1,3-pentadiene).

5.3 Structure of Alkenes

Vinylic and Allylic Carbons

• Vinylic carbons: carbons involved in the double bond.

• Allylic carbons: adjacent to vinylic carbons.

• Each sp² carbon of an alkene has three sp² orbitals lying in the same plane, promoting optimal π bond formation.

5.4 Electrophilic Addition Reactions

• Alkenes react as nucleophiles; they are attracted to electrophiles (electron-deficient species).

• The double bond breaks, forming a carbocation (positively charged carbon).

• Mechanism: Curved arrows demonstrate electron flows during bond formation and breaking.

5.5 Thermodynamics and Kinetics

Reaction Energy Profile

• Reaction coordinate diagrams illustrate energy changes throughout a reaction.

• Stability of reactants/products can be assessed with Gibbs free energy and Keq (equilibrium constant).

Equilibria and Stability

• Exergonic (negative AG°) reactions favor products; endergonic (positive AG°) reactions have more stable reactants.

Factors Affecting Reaction Rate

1. Concentration of Reactants (increase frequency of collisions).

2. Temperature (kinetic energy impacts collision frequency and energy).

3. Catalysts (provide alternate pathways and lower activation energy).

5.6 Catalysis in Reactions

Types of Catalysis

• Acid-base catalysis or nucleophilic catalysis involves acids donating protons, bases removing protons, and nucleophiles forming bonds.

• Enzymes are biological catalysts that exhibit specificity for their substrates.

Summary of Key Concepts

• Degree of Unsaturation and Molecular Formulas

• Functional Groups and Nomenclature

• Stability of Alkenes and Kinetics of Reactions

• The role of Catalysis in chemical and biological systems.

Problem-Solving Examples

1. Molecular formula for alkenes: C₄H₆ (2 double bonds).

2. Determine substitution: Assign proper numbers to double bonds and substituents based on IUPAC nomenclature.