Organic Chemistry I: Free Radical Halogenation, Kinetics, Thermodynamics, and Reaction Mechanisms

Unit 2: Ethane to Ethanol – Free Radical Halogenation

Introduction to Organic Reaction Mechanisms

Organic chemistry involves understanding how molecules react and transform. Rather than memorizing reactions, students should focus on the underlying mechanisms, which often involve multiple steps and intermediates. Energy diagrams help visualize these processes, showing the relative energies of reactants, products, and transition states.

• Reaction Mechanism: The step-by-step sequence of elementary reactions by which overall chemical change occurs.

• Transition State: A high-energy state during a reaction where bonds are partially formed and broken.

• Intermediate: A species formed during the reaction that is not the final product.

Example: Free radical halogenation of alkanes involves initiation, propagation, and termination steps.

Kinetics and Thermodynamics

Key Concepts

Kinetics describes how fast a reaction occurs, while thermodynamics describes the energy change and equilibrium position.

• Kinetics: Depends on activation energy ($E_a$), the minimum energy required for a reaction to occur.

• Thermodynamics: Depends on the energy difference between reactants and products ($riangle H$).

Equations:

• Activation Energy: $E_a$

• Enthalpy Change: $\triangle H = E_{products} - E_{reactants}$

Example: A reaction may be thermodynamically favorable but slow if the activation energy is high.

Combustion and Climate Change

Combustion Reaction

Combustion of hydrocarbons releases energy and carbon dioxide, contributing to climate change.

• General Equation: $C_6H_{14} + 9.5 O_2 \rightarrow 6 CO_2 + 7 H_2O$

• CO2 Role: Greenhouse gas, traps heat in the atmosphere.

• Historical Context: Heat retention by CO2 described by John Tyndall and Eunice Foote; anthropogenic climate change predicted by Svante Arrhenius.

Free Radical Halogenation

Mechanism Overview

Free radical halogenation is a method to introduce halogens into alkanes. The process involves three main steps: initiation, propagation, and termination.

• Initiation: Formation of free radicals, often by homolytic cleavage using light or heat.

• Propagation: Radicals react with alkanes to form new radicals and products.

• Termination: Two radicals combine to form a stable molecule.

Example: Chlorination of ethane:

• Initiation: $Cl_2 \xrightarrow{hv} 2 Cl\cdot$

• Propagation: $Cl\cdot + CH_3CH_3 \rightarrow CH_3CH_2\cdot + HCl$

• Propagation: $CH_3CH_2\cdot + Cl_2 \rightarrow CH_3CH_2Cl + Cl\cdot$

• Termination: $Cl\cdot + Cl\cdot \rightarrow Cl_2$

Reactive Forms of Carbon

Types of Reactive Carbon Species

Organic reactions often involve reactive carbon species:

• Carbocation: Positively charged carbon, electron-deficient.

• Carbanion: Negatively charged carbon, electron-rich.

• Carbon Radical: Carbon with an unpaired electron, highly reactive.

• Alkane: Saturated hydrocarbon, relatively unreactive.

Bond Dissociation Energies (BDE)

Homolytic Cleavage

BDE is the energy required to break a bond evenly, forming two radicals. It helps predict reactivity and selectivity in halogenation reactions.

• Homolytic Cleavage: $R-H \rightarrow R\cdot + H\cdot$

• BDE Table (Selected):

Bond	BDE (kJ/mol)
H-H	436
C-H (methyl)	435
C-H (1°)	410
C-H (2°)	397
C-H (3°)	381
Cl-Cl	243
Br-Br	193

Application: Lower BDE means easier bond cleavage and higher reactivity.

Chlorination vs. Bromination

Reactivity and Selectivity

Chlorination is less selective but more reactive than bromination. Bromination favors formation of the most stable radical (tertiary > secondary > primary).

• Chlorination: Fast, less selective, forms mixtures.

• Bromination: Slow, highly selective, favors tertiary radicals.

Hydrogen Type	Relative Reactivity (Chlorination)	Relative Reactivity (Bromination)
Primary	1	1
Secondary	3.8	82
Tertiary	5	1600

Example: Bromination of isobutane yields almost exclusively tert-butyl bromide.

Transition States and Hammond's Postulate

Transition State Structure

The nature of the transition state affects selectivity. Hammond's Postulate states that the transition state resembles the species (reactant or product) to which it is closer in energy.

• Exothermic Reaction: Early transition state, resembles reactants.

• Endothermic Reaction: Late transition state, resembles products (radicals).

Application: Bromination has a late transition state, so selectivity is higher for the most stable radical.

Substitution and Elimination Reactions

SN2 and E2 Mechanisms

Alkyl halides can undergo nucleophilic substitution (SN2) or elimination (E2) reactions, often competing depending on conditions and the structure of the substrate.

• SN2: Bimolecular nucleophilic substitution, single step, inversion of configuration.

• E2: Bimolecular elimination, single step, formation of alkene.

Example: Reaction of bromoethane with hydroxide can yield ethanol (SN2) or ethene (E2).

Nucleophilicity and Basicity

Definitions and Trends

Nucleophiles and bases both possess lone pairs, but their strength and reactivity depend on structure, charge, and resonance stabilization.

• Nucleophile: Species that donates an electron pair to form a new bond.

• Base: Species that accepts a proton.

• Resonance: Delocalization of electrons stabilizes anions, reducing basicity but often enhancing nucleophilicity.

Base/Nucleophile	Conjugate Acid	pKa	Strength
Acetate (OAc-)	Acetic acid	5	Weak
Hydroxide (OH-)	Water	16	Strong

Example: Acetate is a weaker base than hydroxide due to resonance stabilization.

Resonance Structures

Representation and Importance

Resonance structures show delocalization of electrons, which stabilizes molecules and ions. Curved arrows indicate electron movement between resonance forms.

• Correct Representation: All possible resonance forms should be shown, with delocalized charge.

• Arrows: Used to indicate electron movement in resonance and reaction mechanisms.

Summary of Key Concepts

• Energy diagrams: Visualize enthalpy and entropy changes.

• Substitution (SN2) and elimination (E2) reactions: Compete depending on substrate and conditions.

• Steric hindrance: Crowding affects reactivity.

• Kinetics vs. thermodynamics: Rate vs. favorability.

• Types of reactive carbons: Carbocation, carbanion, free radical, alkane.

• Homolytic bond dissociation energies: Predict reactivity.

• Nucleophilicity vs. basicity: Related but distinct concepts.

• Resonance: Stabilizes anions, affects reactivity.

Practice and Further Study

• Review textbook sections: Klein, Chapters 2, 3, 6, 7, 10 (selected sections).

• Practice problems: Chapter 6 (#6.24-6.26), Chapter 10 (#10.26, 10.28-10.29, 10.33-10.34, 10.36), Chapter 7 (#7.50, 7.55).

Additional info:

• Some content inferred from context and standard Organic Chemistry curriculum (e.g., Klein textbook references, mechanism details).

• Tables reconstructed from typical values and trends in Organic Chemistry.