Chapter 4: The Study of Chemical Reactions

Introduction to Chemical Reactions

Chemical reactions in organic chemistry involve the transformation of reactants into products. Understanding these reactions requires knowledge of thermodynamics, kinetics, and mechanisms.

• Thermodynamics: Studies the energy changes accompanying chemical and physical transformations.

• Kinetics: Examines the rates at which reactions occur.

• Mechanism: Describes the step-by-step process by which reactants are converted to products.

Chlorination of Methane

The chlorination of methane is a classic example of a free-radical chain reaction. The overall reaction is:

• Requires heat or light for initiation.

• Most effective wavelength is blue, absorbed by chlorine gas.

• Many molecules of product are formed from absorption of only one photon of light.

Free-Radical Chain Reaction Mechanism

Steps in the Free-Radical Chain Reaction

• Initiation: Generates a radical intermediate.

• Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate and a product molecule.

• Termination: Side reactions that destroy the reactive intermediate, ending the chain process.

Initiation Step: Formation of Chlorine Atom

Chlorine molecules split homolytically into chlorine atoms (free radicals) upon absorption of a photon:

• Each chlorine atom has an unpaired electron, making it a radical.

Lewis Structures of Free Radicals

Free radicals are reactive species with odd numbers of electrons. Halogens have seven valence electrons; one will be unpaired in the radical form.

• Examples:

  • Chlorine atom:

  • Hydroxyl radical:

  • Methyl radical:

Propagation Steps

Step 1: Carbon Radical Formation

• Chlorine atom collides with methane, abstracts a hydrogen atom, forming HCl and a methyl radical:

Step 2: Product Formation

• Methyl radical reacts with another chlorine molecule, producing methyl chloride and regenerating a chlorine radical:

Overall Reaction

• Combines the propagation steps to show the net transformation:

Termination Steps

• Occurs when two free radicals combine, producing a nonradical compound.

• Examples:

  • Radical collides with a wall or contaminant, removing it from the reaction.

Thermodynamics of Chemical Reactions

Equilibrium Constant ()

The equilibrium constant quantifies the ratio of product concentrations to reactant concentrations at equilibrium.

• General form:

• For chlorination of methane:

Free Energy Change ()

Free energy change determines the spontaneity of a reaction.

• Negative indicates a favorable, spontaneous reaction.

• Equation: where J/K·mol and is temperature in kelvins.

Factors Determining

• Enthalpy (): Heat released or absorbed during a reaction.

• Entropy (): Change in randomness, disorder, or freedom of movement.

• Relationship:

Table 4-1: Product Composition as a Function of at 25°C

This table shows how the value of affects the percentage of product formed at equilibrium.

Enthalpy

• : Heat released or absorbed at standard conditions.

• Exothermic (): Heat is released.

• Endothermic (): Heat is absorbed.

• Reactions favor products with the lowest enthalpy (strongest bonds).

Entropy

• : Change in disorder or randomness.

• Increasing heat, volume, or number of particles increases entropy.

• Spontaneous reactions maximize disorder and minimize enthalpy.

• In , entropy is often a small value.

Solved Problem 1: Calculating

• Given for chlorination of methane, calculate :

• For at 298 K: J/K·mol K kJ/mol

Bond-Dissociation Enthalpies (BDE)

Definition and Application

• BDE: Energy required to break a bond in a gaseous molecule.

• Bond formation releases energy ().

• BDE can estimate for a reaction.

Types of Cleavage

• Homolytic cleavage: Each atom gets one electron; free radicals result.

• Heterolytic cleavage: Most electronegative atom gets both electrons; ions result.

Homolytic and Heterolytic Cleavages

• Homolytic:

• Heterolytic: varies with solvent.

Enthalpy Changes in Chlorination

• Overall reaction:

• Bond breaking: (435 kJ/mol), (243 kJ/mol)

• Bond forming: (352 kJ/mol), (431 kJ/mol)

• Total : (breaking) (forming) kJ/mol

Summary

• Organic reactions are governed by thermodynamics, kinetics, and mechanisms.

• Free-radical chain reactions, such as methane chlorination, proceed via initiation, propagation, and termination steps.

• Thermodynamic quantities (, , , ) determine reaction favorability and product composition.

• Bond-dissociation enthalpies are essential for understanding reaction energetics and mechanisms.