Chapter 04: The Study of Chemical Reactions

4.1 Introduction

This section introduces the fundamental concepts of chemical reactions in organic chemistry, focusing on the movement of electrons and the breaking and formation of bonds.

• Radical Reactions: Involve single-headed arrows ("fishhooks") to represent the movement of one electron.

• Bond Breaking: Can occur homolytically (each atom takes one electron) or heterolytically (one atom takes both electrons).

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

• Kinetics: Studies the rate of reactions and how fast they occur.

• Reaction Mechanism: Shows the step-by-step movement of electrons using curved arrows.

Example: Homolytic cleavage of a bond:

Example: Heterolytic cleavage of a bond:

4.2 Chlorination of Methane

Chlorination of methane is a classic example of a free-radical halogenation reaction, which proceeds via a chain mechanism.

• Free-radical halogenation: Produces alkyl halides and is a substitution reaction.

• Initiation: Formation of radicals, usually by heat or light.

• Propagation: Radicals react with stable molecules to produce new radicals and products.

• Termination: Two radicals combine to end the chain reaction.

• Most effective wavelength: Absorbed by chlorine gas; most product molecules are formed from one photon of light.

Example Reaction:

• Mixture of alkyl halides forms when alkanes are treated with and in the presence of heat or light.

4.3 Free-Radical Chain Reactions

Free-radical chain reactions consist of three fundamental steps: initiation, propagation, and termination.

1. Initiation: Creation of a reactive radical species, often by homolytic bond cleavage (e.g., ).

2. Propagation: Radical reacts with a stable molecule to produce a new radical and product (e.g., ).

3. Termination: Two radicals combine to form a stable molecule (e.g., ).

Additional info: In propagation, the radical abstracts a hydrogen atom, and the newly formed radical continues the chain.

4.4 Thermodynamics of Chemical Reactions

Thermodynamics is the study of energy changes in chemical reactions, determining whether a reaction is energetically favorable.

• Equilibrium Constant (): Indicates the position of equilibrium.

• Gibbs Free Energy (): Determines spontaneity of a reaction.

• Enthalpy (): Heat absorbed or released at constant pressure.

• Entropy (): Measure of disorder or randomness.

Key Equations:

Example: A negative indicates a spontaneous reaction.

Additional info: Lower energy products are more stable; reactions tend to favor formation of more stable (lower energy) products.

4.5 Bond-Dissociation Enthalpy (BDE)

BDE is the energy required to break a particular bond homolytically in a gaseous molecule.

• BDE values: Used to estimate enthalpy changes in reactions.

• Bond breaking: Requires energy (endothermic).

• Bond formation: Releases energy (exothermic).

Equation:

4.7 Enthalpy Changes in Chlorination

Enthalpy changes for the chlorination of methane can be calculated using bond-dissociation enthalpies.

Overall enthalpy change: (exothermic)

Additional info: Exothermic reactions release energy and are generally more favorable.

4.8 Kinetics and the Rate Equation

Kinetics studies the rate of chemical reactions and the factors that affect it.

• Rate equation: Relates the concentration of reactants to the observed reaction rate.

• General form:

• Rate constant (): Depends on temperature and other conditions.

• Arrhenius equation:

• Activation energy (): Minimum energy required to initiate a reaction.

Example: Higher temperature increases the number of molecules with sufficient energy to react.

4.10 Transition States and 4.11 Rates of Multistep Reactions

Transition states are high-energy states that occur during the conversion of reactants to products. Multistep reactions involve several transition states and intermediates.

• Reaction energy diagrams: Graphically represent the energy changes during a reaction.

• Rate-determining step: The slowest step in a multistep reaction, which controls the overall rate.

• Intermediates: Species formed during the reaction that are not present in the final products.

Example: Energy diagram for the chlorination of methane shows multiple steps and transition states.

4.13 Selectivity in Halogenation

Halogenation reactions can occur at different positions on an alkane, depending on the type of hydrogen atom (primary, secondary, tertiary).

• Primary, secondary, tertiary hydrogens: Classified based on the carbon to which they are attached.

• Product ratio: Depends on the number and reactivity of each type of hydrogen.

• Halogenation of propane: Provides information about the relative reactivity of primary vs. secondary hydrogens.

Example: Chlorination of propane yields both 1-chloropropane and 2-chloropropane, with the latter being favored due to the greater stability of the secondary radical.

Additional info: More substituted radicals (secondary, tertiary) are generally more stable and form preferentially.