Learning Outcomes

• Propose mechanisms and explain the steps for simple reactions.

• Draw a reaction-energy diagram and explain the factors that control the thermodynamics and kinetics of a reaction.

• Use the mechanism, thermodynamics, and kinetics of a reaction to predict the major product.

• Identify reactive intermediates and explain their properties.

Introduction

Understanding organic reactions requires more than just knowing the reactants and products. To fully comprehend a reaction, it is essential to study the mechanism (the step-by-step pathway), thermodynamics (energy changes and equilibrium), and kinetics (reaction rates and factors affecting them).

• Mechanism: The detailed, stepwise process by which reactants are converted to products.

• Thermodynamics: The study of energy changes, stability of reactants and products, and the position of equilibrium.

• Kinetics: The study of reaction rates and how they are influenced by various factors.

Chlorination of Methane

Overview of the Reaction

The chlorination of methane is a classic example of a free-radical halogenation reaction. Methane reacts with chlorine in the presence of heat or light to produce a mixture of products, primarily chloromethane and hydrogen chloride.

• General reaction:

• The reaction continues, leading to further substitution and formation of dichloromethane, trichloromethane, and tetrachloromethane.

• Heat or light is required to initiate the reaction.

Key Observations

• Chlorination does not occur at room temperature in the absence of light.

• The most effective wavelength is blue light, which is strongly absorbed by chlorine gas.

• The reaction has a high quantum yield, meaning many molecules of product are formed for each photon of light absorbed.

Three Aspects of the Reaction

Mechanism

The mechanism provides a complete, step-by-step description of which bonds break and which bonds form, explaining the observed products.

Thermodynamics

• Thermodynamics studies the energy changes that accompany chemical and physical transformations.

• It allows comparison of the stability of reactants and products and predicts which compounds are favored at equilibrium.

Kinetics

• Kinetics is the study of reaction rates and determines which products are formed fastest.

• Kinetics helps predict how the rate will change if reaction conditions are altered.

The Free-Radical Chain Reaction

Free-radical halogenation proceeds via a chain reaction mechanism, which consists of three main steps: initiation, propagation, and termination.

1. Initiation

• A chlorine molecule absorbs blue light and splits homolytically into two chlorine atoms (radicals).

• This produces two highly reactive chlorine atoms (free radicals).

2. Propagation

• The reactive intermediate (chlorine radical) reacts with a stable molecule (methane) to form a product and another reactive intermediate.

• There are two main propagation steps:

  1. First Propagation Step: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical.

  2. Second Propagation Step: The methyl radical reacts with a chlorine molecule to form chloromethane and another chlorine radical.

• This chain reaction continues until the reactants are depleted or radicals are destroyed in termination steps.

3. Termination

• Side reactions that destroy reactive intermediates and slow or stop the reaction.

• Examples include the combination of two radicals or reaction of radicals with the vessel wall.

• Termination steps decrease the number of free radicals, thus breaking the chain.

Some Important Terms

• Half-Arrows: Used to show the movement of single unpaired electrons.

• Curved Arrows: Used to represent the movement of electron pairs.

• Reactive Intermediate: A short-lived species that is never present in high concentration because it reacts as quickly as it is formed.

• Odd Electron: Each reactive intermediate has an odd number of valence electrons, one of which is unpaired.

• Species with unpaired electrons are called radicals or free radicals.

• Radicals are electron-deficient because they lack an octet.

• Radicals are represented by a structure with a single dot.

Mechanism of Free-Radical Chlorination

• Free-radical chlorination is a chain reaction involving initiation, propagation, and termination steps.

• Propagation steps continue the chain by generating new radicals as products.

• Termination steps remove radicals from the system, ending the chain reaction.

Summary Table: Steps in Free-Radical Chlorination

Key Concepts and Applications

• Substitution Reaction: The first step of chlorination is a substitution, where a chlorine atom replaces a hydrogen atom in methane.

• Homolytic Cleavage: The breaking of a bond so that each atom retains one electron, forming radicals.

• Chain Reaction: A process where the product of one step initiates the next, allowing the reaction to continue.

• Quantum Yield: The efficiency of the reaction in terms of the number of molecules formed per photon absorbed.

Example

In the chlorination of methane, the following sequence occurs:

• Initiation:

• Propagation 1:

• Propagation 2:

• Termination: or

Additional info:

• Further substitution can occur, leading to dichloromethane, trichloromethane, and tetrachloromethane as minor products.

• The concentration of radicals during the reaction is very low because they react quickly.

• The probability of two radicals meeting (termination) is much lower than a radical meeting a reactant (propagation).