Lecture Notes: Chemical Reactions and Radicals

Introduction

• Dynamic social scenes are analogous to chemical reactions.
• Focus: formation and breaking of covalent bonds, radicals and their reactions.

Covalent Bonds

• Formed and broken during chemical reactions.
• Atoms form covalent bonds to fill their outer electron shells and achieve stability.
• Bonds form by sharing pairs of electrons, often involving unpaired electrons from atoms.

Radicals

• Atoms with unpaired electrons, highly reactive and unstable.
• Can be charged or often uncharged.
• Reaction mechanisms use half arrows or fish hooks to indicate interaction.
• Lone pairs of electrons are typically not shown as they don't participate.

Bond Formation and Breaking

• Radicals: Half arrows show sharing of unpaired electrons between radicals.
• Lone Pair Sharing: Full or curly arrows indicate lone pair sharing from nucleophile to electrophile.
• Homolysis (Homolytic Fission): Bonding electron pair splits equally, each atom gets one electron, forming radicals.
• Heterolysis (Heterolytic Fission): Polar bonds, electrons move to more electronegative atom, creating charged particles.
• Focus is on homolytic fission and radical reactions.

Importance of Radicals

• Radicals form when molecules absorb UV light causing homolytic fission.
• Example: Cl2 absorbs UV light forming two chlorine radicals.
• Radicals play a role in mass spectrometry (e.g., methanol forming radical cations).
• Radicals are involved in ozone breakdown in the atmosphere.

Ozone Layer and Radicals

• Ozone absorbs UV light, undergoes homolytic fission, producing O2 and an oxygen radical.
• Introduction of CFCs leads to ozone layer reduction.
• CFCs release chlorine radicals upon UV exposure, initiating chain reactions.
• Propagation steps involve producing new radicals, continuing reactions.
• Termination occurs when radicals react to form non-radical products.
• Chlorine radicals act as catalysts, expediting ozone destruction.

Radical Reactions with Alkanes

• Conversion of alkanes to halogenoalkanes occurs in gas phase.
• Example: Methane reacts with chlorine to form chloromethane, hydrochloric acid.
• Mechanism involves initiation, propagation, and termination steps.
• Free radical substitution can be difficult to control, leading to by-products.

Summary

• Key concepts: covalent bond formation and breaking, radical roles.
• Understanding these concepts reveals insights into molecular mechanisms.
• Critical for grasping atmospheric chemistry and chemical production processes.

Mastery of bond dynamics is crucial for advanced chemistry study.