Chemical reactions are like intricate dances of atoms and electrons. Reaction mechanisms break down these complex moves, showing how bonds break and form. Understanding these steps helps predict outcomes and design new reactions.

Bond cleavage can be heterolytic or homolytic, determining how electrons are distributed. This affects whether reactions are polar or radical, influencing the types of intermediates formed and the overall reaction pathway.

Reaction Mechanisms and Bond Cleavage

Process of chemical transformations

• Reaction mechanism provides step-by-step description of how reactants convert into products
  • Illustrates order of bond breaking and forming (breaking C-H bond, forming C-Br bond)
  • Shows flow of electrons during each step (electron pair from C-H bond moves to form H-Br)
  • Identifies intermediate species formed along reaction pathway (carbocation, carbanion)
• Each step involves breaking and/or making chemical bonds
  • Bond-breaking removes electrons from a bond (C-H bond cleaves)
  • Bond-making adds electrons to form new bond (C-Br bond forms)
• Mechanism provides detailed understanding of reaction at molecular level
  • Helps predict outcome of similar reactions (substitution, elimination)
  • Allows design of new synthetic pathways (drug discovery, materials science)

Heterolytic vs homolytic bond cleavage

• Heterolytic bond cleavage unequally distributes electrons between atoms
  • One atom retains both electrons from broken bond, forming ion pair
    • Atom gaining electrons becomes negatively charged anion (bromide ion)
    • Atom losing electrons becomes positively charged cation (carbocation)
  • Occurs in polar reactions more common in organic chemistry (SN1, SN2, E1, E2)
• Homolytic bond cleavage equally distributes electrons between atoms
  • Each atom retains one electron from broken bond, forming two neutral radicals
    • Radicals are species with unpaired electron (chlorine radical, alkyl radical)
  • Occurs in radical reactions less common in organic chemistry (halogenation, polymerization)

Polar vs radical reactions

• Polar reactions involve heterolytic cleavage of bonds
  • Electrons move in pairs from one atom to another (nucleophile to electrophile)
  • Characterized by formation of charged intermediates (carbocation, carbanion)
  • Prevalent in organic chemistry due to presence of polarized bonds
    • $C-X$ bonds where $X$ is electronegative atom ($N$, $O$, halogen)
    • Examples: nucleophilic substitution, elimination, addition reactions
  • Often involve a leaving group, which is a stable species that departs during the reaction
• Radical reactions involve homolytic cleavage of bonds
  • Electrons move individually, each atom retains one electron (initiator cleaves to form two radicals)
  • Characterized by formation of neutral, highly reactive radical intermediates
    • Radicals have unpaired electron (hydroxyl radical, phenyl radical)
  • Less common in organic chemistry compared to polar reactions
    1. Radicals are generally less stable and more difficult to control
    2. Radical reactions are important in specific areas (polymer chemistry, biochemical processes)
       • Examples: halogenation of alkanes, polymerization of alkenes, lipid peroxidation

Reaction Energy Profile

• Reaction coordinate diagram illustrates energy changes during a reaction
  • x-axis represents reaction progress, y-axis represents energy
  • Shows reactants, products, and any intermediates or transition states
• Transition state is the highest energy point on the reaction pathway
  • Represents the arrangement of atoms at the peak of the energy barrier
• Activation energy is the minimum energy required for a reaction to occur
  • Represented by the energy difference between reactants and transition state
• Rate-determining step is the slowest step in a multi-step reaction
  • Usually the step with the highest activation energy
  • Controls the overall rate of the reaction