CHM030 3.2: Overview of Organic Reactions

• Kinds of Organic Reactions (SEARS)

  1 Substitutions

  2 Elimination reactions

  3 Addition reactions

  4 Rearrangement reactions
• Substitution


  parts of two molecules exchange
• Elimination Reactions


  one molecule splits into two
• Addition reactions


  two molecules combine
• Rearrangement reactions


  a molecule undergoes changes in the way its atoms are connected
• Mechanism

  • an overall description of how a reaction occurs
  • describes what takes place at each stage of a chemical transformation
  • which bonds are broken and in what order
  • which bonds are formed and in what order
  • what are the relative rates of the steps
• How Organic Reactions Occur: Mechanisms

  All chemical reactions involve:

  • bond-breaking in the reactant molecules
  • bond-making in the product molecules
• Symmetrical (homolytic)


  one electron remains with each of the product fragment

  : symmetrical bond-breaking (radical) - one bonding electron stays with each product
• unsymmetrical (homolytic)


  both electrons remain with one product fragment, leaving the other fragment with a vacant orbital

  : unsymmetrical bond-breaking (polar) - two bonding electrons stay with one product
• How Organic Reactions Occur: Mechanisms

  Two possible ways of bond-breaking:

  1 symmetrical (homolytic)

  2 unsymmetrical (heterolytic)

  Two possible ways of bond formation:

  1 symmetrical

  2 unsymmetrical
• unsymmetrical


  both bonding electrons are donated by one reactant

  : unsymmetrical bond making (polar) - two bonding electrons are donated by one reactant
• curved arrows


  these arrows indicate breaking and forming of bonds
• half-head arrowheads


  • also known as fish-hook
  • these arrowheads indicate homolytic and homogenic steps called radical processes
• Complete-head arrowheads

  these arrowheads indicate heterolytic and heterogenic steps called polar processes
• Radical Reaction


  a process that involves symmetrical bond breaking and bond formation
• Radical or free radical

  neutral chemical species

  contains an odd number of electrons

  has a single, unpaired electron in one of its orbitals
• Polar Reaction


  a process that involves unsymmetrical bond-breaking and bond-formation
• Polar reactions


  involve species that have an even number of electrons and

  thus have only electron pairs in their orbitals

  note: polar processes are the more common reaction type in organic and biological chemistry.
• Nucleophile 


  • a substance that is “nucleus loving” and thus attracted to a positive charge
  • has a negatively polarized, electron-rich atom
  • can form a bond by donating an electron pair to a positively polarized, electron-poor atom
  • can be either neutral or negatively charged and usually have lone pairs of electrons
  • strong nucleophiles are recognizable by the presence of sodium, lithium, or potassium ions (NaOCH3, LiCH3, NaOH or KOH, NaCN or KCN, NaNH2, NaNHR, NaNR2, NaI, KI, NaN3)
  • neutral nucleophiles (H2O, ROH, H2 S, RSH)
• Hydrogen chloride: HCl

  • strong acid
  • powerful proton donor
  • good electrophile
  • the reaction of HCl with ethylene is a typical electrophile-nucleophile combination
• transition state


  represents the highest-energy structure involved in the step of the reaction
• Eact


  the energy difference between the reactants and the transition state

  a measure of how rapidly the reaction occurs
• large Eact


  this type of Eact results in a slow reaction

  • because few of the reacting molecules collide with enough energy to reach the transition state
• small Eact


  this type of Eact results in a rapid reaction

  • almost all reacting molecules are energetic enough to climb to the transition state
• minimum energy


  the energy level of the carbocation
• carbocation


  formed transiently during the course of the multistep reaction
• energy is released , exothermic reaction , favorable reaction


  if the energy level of the product is lower than that of the reactants
• energy is absorbed, endothermic reaction, unfavorable reaction


  if the energy level of the product is higher than that of the reactants
• catalysts


  • a substance that increases the rate of a chemical transformation by providing an alternative mechanism pathway with lower activation energy for the reaction
  • takes part in the reaction but is regenerated at some point
  • undergoes no net change
• Enzymes


  large, structurally complex catalysts

  • provide reaction mechanisms that proceed through a series of small steps rather than through one or two large steps
• exergonic reaction


  the reactants are at a higher free energy level than the products (reaction goes energetically downhill)
• endergonic reaction


  the reactants are at a lower free energy level than the products (reaction goes energetically uphill)
• Describing a reaction: Catalysis

  In the absence of palladium, an alkene undergoes no reaction with H2 gas even at high temperature.
• Describing a reaction: Catalysis

  In the presence of palladium, reaction occurs rapidly at room temperature.