Organic chemistry m4-5

• Chemical reaction


  A process in which one or more substances (called reactants) are converted to one or more different substances (called products)
• Substances are either chemical elements or compounds
• A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products
• Chemical reactions are an integral part of technology, of culture, and indeed of life itself
• Chemical reactions must be distinguished from physical changes
• If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same
• If water, as ice, liquid, or vapor, encounters sodium metal (Na), the atoms will be redistributed to give the new substances molecular hydrogen (H2) and sodium hydroxide (NaOH), indicating a chemical change or reaction
• Writing chemical equations


  1. Know the facts about the chemical reaction

  2. Write the correct formula of the reactants and products

  3. Balance the chemical equation
• A balanced chemical equation shows equal number of atoms of elements on the reactant side and product side
• Main types of organic reactions

  • Substitution Reactions
  • Elimination Reactions
  • Addition Reactions
  • Rearrangement Reactions
  • Reduction-Oxidation Reactions
• Substitution reaction


  Any of a class of chemical reactions in which an atom, ion, or group of atoms or ions in a molecule is replaced by another atom, ion, or group
• Elimination reaction


  Any of a class of organic chemical reactions in which pair of atoms or groups of atoms are removed from a molecule, usually through the action of acids, bases, or metals and, in some cases, by heating to a high temperature
• Types of elimination reactions

  • Dehydration - removal of water
  • Dehydrogenation - removal of hydrogen
  • Dehalogenation - removal of halogen
  • Dehydrohalogenation - removal of hydrogen and halogen
• Addition reaction


  Any of a class of chemical reactions in which an atom or group of atoms is added to a molecule
• Types of addition reactions

  • Hydration - addition of water
  • Hydrogenation - addition of hydrogen
  • Halogenation - addition of halogen (bromination, chlorination, or fluorination)
  • Hydrohalogenation - addition of hydrogen and halogen
• Rearrangement reaction


  A reaction that involves the one step migration of an H atom or of a larger molecular fragment within a relatively short lived intermediate, or a multi-step reaction that includes the migration of an H atom or of a larger molecular fragment as one of its steps
• Types of rearrangement reactions

  • Curtius Rearrangement
  • Claisen Rearrangement
  • Beckmann Rearrangement
  • Hofmann Rearrangement
  • Pericyclic Rearrangement
  • Photochemical rearrangements
• Oxidation-reduction (redox) reaction


  Any chemical reaction in which the oxidation number of a participating chemical species changes
• Oxidation-reduction reactions are as common and familiar as fire, the rusting and dissolution of metals, the browning of fruits, and respiration and photosynthesis
• Photoreactions


  Interconvert isomeric compounds
• Rearrangement


  More general than "isomerization"
• Primary photochemical rearrangements


  • Cis-trans isomerization
  • Sigmatropic rearrangements
  • Electrocyclic rearrangements
  • Structural rearrangements which result from intramolecular cycloadditions
• Oxidation-reduction reaction (redox reaction)

  Any chemical reaction in which the oxidation number of a participating chemical species changes
• Reaction mechanism


  A step-by-step description of what occurs during a reaction on the molecular level
• Elementary process


  A single moment during a reaction in which molecules break and/or form new bonds
• It is impossible to prove a reaction mechanism for certain
• Requirements for a valid reaction mechanism


  • The sum of each elementary step must yield the overall reaction equation
  • The rate law of the rate-determining step must agree with the experimentally determined rate law
• Benzoin condensation


  1. Cyanide ion (CN-) acts as a catalyst

  2. Proton (H+) transfers occur

  3. Arrow pushing method is used to show where electron pairs go

  
• Reaction intermediate


  A species which appears in the mechanism of a reaction, but not in the overall balanced equation
• Chemical intermediate


  Any chemical substance produced during the conversion of some reactant to a product
• Types of reaction intermediates

  • Carbanions
  • Carbocations
  • Free radicals
  • Carbenes

  
• Carbanion


  A reaction intermediate with a negative charge on a carbon atom, formed by treating an organic compound with a strong base
• Carbocation


  A molecule with a positive charge on a carbon atom and three bonds
• Free radical


  Contains a single, unpaired electron, formed when a covalent bond is broken
• Carbene


  Has a six-electron sp2 structure with a lone pair but no overall charge, can be electrophilic or nucleophilic
• Methods of forming reaction intermediates

  • Homolytic cleavage
  • Heterolytic cleavage
• Sites of organic reactions

  • Multiple bonds
  • Polar bonds
  • Lewis acids and bases
• Markovnikov's rule


  When halogen acid (HX) is added to an unsymmetrical alkene, the negative component attaches to the carbon with fewer hydrogen atoms

  
• Zaitsev's rule


  In an elimination reaction, the major product is the more stable alkene with the more highly substituted double bond

  
• Benzene


  Though unsaturated, it undergoes substitution reactions rather than addition reactions