Basic rxns in org chem


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Basic rxns in org chem

  2. 2. Chemical Reaction: A transformation resulting in a change of composition, constitution and/or configuration of a compound. Organic reactions occur between organic molecules (molecules containing carbon and hydrogen). Since there is a virtually unlimited number of organic molecules, the scope of organic reactions is very large. However, many of the characteristics of organic molecules are determined by functional groups—small groups of atoms that react in predictable ways. TYPES OF REACTIONS: A. By Structural Change  Addition- the number of σ-bonds in the substrate molecule increases, usually at the expense of one or more π-bonds
  3. 3.  Elimination- the number of σ-bonds in the substrate decreases, and new π-bonds are often formed. Substitution-characterized by replacement of an atom or group (Y) by another atom or group (Z). Aside from these groups, the number of bonds does not change.
  4. 4. Rearrangement- generates an isomer, and again the number of bonds normally does not change. EXAMPLE: addition and elimination reactions
  5. 5. B. Classification by Reaction Type  Acid- Base Reactions- also called neutralization reactions. Uses the Bronsted Theory and the Lewis Theory . NH4 (+) + H2O H3O(+) + NH3  Redox Reactions-the change in oxidation state of those carbons involved in a chemical transformation is taken into account. To determine whether a carbon atom has undergone a redox change during a reaction we simply note any changes in the number of bonds to hydrogen and the number of bonds to more electronegative atoms such as O, N, F, Cl, Br, I, & S that has occurred
  6. 6. INTERMEDIATES- The products of bond breaking, shown above, are not stable in the usual sense, and cannot be isolated for prolonged study.  Carbocations are compounds in which carbon bears a positive charge. Classical carbocations or carbenium ions are trivalent, and have only six valence electrons. Nonclassical carbocations or carbonium ions are tetra- or pentavalent, and have eight valence electrons. Examples are the methyl cation (classical) CH3 +, and the methonium ion (nonclassical) CH5
  7. 7.  Carbanions are compounds in which carbon bears a negative charge. A carbanion will always have a positive counterion in association with it; depending upon the particular cation and the stability of the carbanion, the association may be ionic, covalent, or some intermediate combination of ionic and covalent bonding, as shown below (M = metal). Carbanions are trivalent, with eight valence electrons. Free radicals are neutral compounds having an odd number of electrons and therefore one unpaired electron. Carbon free radicals are trivalent, with seven valence electrons, and typically assume a planar structure.
  8. 8. Free radicals are primarily electron-deficient species and are stabilized by structural features which donate electron density or delocalize the odd electron by resonance Radical ions - charged compounds with an unpaired electron, and are either radical cations or radical anions. They are derived from a stable neutral molecule by addition of one electron or removal of one electron Carbenes - compounds which have a divalent carbon. The divalent carbon also has two nonbonded electrons, for a total of six valence electrons. The two nonbonded electrons may have either the same spin quantum number, which is a triplet state, or an opposite spin quantum number, which is a singlet state. Generation of carbenes is most commonly by photolysis or thermolysis of diazo compounds or ketenes, or by alpha-elimination reactions
  9. 9. REAGENTS- common partner of the reactant in many chemical reactions. It may be organic or inorganic; small or large; gas, liquid or solid. The portion of a reagent that ends up being incorporated in the product may range from all to very little or none. TYPES: •Electrophiles- Reagents that seek electrons so as to achieve a stable shell of electrons like that of a noble gas. •Nucleophiles- On the other hand, these are reagents that seek a positive centre other than a proton.
  10. 10. Factors that Influence Reactions A. Energetics: The potential energy of a reacting system changes as the reaction progresses. The overall change may be exothermic ( energy is released ) or endothermic ( energy must be added ), and there is usually an activation energy requirement as well. As a rule, compounds constructed of strong covalent bonds are more stable than compounds incorporating one or more relatively weak bonds. B. Electronic Effects: The distribution of electrons at sites of reaction (functional groups) is a particularly important factor. Electron deficient species or groups, which may or may not be positively charged, are attracted to electron rich species or groups, which may or may not be negatively charged.
  11. 11. We refer to these species as electrophiles & nucleophiles respectively. In general, opposites attract and like repel. The charge distribution in a molecule is usually discussed with respect to two interacting effects: An inductive effect, which is a function of the electronegativity differences that exist between atoms (and groups); and a resonance effect, in which electrons move in a discontinuous fashion between parts of a molecule. C. Steric Effects: Atoms occupy space. When they are crowded together, van der Waals repulsions produce an unfavorable steric hindrance. Steric hindrance may influence conformational equilibria, as well as destabilizing transition states of reactions.
  12. 12. D. Stereoelectronic Effects: In many reactions atomic or molecular orbitals interact in a manner that has an optimal configurational or geometrical alignment. Departure from this alignment inhibits the reaction. E. Solvent Effects: Most reactions are conducted in solution, not in a gaseous state. The solvent selected for a given reaction may exert a strong influence on its course. Remember, solvents are chemicals, and most undergo chemical reaction under the right conditions. .
  13. 13. ACTIVATION ENERGY Every reaction in which bonds are broken will have a high energy transition state that must be reached before products can form. In order for the reactants to reach this transition state, energy must be supplied and reactant molecules must orient themselves in a suitable fashion. The energy needed to raise the reactants to the transition state energy level is called the activation energy, ΔE‡. Exothermic Endothermic Exothermic Single Step Reaction Single Step Reaction Two Step Reaction
  14. 14. What is the source of the activation energy that enables a chemical reaction to occur? Often it is heat, as noted above in reference to the flame or spark that initiates methane combustion. At room temperature, indeed at any temperature above absolute zero, the molecules of a compound have a total energy that is a combination of translational (kinetic) energy, internal vibrational and rotational energies, as well as electronic and nuclear energies. The temperature of a system is a measure of the average kinetic energy of all the atoms and molecules present in the system.