Reactions of Alkenes Chapter 6 1 Reactions Mechanisms • A reaction mechanism describes how a reaction occurs and explains the following. (cid:1) Which bonds are broken and which new ones are formed. (cid:1) The order and relative rates of the various bond-breaking and bond-forming steps. (cid:1) If in solution, the role of the solvent. (cid:1) If there is a catalyst, the role of a catalyst. (cid:1) The position of all atoms and energy of the entire system during the reaction. 2 Gibbs Free Energy • Gibbs free energy change, ∆G0: – A thermodynamic function relating enthalpy, entropy, and temperature. ∆∆∆∆G0 = ∆∆∆∆H0 –T∆∆∆∆S0 – Exergonic reaction: A reaction in which the Gibbs free energy of the products is lower than that of the reactants; the position of equilibrium for an exergonic reaction favors products. – Endergonic reaction: A reaction in which the Gibbs free energy of the products is higher than that of the reactants; the position of equilibrium for an endergonic reaction favors starting materials. 3 Energy Diagrams ∆Η0 • Enthalpy change, : The difference in total bond energy between reactants and products. – a measure of bond making (exothermic) and bond breaking (endothermic). ∆Η0 • Heat of reaction, : The difference in enthalpy between reactants and products. – Exothermic reaction: A reaction in which the enthalpy of the products is lower than that of the reactants; a reaction in which heat is released. – Endothermic reaction: A reaction in which the enthalpy of the products is higher than that of the reactants; a reaction in which heat is absorbed. 4 Energy Diagrams • Energy diagram: A graph showing the changes in energy that occur during a chemical reaction. • Reaction coordinate: A measure in the change in positions of atoms during a reaction. y g r e n E Reaction coordinate 5 Activation Energy • Transition state ‡: – An unstable species of maximum energy formed during the course of a reaction. – A maximum on an energy diagram. • Activation Energy, ∆G‡: The difference in Gibbs free energy between reactants and a transition state. – If ∆G‡ is large, few collisions occur with sufficient energy to reach the transition state; reaction is slow. – If ∆G‡ is small, many collisions occur with sufficient energy to reach the transition state; reaction is fast. 6 Energy Diagrams – An energy diagram for a one-step reaction with no intermediate. 7 Energy Diagrams • An energy diagram for a two-step reaction with one intermediate. 8 Electron Pushing • Organic chemists use a technique called electron pushing, alternatively called arrow pushing, to depict the flow of electrons during a chemical reaction. • Rule 1: Arrows are used to indicate movement of electrons. 9 Electron Pushing • Rule 2: Arrows are never used to indicate the movement of atoms. 10 Electron Pushing • Rule 2: Arrows are never used to indicate the movement of atoms. 11 Electron Sources & Sinks (cid:2) Rule 3 Arrows always start at an electron source and end at an electron sink. (cid:2) π Electron source: Most commonly a bond or a lone pair of electrons on an atom. (cid:2) Electron sink: An atom in a molecule or ion that can accept a new bond or a lone pair of electrons. 12 Electron Sources & Sinks (cid:2) Rule 4 Bond breaking will occur to avoid overfilling valence (hypervalence) on an atom serving as an electron sink 13 Patterns of e- Movement π 1. Redistribution of bonds and/or lone pairs. σ 2. Formation of a new bond from a π lone pair or a bond. σ 3. Breaking a bond to give a new lone π pair or a bond. 14 Mechanisms: Make-a-bond • Pattern 1: Make a new bond between a nucleophile (source for an arrow) and an electrophile (sink for an arrow). 15 Mechanisms: Break-a-bond • Pattern 2: Break a bond so that relatively stable molecules or ions are created. 16 Proton transfer • Pattern 3: Add a proton Use this pattern when there is a strong acid present or a molecule that has a strongly basic functional group. 17 Proton transfer • Pattern 4: Take a proton away. Use this pattern when a molecule has a strongly acidic proton or there is a strong base present. 18 Electrophilic Additions – Hydrohalogenation using HCl, HBr, HI – Hydration using H O in the presence of 2 H SO 2 4 – Halogenation using Cl , Br 2 2 – Halohydrination using HOCl, HOBr – Oxymercuration using Hg(OAc) , H O 2 2 followed by reduction 19 Characteristic Reactions Reaction Descriptive Name(s ) H C C + HCl C C Hydrochlorination (HX) (hydrohalogenation) Cl(X) H + H O C C Hydration 2 C C OH (X)Br C C + Br C C Bromination 2 (X ) Br( X) (halogenation) 2 HO H O 2 C C + Br C C Halohydrin formation 2 (X ) Br( X) (Bromohydrin 2 formation) 20
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