MWF_15 - Organic Lecture Series CH 310/318 M Textbook...

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Unformatted text preview: Organic Lecture Series CH 310/318 M Textbook Assignment: Chapter 6 LECTURE 15 Today's Topics: Alkenes-structure; nomenclature Notice & Announcements: NO HW this week-XM1 on Wednesday 1 Organic Lecture Series Reactions of Alkenes Chapter 6 2 Organic Lecture Series Reactions of Alkenes 3 Organic Lecture Series Characteristic Reactions 4 Organic Lecture Series Reactions Mechanisms A reaction mechanism describes how a reaction occurs and explains the following. Which bonds are broken and which new ones are formed. The order and relative rates of the various bond-breaking and bond-forming steps. If in solution, the role of the solvent. If there is a catalyst, the role of a catalyst. The position of all atoms and energy of the entire system during the reaction. 5 Organic Lecture Series Gibbs Free Energy Gibbs free energy change, G0: A thermodynamic function relating enthalpy, entropy, and temperature. 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. 6 Organic Lecture Series Energy Diagrams Enthalpy change, 0: The difference in total bond energy between reactants and products. a measure of bond making (exothermic) and bond breaking (endothermic). Heat of reaction, 0: 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. 7 Organic Lecture Series 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. 8 Organic Lecture Series 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. 9 Organic Lecture Series Energy Diagrams An energy diagram for a one-step reaction with no intermediate. 10 Organic Lecture Series Energy Diagrams An energy diagram for a two-step reaction with one intermediate. 11 Organic Lecture Series 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. 12 Organic Lecture Series Electron Pushing Rule 2: Arrows are never used to indicate the movement of atoms. 13 Organic Lecture Series Electron Pushing Rule 2: Arrows are never used to indicate the movement of atoms. 14 Organic Lecture Series Electron Sources & Sinks Rule 3 Arrows always start at an electron source and end at an electron sink. Electron source: Most commonly a bond or a lone pair of electrons on an atom. Electron sink: An atom in a molecule or ion that can accept a new bond or a lone pair of electrons. 15 Organic Lecture Series Electron Sources & Sinks Rule 4 Bond breaking will occur to avoid overfilling valence (hypervalence) on an atom serving as an electron sink 16 Organic Lecture Series Patterns of e- Movement 1. Redistribution of bonds and/or lone pairs. 2. Formation of a new bond from a lone pair or a bond. 3. Breaking a bond to give a new lone pair or a bond. 17 Mechanisms: Make-a-bond Organic Lecture Series Pattern 1: Make a new bond between a nucleophile (source for an arrow) and an electrophile (sink for an arrow). 18 Organic Lecture Series Mechanisms: Break-a-bond Pattern 2: Break a bond so that relatively stable molecules or ions are created. 19 Organic Lecture Series 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. 20 Organic Lecture Series 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. 21 ...
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