Day_Lecture_III

Day_Lecture_III - ORGANIC CHEMISTRY 307 LECTURE NOTES III...

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1 ORGANIC CHEMISTRY 307 LECTURE NOTES III R. Boikess II. Principles of Organic Reactions 1. Chemical reactions are the result of bond breaking and bond making. a. Most (but not all) bond making and bond breaking tends to be associated with a functional group. b. Two types of bond breaking (oversimplified) i. Heterolytic, most common. Shared pair winds up with one of the atoms. This atom will therefore have an electron excess (and often a negative charge) and the other one will have an electron deficiency (and often a positive charge). Example HCl or ROH. The more EN atom gets the pair. These reactions tend to take place in solution. ii. Homolytic, less common, but simpler. The two atoms of the bond split the shared pair, each one gets its electron back. No charges generated. Example HCl and Cl 2 . These reactions tend to take place in the gas phase or in nonpolar solvents. So complications due to solvation and other intermolecular interactions tend to be minimized. c. Two types of bond making (oversimplified) i. Heterolytic, Much more important. One of the atoms going to form the bond comes with an available electron pair, the other comes with an electron deficiency or incomplete octet that permits it to accept the pair. Example NH 3 and H + or AlCl 3 and Cl - . ii. Homolytic, each atom brings an electron (note that each atom must therefore have an odd number of valence shell electrons. 2. Understand and systematize reactions by keeping track of the electrons that are involved in bond breaking and making. We show what happens to them using curved arrows. An arrow with one
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2 hook tracks one electron, with two hooks tracks an electron pair. The direction is always the one in which the electrons are moving. Discuss what the arrows mean in detail. Simple examples: homolytic cleavage of a Cl-Cl bond. Heterolytic cleavage of H-Cl bond and formation of an O-H bond with water in a typical acid base reaction. Essential skill to be mastered. 3. More about understanding reactions. Kinetics and Thermodynamics, Key Concepts a. Free Energy criterion for spontaneity and stability i. ΔG = ΔH – TΔS discuss H (standard heat of formation and bond energies) and S (microstates, disorder) ii. ΔG = -RTlnK b. Kinetics: time and reactivity i. rate laws and mechanistic information, rate determining step ii. Arrhenius and barriers c. Tie kinetics and thermo together with a simple graphical picture called reaction coordinate diagram. Very useful (See Figure 2.1 in Vollhardt) even if not very precise. i. y-axis, choice of what to plot, we can do free energy or potential energy ii. x-axis “reaction coordinate”
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3 iii. Examples: one step exothermic and endothermic. Two steps to show an intermediate. d. Transition state theory. Discuss meaning and importance.
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This note was uploaded on 04/04/2008 for the course CHEM 307 taught by Professor Several during the Spring '08 term at Rutgers.

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Day_Lecture_III - ORGANIC CHEMISTRY 307 LECTURE NOTES III...

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