5.1_nano_substitution_bimolecular_post_

5.1_nano_substitutio - Overview of Chemical Reactions A few Different Types of Chemical Reactions All involve bond breaking and bond formation and

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5.1 - SN2 1 Substitution Additions Eliminations Rearrangements A few Different Types of Chemical Reactions -Cl on -CH 3 has been substituted with an -OH Br 2 has been added to C 2 H 2 HBr has been eliminated or removed form C 2 H 5 Br The carbon skeletal system, C 6 H 12 , has been rearranged All involve bond breaking and bond formation and the flow of electrons Overview of Chemical Reactions
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5.1 - SN2 2 Homolysis and Heterolysis of Covalent Bonds Heterolysis – Cleavage of sigma bond occurs so that one fragment take away both electrons Double headed arrow shows the movement of two electrons Heterolysis requires that the bond be polarized which is due to the different electronegativities between the atoms Often heterolysis is assisted by a molecule with an unshared pair of electrons (a base) that can form a bond to on to the atoms. (-ve goes to +ve) Double headed arrow shows the movement of two electrons Heterolytic bond cleavage Homolysis – Cleavage of sigma bond occurs so that each fragment takes away one electron. Single headed arrow shows the movement of one electrons
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5.1 - SN2 3 Heterolysis of Bonds to Carbons: Carbanions and Carbocations Reaction can occur to give a carbocation or carbanion depending on the nature of Z Carbocations have only 6 valence electrons and a positive charge Carbanions have 8 valence electrons and a negative charge
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5.1 - SN2 4 Transition State Theory: Free Energy Diagrams Why does a reaction occur? Δ = Δ -T Δ Remember from 1 st year For a spontaneous energy to occur the Gibbs free energy ( Δ G°) must be negative. Thus you can have different combinations of Δ H° and Δ S° values for a reaction to occur. A process that gives off heat ( Δ H° is negative, exothermic) and becomes more disordered ( Δ S° is positive) a spontaneous reaction will occur and Δ will be negative for all temperatures A process that has an increase in enthalpy ( Δ H° is positive, endothermic) and a decrease increase in order ( Δ S° is negative) a reaction will be nonspontaneous at all temperatures. The questionable cases are those in which the entropy and enthalpy factors work in opposition. In general the Δ H factor dominates al low temperature and the T Δ S term at high temperatures. total or free energy change change in bonding energy change in disorder Volhardt: 2-1
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5.1 - SN2 5 There is an energy barrier to overcome before the reaction will occur Energy Barrier that the reactants must surmount in order to reach the lower energy level Exergonic: The free energy level of the products is lower than that of the reactants
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5.1 - SN2 6 Endergonic Reaction Positive free-energy change Energy Barrier ( Δ G )that the reactants must surmount is much larger for a endergonic reaction than for an exergonic reaction
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5.1 - SN2 7 Relationship between Temperature and Rate of Reaction For many reactions taking place at room temperature, a 10°C increase in temperature will cause the reaction rate to double.
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This note was uploaded on 07/18/2011 for the course NE 122 taught by Professor Nano during the Winter '09 term at Waterloo.

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5.1_nano_substitutio - Overview of Chemical Reactions A few Different Types of Chemical Reactions All involve bond breaking and bond formation and

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