section_5_Sub-Elim_complete_to_What_abou

section_5_Sub-Elim_complete_to_What_abou - Overview of...

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Section 5: Substitution and Elimination 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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Section 5: Substitution and Elimination 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 (a polarized bond is due to a difference in electronegativities between the two atoms which are bonded) Often heterolysis is assisted by a molecule with an unshared pair of electrons (Lewis base) that can form a new bond (with the Lewis acid): -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 electron
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Section 5: Substitution and Elimination 3 Heterolysis of Bonds to Carbons: Carbanions and Carbocations Reactions can occur to give a carbocation or carbanion depending on the nature of Z (partial charge due to polarization of bond) Carbocations have only 6 valence electrons and a positive charge: Carbanions have 8 valence electrons and a negative charge (one lone pair):
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Section 5: Substitution and Elimination 4 Substitution and Elimination Reactions Unit Overview: In this unit we will examine 4 different types of reactions: 2 types of substitution reactions: S N 1 and S N 2 and 2 types of elimination reactions: E1 and E2. Overview of the Reaction Mechanism Reaction Rates: Energy, Transition State, Collisions, Sterics, Catalysts Reaction Terminology Reaction Mechanisms: What is a mechanism? Substitution versus Elimination Reactions; Bimolecular versus Unimolecular Reactions; Stereochemistry. Factors Determining the Rate of Reaction: which reaction pathway is favoured given a set of reaction conditions: S N 1 versus S N 2 versus E1 versus E2. Type of Nucleophile Type of Leaving Group Solvent Effects Type of Substrate Other Reaction Conditions
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Section 5: Substitution and Elimination 5 Transition State Theory: Free Energy Diagrams Why Does a Reaction Occur? Δ = Δ -T Δ Remember from 1 st year For a spontaneous reaction 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) will produce a spontaneous reaction and Δ will be negative for all temperatures.
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This note was uploaded on 09/13/2011 for the course CHEM 228 taught by Professor Jacobfischer during the Spring '09 term at Waterloo.

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section_5_Sub-Elim_complete_to_What_abou - Overview of...

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