lecture 11 - 11. Reactions of Alkyl Halides: Nucleophilic...

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11. Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations Based on McMurry’s Organic Chemistry , 7 th edition
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2 Alkyl Halides React with Nucleophiles and Bases Alkyl halides are polarized at the carbon-halide bond, making the carbon electrophilic Nucleophiles will replace the halide in C-X bonds of many alkyl halides(reaction as Lewis base) Nucleophiles that are Brønsted bases produce elimination
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3 Why this Chapter? Nucleophilic substitution, base induced elimination are among most widely occurring and versatile reaction types in organic chemistry Reactions will be examined closely to see: - How they occur - What their characteristics are - How they can be used
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4 11.1 The Discovery of Nucleophilic Substitution Reactions In 1896, Walden showed that (-)-malic acid could be converted to (+)-malic acid by a series of chemical steps with achiral reagents This established that optical rotation was directly related to chirality and that it changes with chemical alteration Reaction of (-)-malic acid with PCl 5 gives (+)- chlorosuccinic acid Further reaction with wet silver oxide gives (+)-malic acid The reaction series starting with (+) malic acid gives (-) acid
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5 Reactions of the Walden Inversion
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6 Significance of the Walden Inversion The reactions alter the array at the chirality center The reactions involve substitution at that center Therefore, nucleophilic substitution can invert the configuration at a chirality center The presence of carboxyl groups in malic acid led to some dispute as to the nature of the reactions in Walden’s cycle
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7 11.2 The S N 2 Reaction Reaction is with inversion at reacting center Follows second order reaction kinetics Ingold nomenclature to describe characteristic step: S=substitution N (subscript) = nucleophilic 2 = both nucleophile and substrate in characteristic step (bimolecular)
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8 Kinetics of Nucleophilic Substitution Rate (V) is change in concentration with time Depends on concentration(s), temperature, inherent nature of reaction (barrier on energy surface) A rate law describes relationship between the concentration of reactants and conversion to products A rate constant (k) is the proportionality factor between concentration and rate Example: for S converting to P V = d[S]/dt = k [S]
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9 Reaction Kinetics The study of rates of reactions is called kinetics Rates decrease as concentrations decrease but the rate constant does not Rate units: [concentration]/time such as L/(mol x s) The rate law is a result of the mechanism The order of a reaction is sum of the exponents of the concentrations in the rate law – the example is second order
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10 S N 2 Process The reaction involves a transition state in which both reactants are together
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lecture 11 - 11. Reactions of Alkyl Halides: Nucleophilic...

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