Chapter11 - 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 , 6 th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto
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Based on McMurry, Organic Chemistry, 6th ed 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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Based on McMurry, Organic Chemistry, 6th ed 3 11.1 The Discovery of the Walden Inversion 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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Based on McMurry, Organic Chemistry, 6th ed 4 Reactions of the Walden Inversion
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Based on McMurry, Organic Chemistry, 6th ed 5 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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Based on McMurry, Organic Chemistry, 6th ed 6 11.2 Stereochemistry of Nucleophilic Substitution Isolate step so we know what occurred (Kenyon and Phillips, 1929) using 1- phenyl-2-propanol Only the second and fifth steps are reactions at carbon So inversion certainly occurs in the substitution step
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Based on McMurry, Organic Chemistry, 6th ed 7 Hughes’ Proof of Inversion React S-2-iodo-octane with radioactive iodide Observe, initially that racemization of mixture is twice as fast as incorporation of label so it must be with inversion Racemization in one reaction step would occur at same rate as incorporation
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Based on McMurry, Organic Chemistry, 6th ed 8 The Nature of Substitution Substitution, by definition, requires that a "leaving group", which is also a Lewis base, departs from the reacting molecule. A nucleophile is a reactant that can be expected to participate effectively in a substitution reaction .
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Based on McMurry, Organic Chemistry, 6th ed 9 Substitution Mechanisms S N 1 Two steps with carbocation intermediate Occurs in 3°, allyl, benzyl S N 2 Two steps combine - without intermediate Occurs in primary, secondary
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10 Reactant and Transition-state Energy Levels Higher reactant energy level (red curve) = faster reaction ( smaller G ). Higher
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Chapter11 - 11 Reactions of Alkyl Halides Nucleophilic...

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