Chapter11 - Alkyl Halides React with Nucleophiles and Bases...

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1 11. Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations Based on McMurry’s Organic Chemistry , 6 th edition 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 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 4 Reactions of the Walden Inversion 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 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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2 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 8 What Hughes Proposed to Observe ± Optical rotation will decrease to 0º as + and – forms become equal (racemized). ± Radioactivity will become uniformly distributed. ± If R goes to R + S then the above occur at equal rates ± If R goes to S then racemization is twice as fast (since each new R removes one S and cancels another) ± If R stayed as R, there would be no racemization but radioactivity would be incorporated. 9 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. 10 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 11 Reactant and Transition-state Energy Levels Higher reactant energy level (red curve) = faster reaction ( smaller G ). Higher
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Chapter11 - Alkyl Halides React with Nucleophiles and Bases...

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