Chapter 11 - Chapter 11 Reactions of Alkyl Halides...

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Unformatted text preview: Chapter 11: Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations 11.1 The Discovery of Nucleophilic Substitution Reactions 11.2 The SN2 Reaction Substitution, Nucleophilic, and Bimolecular (second order kinetics- rate is dependent on two species) rate = k * [substrate RX] * [Nucleophile] because reaction is concerted 1. The nucleophile attacks the alkyl halide from the backside and the stereochemistry at carbon is fully inverted remember- the CIP priority may be altered, so the R or S designation may or may not switch 11.3 Characteristics of the SN2 Reaction *will never occur on an sp2 hybridized carbon The Substrate: must be relatively unhindered, never tertiary The Nucleophile: better nucleophiles are negatively charged, nucleophilicity trends are opposite of electronegativity nucleophiles attack carbon, bases attack hydrogen The Leaving Group: best able to stabilize negative charge- oTOS, I, Br, Cl The Solvent: polar aprotic 11.4 The SN1 Reaction substitution, nucleophilic, unimolecular (first order kindetics- rate is dependent on substrate concentration only) rate = k * [RX substrate] 1.spontaneous dissociation of the alkyl bromide to generate C+ and Br- (slow, rate limiting step) 2.carbocation intermediate reacts with the nucleophile in a fast step will yield a near racemate due to ion pair effect 11.5 Characteristics of the SN1 Reaction The Substrate: favors a stable carbocation (highly substituted, allylic, benzylic) Leaving Group: basically the same as SN2, neutral water is often a leaving group (protonated alcohols) Nucleophile: does not determine rate, same qualities as SN2 though Solvent: opposite SN2 to stabilize the transition state (protic, polar) 11.6 Biological Substitution Reactions 11.7 Elimination Reactions of Alkyl Halides: Zaitsev's Rule Zaitsev's Rule: base induced elimination reactions generally (although not always) give the more stable alkene product- the one with more alkyl substituents on double-bond carbons) unless: dictated by E2 geometry requirements or "bulky bases" E1: C-X bond breaks, giving C+ intermediate, and a proton is removed to form and alkene E2: C-H and C-X bonds break simultaneously, giving the alkene in a single step 11.8 The E2 Reaction and the Deuterium Isotope Effect Second order kinetics: rate = k * [RX] * [base] one concerted step must have antiperiplanar geometry- easy to get in linear molecules because of free rotation about C-C double bonds, but in cyclic molecules, geometry constraints may lead to non-Zaitsev product Solvent: strong base heat drives elimination reactions 11.9 The E2 Reaction and Cyclohexane Conformation antiperiplanar = trans diaxial, may override Zaitsev's rule 11.10 The E1 and E1cB Reactions 1. C-X bond spontaneously breaks (same as SN1) 2. Loss of neighboring H+ yields alkene product Solvent: weak base Heat drives elimination reactions No reason to yield non-Zaitsev product 11.11 Biological Elimination Reactions 11.12 A Summary of Reactivity: SN1, SN2, E1, E1cB, E2 Primary: Good nucleophile: SN2 Strong base: E2 Secondary: Good nucleophile/polar aprotic solvent: SN2 Strong Base: E2 allylic and benzylic halides: SN1 or E1 Tertiary strong base: E2 SN1 or E1...
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This note was uploaded on 04/26/2009 for the course CHEM 227 taught by Professor Santander during the Fall '08 term at Texas A&M.

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