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Chapter 7 More Haloalkane Reactions

Chapter 7 More Haloalkane Reactions. Unimolecular Nucleophilic Substitution: S N 1 Solvolysis 2-bromo-2-methylpropane reacts slowly in S N 2 Reaction with solvent (water) is fast Solvolysis = nucleophilic substitution by solvent Hydrolysis = solvolysis by water

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Chapter 7 More Haloalkane Reactions

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  1. Chapter 7 More Haloalkane Reactions • Unimolecular Nucleophilic Substitution: SN1 • Solvolysis • 2-bromo-2-methylpropane reacts slowly in SN2 • Reaction with solvent (water) is fast • Solvolysis = nucleophilic substitution by solvent • Hydrolysis = solvolysis by water • SN2 should have reverse rates. What is happening?

  2. Solvolysis follows SN1 Mechanism • Rate = k[ ] • Unimolecular rate, depends only on the haloalkane • We know water must eventually be involved • Rate-Determining Step = slowest step of a stepwise reaction • SN1 Mechanism • Step 1 is Dissociation (rate determining step) • Formation of a Carbocation • Heterolytic Cleavage • Slow because must separate charges

  3. Step 2 is fast reaction with solvent • Step 3 is fast deprotonation of the alkyloxonium ion (Only if Needed) • Like H3O+, ROH2+ and ROHR+ are strong acids • Fast deprotonation by the solvent gives the alcohol product

  4. Potential Energy Diagram for SN2 vs. SN1 7) Stereochemistry and SN1 Planar carbocation intermediate leads to racemic products

  5. Factors affecting SN1rate of reaction • Polar Solvents accelerate the rate of SN1 reactions • Polar solvents help charges separate • Polar solvent stabilizes the charged carbocation intermediate • Protic Solvents accelerate SN1 reactions • Hydrogen bonding from solvent to d- L.G. helps it leave • SN2 best solvent was Polar, Aprotic (doesn’t block nucleophile) • SN1 best solvent is Polar, Protic • Better L.G. Accelerates SN1 • Rate Determining Step is the dissociation of the L.G. • --OSO2R > --I > --Br > --Cl

  6. Nucleophile Strength has NO EFFECT on rate • Rate depends only on the slowest step (Dissociation) • Once the carbocation is formed, it will react quickly with any Nu • If multiple nucleophiles are present, major product will be best Nu • Alkyl Group Effects on SN1 and SN2 • Carbocation Stability leads to fastest SN1 for Tertiary Carbons • 3o C+ > 2o C+ > 1o C+ • Hyperconjugation is the reason, just like for radical stability

  7. Conclusions • Tertiary carbons can only do SN1 nucleophilic substitutions • Primary carbons can only do SN2 nucleophilic substitutions • Secondary carbons can do SN2 or SN1 depending on conditions • Best SN1 conditions = Good L.G., Polar Protic Solvent • Best SN2 conditions = Good Nu, Polar Aprotic Solvent

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