Chapter 7 More Haloalkane Reactions

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Presentation transcript:

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?

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

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

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

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

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

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