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