Chapter 7 More Haloalkane Reactions
I.
Unimolecular Nucleophilic Substitution: SN1
A.
Solvolysis
1) 2-bromo-2-methylpropane reacts slowly in SN2
Nu:- +
2)
5)
slow
Nu
+ Br-
Reaction with solvent (water) is fast
H2O:
3)
4)
Br
+
Br
fast
OH
+ Br-
Solvolysis = nucleophilic substitution by solvent
Hydrolysis = solvolysis by water
H2O
+
Br
H2O
+
Br
medium
slow
OH + Br
OH + Br
SN2 should have reverse rates. What is happening?
B.
H2O:-
+
Br
acetic
acid
Solvolysis follows SN1 Mechanism
Br ]
1) Rate = k[
2) Unimolecular rate, depends only on the haloalkane
3) We know water must eventually be involved
4) Rate-Determining Step = slowest step of a stepwise reaction
5)
SN1 Mechanism
a) Step 1 is Dissociation (rate determining step)
Br
+ Br-
i. Formation of a Carbocation
ii. Heterolytic Cleavage
iii. Slow because must separate charges
OH
+ Br-
b)
Step 2 is fast reaction with solvent
+ OH2
fast
alkyloxonium ion
electrophile nucleophile
+ HOR
c)
OH2
OHR
Step 3 is fast deprotonation of the alkyloxonium ion (Only if Needed)
i. Like H3O+, ROH2+ and ROHR+ are strong acids
ii. Fast deprotonation by the solvent gives the alcohol product
H
O
H
+
OH2
fast
OH + H3O+
6)
Potential Energy Diagram for SN2 vs. SN1
7)
Stereochemistry and SN1
Planar carbocation intermediate leads to racemic products
C.
Factors affecting SN1rate of reaction
1) Polar Solvents accelerate the rate of SN1 reactions
a) Polar solvents help charges separate
b) Polar solvent stabilizes the charged carbocation intermediate
2)
Protic Solvents accelerate SN1 reactions
a) Hydrogen bonding from solvent to d- L.G. helps it leave
b) SN2 best solvent was Polar, Aprotic (doesn’t block nucleophile)
c) SN1 best solvent is Polar, Protic
3)
Better L.G. Accelerates SN1
a) Rate Determining Step is the dissociation of the L.G.
b) --OSO2R > --I > --Br > --Cl
4)
Nucleophile Strength has NO EFFECT on rate
a) Rate depends only on the slowest step (Dissociation)
b) Once the carbocation is formed, it will react quickly with any Nu
c) If multiple nucleophiles are present, major product will be best Nu
-
Br
N3
N3
CH3OH
+
OCH3
major
D.
Alkyl Group Effects on SN1 and SN2
1) Carbocation Stability leads to fastest SN1 for Tertiary Carbons
a) 3o C+ > 2o C+ > 1o C+
b) Hyperconjugation is the reason, just like for radical stability
2)
Conclusions
a) Tertiary carbons can only do SN1 nucleophilic substitutions
N3-
Br
b)
CH3OH
Primary carbons can only do SN2 nucleophilic substitutions
OH- + CH3
c)
N3
CH3
Cl
OH + Cl
Secondary carbons can do SN2 or SN1 depending on conditions
i. Best SN1 conditions = Good L.G., Polar Protic Solvent
H2O
ii.
+
Br
water
OH + Br
Best SN2 conditions = Good Nu, Polar Aprotic Solvent
CN
-
acetone
+
I
+ ICN