Movie time mechanism

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Movie time (mechanism)
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SN2 involves a transition state
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SN1 involves an intermediate (carbocation)
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Carbocations

• Trigonal planar, sp2
• Stability (tertiary gt secondary gt primary gt
  methyl)
• hyperconjugation

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Movie time (SN1 stereochemistry)
(mechanism and stereochemistry)
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Solvolysis reactions
Examples
(CH3)3C-Cl H2O
(CH3)3C-OH HCl
(CH3)3C-Cl CH3OH
(CH3)3C-OCH3 HCl
(CH3)3C-Cl RC(O)OH
(CH3)3C-OC(O)CH3 HCl
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Mechanism
Step 1 (CH3)3-Cl (CH3)3
Cl- Step 2
O () OH (CH3)3
H-O-C-R (CH3)3-OC-R Step 3
() O-H
O (CH3)3-OC-R Cl
(CH3)3-O-C-R HCl
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Can we make any prediction about SN1 and SN2 ?
Several factors concur to determine which
mechanism will be followed 1. Structure of the
substrate.
SN2
Me gt primary gt secondary gtgt tertiary
(unreactive) Spatial arrangement at or near the
reacting site hinders or retard the reaction
(steric effect). Large bulky groups introduce
what is called steric hindrance.
Steric hindrance of the substrate is a primary
factor affecting SN2
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Movie (steric hindrance)
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1. Structure of the substrate.
SN1
Relative stability of the carbocation is the
primary factor
tertiary, allylic, benzylic
hyperconjugation
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SN1
The carbocations are thermodynamically uphill.
Any factor stabilizing the carbocation makes the
reaction faster.
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2. Concentration of nucleophile and its
strenght. SN2
The kinetic law Clearly shows that the higher
is the concentration, the higher is the rate
dconc dt
Reaction rate
kNusubst
CH3O- is a good nucleophile while
CH3OH is a poor nucleophile OH- is
a good nucleophile while H2O is a
poor nucleophile

• A negatively charged Nu is more reactive than its
  neutral conj. acid
• 2. Given the same Nu atom, nucleophilicity
  parallels basicity.
• RO- gt HO- gtgt RCO2- gtROH gtH2O

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3. Solvent Effect on SN2
While in protic solvent the nucleophilicity
parallels atom dimension rather than
bacicity Basicity RO-
gt RS- F gt Cl gt Br gt I Nucleophilicity
RO- lt RS- F lt Cl lt Br lt I Why ???
Small basic atoms O and F engage in strong H
bonding with the protic solvent thus quenching
the nucleophilicity
In addition, larger atoms are more polarizable so
they can donate more electron density to the
substrate
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Basicity and Nucleophilicity are related but
measured in different manners
Position of an equilibrium determined by pKa
Relative reaction rates CN- less basic than
OH- CN- more nucleoph. than OH-
In protic solvents Nucleophilicity SH gt CN gt I
gt OH gt N3 gt Br gt CH3CO2 gt Cl gt F gt H2O
In polar aprotic solvents (no H bond
possible) Cations are well solvated and the
naked counteranions display nucleophilicity that
parallels basicity F gt Cl gt Br gt I Thus,
depending on the reaction conditions the
order can be completely reversed.
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Solvent Effect on SN1
Use of polar solvents greatly stabilizes
carbocations and increases the reaction rate
More polar solvent faster SN1
80 59 49 37 24 21
Water Formic acid DMSO DMF Ethanol acetone
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Nature of the Leaving Group
In both SN1 and SN2 the leaving group is X-.
Every factor stabilizing the anion by
delocalization of the negative charge, will
increase the reaction rate. So I gt Br gt
Cl gt F (reverse of the basicity) I- will
delocalize better than F the extra electron by
placing it in a much larger volume where
electrons necessarily are more far away from the
nucleus - Anion stabilized by resonance are
excellent leaving groups - Strongly basic anions
are NEVER good leaving groups
(R- and H- NEVER work as leaving groups.)
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Summary (SN1 versus SN2)
SN1 SN2 Substrate
tertiary Me gt 1 gt 2 (low hindrance) Nucleophile
weak lewis base (neutral)
Strong lewis base, high conc. Solvent polar
protic Polar aprotic (or protic) Leaving group
I gt Br gt Cl gt F (less
basic, more stable, the best)
Vinyl and phenyl halides are UNREACTIVE
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Elimination Reactions
Lets treat a tertiary R-X with a very strong
base / nucleophile (SN2 conditions except for the
nature of the substrate)
EtONa
CH3CHCH3 CH2CH-CH3
NaBr EtOH
EtOH
Br
CH3
EtONa
CH3-C-Br CH3-CCH2
NaBr EtOH
EtOH
CH3
Dehydrohalogenation an Elimination reaction
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Elimination reaction Rate
kR-Xbase, 2nd order, thus bimolecular Thus
E2
Movie
Stereochemical implications (formation of cis
versus trans or viceversa)
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(No Transcript)
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A consequence
cis
trans