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Nucleophilic Substitution Reactions: S N 1 Mechanism . Two Step Mechanism Which step is rate determining?. A) Step #1 B) Step #2. Solvolysis Reactions. Tertiary alkyl halides are very unreactive in substitutions that proceed by the S N 2 mechanism. - PowerPoint PPT Presentation
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Tertiary alkyl halides are very unreactive in substitutions that proceed by the SN2 mechanism.
But they are highly reactive in solvolysis reactions where the solvent is generally one of the reactants.
Solvolysis Reactions
Hydrolysis of tert-butyl bromide.
+
+
H Br.... :
O: :
H
H
C
CH3
CH3
CH3
Br
C OH....
..
.. :
CH3
CH3
CH3
C+
+
O :
H
H
Br.... :: –
CH3
CH3
CH3
+ O: :
H
H
C+
+
O :
H
H
Br.... :: –
CH3
CH3
CH3
C
CH3
CH3
CH3
Br.... :
This is the nucleophilic substitutionstage of the reaction; the one withwhich we are concerned.
Hydrolysis of tert-butyl bromide.
+ O: :
H
H
C+
+
O :
H
H
Br.... :: –
CH3
CH3
CH3
C
CH3
CH3
CH3
Br.... :
The reaction rate is independentof the concentration of the nucleophileand follows a first-order rate law.
rate = k[(CH3)3CBr]
Hydrolysis of tert-butyl bromide.
+ O: :
H
H
+
Br.... :: –
C+O :
H
HCH3
CH3
CH3
C
CH3
CH3
CH3
Br.... :
The mechanism of this step isnot SN2. It is called SN1 and begins with ionization of (CH3)3CBr.
Hydrolysis of tert-butyl bromide.
rate = k[alkyl halide]First-order kinetics implies a unimolecular
rate-determining step.
Proposed mechanism is called SN1, which stands for
substitution nucleophilic unimolecular
Kinetics and Mechanism
first order kinetics: rate = k[RX]unimolecular rate-determining step
carbocation intermediaterate follows carbocation stabilityrearrangements sometimes observed
reaction is not stereospecificracemization occurs in reactions of optically active alkyl halides
Characteristics of the SN1 mechanism
Question
What is the rate-determining step in the reaction of cyclobutanol with HCl?A) protonation of the OH groupB) attack of the bromide on the carbocationC) simultaneous formation of the C-Br bond
and the breaking of the C-OH bondD) carbocation formation
The species shown below represents _____ of the reaction between isopropyl alcohol andhydrogen bromide.
A) the alkyloxonium ion intermediateB) the transition step of the bimolecular
proton transferC) the transition state of the attack of the
nucleophile on the carbocationD) the transition state of the unimolecular
dissociation
Question
The rate of nucleophilic substitutionby the SN1 mechanism is governed
by electronic effects.
Carbocation formation is rate-determining.The more stable the carbocation, the faster
its rate of formation, and the greater the rate of unimolecular nucleophilic substitution.
Electronic Effects Govern SN1 Rates
RBr solvolysis in aqueous formic acid
Alkyl bromide ClassRelative rate
CH3Br Methyl 1
CH3CH2Br Primary 2
(CH3)2CHBr Secondary 43
(CH3)3CBr Tertiary 100,000,000
Reactivity toward substitution by the SN1 mechanism
Nucleophilic substitutions that exhibitfirst-order kinetic behavior are
not stereospecific.
Generalization
R-(–)-2-Bromooctane
H
C
CH3
Br
CH3(CH2)5
Stereochemistry of an SN1 Reaction
(R)-(–)-2-Octanol (17%)
H
C
CH3
OH
CH3(CH2)5
C
H CH3
HO
(CH2)5CH3
(S)-(+)-2-Octanol (83%)
H2O
Leaving group shieldsone face of carbocation;nucleophile attacks faster at opposite face.
+
Figure
Ionization stepgives carbocation; threebonds to chiralitycenter become coplanar
Structure of tert-Butyl Cation
Positively charged carbon is sp2 hybridized.All four carbons lie in same plane.Unhybridized p orbital is perpendicular to plane of four carbons.
CCH3
H3C
CH3
+
Carbocation Capture
+
Cl –
Lewis acid Lewis base
Electrophile Nucleophile
C
CH3
CH3H3C
+ CH3C
CH3H3C
Cl
CH3 C
H
CHCH3
Br
CH3 H2OCH3 C
OH
CH2CH3
CH3
(93%)
Example
CH3 C
H
CHCH3
CH3
+
H2O
CH3 C CHCH3
CH3
H+
Solvent Dielectric Relative constant rate
acetic acidmethanolformic acidwater
SN1 Reactivity versus Solvent Polarity
Most polar Fastest rate
R+
RX
R X
energy of RX not much affected by polarity of solvent
transition state stabilized by polar solvent
R+
RX
R X
energy of RX not much affected by polarity of solvent
transition state stabilized by polar solvent
activation energy decreases;
rate increases
SN2 Reaction Rates Increase inPolar Aprotic Solvents
An aprotic solvent is one that doesnot have an —OH group.
In general...
Solvent Type Relative Rate
CH3OH polar protic 1
H2O polar protic 7
DMSO polar aprotic 1300
DMF polar aprotic2800
Acetonitrile polar aprotic 5000
CH3CH2CH2CH2Br + N3–
SN2 Reactivity versus Type of Solvent
When...
primary alkyl halides undergo nucleophilic substitution, they always react by the SN2 mechanism
tertiary alkyl halides undergo nucleophilic substitution, they always react by the SN1 mechanism
secondary alkyl halides undergo nucleophilic substitution, they react by the
SN1 mechanism in the presence of a weak nucleophile (solvolysis)SN2 mechanism in the presence of a good nucleophile
The most commonly used leaving groups are halides and sulfonate ions.
CONCEPTUAL CHECKPOINT 7.28.
SN1 vs. SN2 – The Leaving Group
To promote an SN1 reaction, use a polar, protic solvent: The protic solvent will hydrogen bond with the nucleophile, stabilizing it, while the leaving group leaves first.
SN2 vs. SN1 – Solvent
SN2 reaction, use a polar, aprotic solvent