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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.

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solvolysis reactions
Solvolysis Reactions

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.
hydrolysis of tert butyl bromide

CH3

CH3

H

H

CH3

CH3

+

..

:

:

:

+

O

O

C

:

C

Br

..

H

H

CH3

CH3

+

..

CH3

:

:

Br

CH3

..

..

..

:

+

Br

H

OH

C

..

..

CH3

Hydrolysis of tert-butyl bromide.
hydrolysis of tert butyl bromide1

CH3

CH3

H

H

CH3

CH3

:

:

O

H

H

CH3

CH3

..

:

:

Br

..

Hydrolysis of tert-butyl bromide.

..

+

+

:

:

C

Br

O

C

..

+

This is the nucleophilic substitutionstage of the reaction; the one withwhich we are concerned.

hydrolysis of tert butyl bromide2

CH3

CH3

H

H

CH3

CH3

:

:

O

H

H

CH3

CH3

..

:

:

Br

..

Hydrolysis of tert-butyl bromide.

..

+

+

:

:

C

Br

O

C

..

+

The reaction rate is independentof the concentration of the nucleophileand follows a first-order rate law.

rate = k[(CH3)3CBr]

hydrolysis of tert butyl bromide3

CH3

CH3

H

H

CH3

CH3

..

+

:

:

:

O

:

C

Br

O

C

..

H

H

CH3

CH3

..

:

:

Br

..

Hydrolysis of tert-butyl bromide.

+

+

The mechanism of this step isnot SN2. It is called SN1 and begins with ionization of (CH3)3CBr.

kinetics and mechanism
Kinetics and Mechanism

rate = k[alkyl halide]

First-order kinetics implies a unimolecularrate-determining step.

  • Proposed mechanism is called SN1, which stands forsubstitution nucleophilic unimolecular
mechanism

CH3

CH3

..

:

Br

C

..

CH3

unimolecular slow

H3C

CH3

..

+

:

:

C

Br

..

CH3

Mechanism

+

mechanism1

H3C

CH3

H

+

C

:

:

O

CH3

H

bimolecular fast

CH3

H

CH3

+

:

O

C

H

CH3

Mechanism
slide12

R+

proton transfer

RX

+

ROH2

ROH

carbocation formation

carbocation capture

characteristics of the s n 1 mechanism
Characteristics of the SN1 mechanism
  • first order kinetics: rate = k[RX]
    • unimolecular rate-determining step
  • carbocation intermediate
    • rate follows carbocation stability
    • rearrangements sometimes observed
  • reaction is not stereospecific
    • racemization occurs in reactions of optically active alkyl halides
question
Question
  • What is the rate-determining step in the reaction of cyclobutanol with HCl?
  • A)protonation of the OH group
  • B)attack of the bromide on the carbocation
  • C)simultaneous formation of the C-Br bond and the breaking of the C-OH bond
  • D)carbocation formation
question1
Question
  • The species shown below represents _____ of the reaction between isopropyl alcohol and
  • hydrogen bromide.
  • A) the alkyloxonium ion intermediate
  • B) the transition step of the bimolecular proton transfer
  • C) the transition state of the attack of the nucleophile on the carbocation
  • D) the transition state of the unimolecular dissociation
electronic effects govern s n 1 rates
Electronic Effects Govern SN1 Rates

The rate of nucleophilic substitutionby the SN1 mechanism is governedby electronic effects.

Carbocation formation is rate-determining.The more stable the carbocation, the fasterits rate of formation, and the greater the rate of unimolecular nucleophilic substitution.

reactivity toward substitution by the s n 1 mechanism
Reactivity toward substitution by the SN1 mechanism

RBr solvolysis in aqueous formic acid

  • Alkyl bromide Class Relative rate
  • CH3Br Methyl 1
  • CH3CH2Br Primary 2
  • (CH3)2CHBr Secondary 43
  • (CH3)3CBr Tertiary 100,000,000
decreasing s n 1 reactivity
Decreasing SN1 Reactivity

(CH3)3CBr

(CH3)2CHBr

CH3CH2Br

CH3Br

question2
Question
  • Select the most stable carbocation.
  • A) B)
  • C) D)
question3
Question
  • Which one of the following reacts with HBr at the fastest rate?
  • A) B)
  • C) D)
generalization
Generalization
  • Nucleophilic substitutions that exhibitfirst-order kinetic behavior are not stereospecific.
stereochemistry of an s n 1 reaction

H

CH3

Br

C

CH3(CH2)5

H

CH3

H

CH3

H2O

OH

C

C

HO

CH3(CH2)5

(CH2)5CH3

(R)-(–)-2-Octanol (17%)

(S)-(+)-2-Octanol (83%)

Stereochemistry of an SN1 Reaction

R-(–)-2-Bromooctane

figure
Figure

Ionization stepgives carbocation; threebonds to chiralitycenter become coplanar

+

Leaving group shieldsone face of carbocation;nucleophile attacks faster at opposite face.

structure of tert butyl cation

CH3

+

C

H3C

CH3

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.
slide28

More than 50%

Less than 50%

+

carbocation capture

H3C

CH3

C

+

Cl

CH3

H3C

CH3

Lewis acid

Lewis base

C

Cl

Electrophile

Nucleophile

H3C

Carbocation Capture

+

because
Because...
  • carbocations are intermediatesin SN1 reactions, rearrangementsare possible.
example

CH3

CH3

H2O

C

CHCH3

C

CH2CH3

CH3

CH3

(93%)

H

Br

OH

H2O

CH3

CH3

C

CHCH3

C

CHCH3

CH3

CH3

+

+

H

H

Example
in general
In general...
  • SN1 Reaction Rates Increase in Polar Solvents
s n 1 reactivity versus solvent polarity

Most polar

Fastest rate

SN1 Reactivity versus Solvent Polarity

Solvent Dielectric Relative

constant rate

acetic acid

methanol

formic acid

water

slide36

R

X 

transition state stabilized by polar solvent

R+

energy of RX not much affected by polarity of solvent

RX

slide37

R

X 

transition state stabilized by polar solvent

activation energy decreases;

rate increases

R+

energy of RX not much affected by polarity of solvent

RX

in general1
In general...
  • SN2 Reaction Rates Increase inPolar Aprotic Solvents

An aprotic solvent is one that doesnot have an —OH group.

s n 2 reactivity versus type of solvent
SN2 Reactivity versus Type of Solvent

CH3CH2CH2CH2Br + N3–

  • Solvent Type Relative Rate
  • CH3OH polar protic 1
  • H2O polar protic 7
  • DMSO polar aprotic 1300
  • DMF polar aprotic 2800
  • Acetonitrile polar aprotic 5000
slide41
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
s n 1 vs s n 2 the leaving group
SN1 vs. SN2 – The Leaving Group
  • The most commonly used leaving groups are halides and sulfonate ions.
  • CONCEPTUAL CHECKPOINT 7.28.
s n 2 vs s n 1 solvent
SN2 vs. SN1 – Solvent

SN2 reaction, use a polar, aprotic solvent

  • 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.
calibrated peer review lab experimentation

Calibrated Peer Review / Lab Experimentation

http://chemconnections.org/organic/chem226/calendar-f-12