Organic Chemistry I

Chem 334 - Fall 2009

Chapter 8 Notes

Professor Carl C. Wamser

Substitution & Elimination

Nucleophilic Substitution

Recognizing Nucleophiles

Solvents

The SN2 Mechanism



SN2 Mechanism - Evidence

SN2 Mechanism - Evidence

SN2 Mechanism - R Groups



SN2 Mechanism - X Groups

SN2 Mechanism - Nucleophiles

Solvent Effects on Nucleophilicity

SN1 Mechanism

SN1 Mechanism - Evidence

SN1 Mechanism - Evidence



SN1 Mechanism - R Groups

SN1 Mechanism - X Groups

Solvolysis Reactions



Neighboring Group Effects - Mustard Gases

Phase-Transfer Catalysis


Beta-Elimination Reactions


E2 Elimination Mechanism

E2 Stereochemistry

Zaitsev Rule

E1 Elimination Mechanism

Substitution or Elimination ?

Nucleophile or Base?

Reactivity Patterns


Nucleophilic Substitution Mechanisms

What's the difference between SN1 and SN2 ?

 

  SN2

  SN1

  Reaction

  RX + Nu --> RNu + X

  same

  Mechanism

  concerted

  two steps

  Intermediate

  none

  carbocation

  Kinetics

  second-order

  first order

  Stereochemistry

  complete inversion

  nonspecific

  Nucleophile

  important

  unimportant

  Leaving Group

  important

  important

  Alkyl Group

  CH3 > 1° > 2° > 3°
(steric hindrance)

  3° > 2° > 1° > CH3
(carbocation stability)

  Occurrence

  CH3 , 1° , some 2°

  3° , some 2°

  Solvent Effects

  variable

  polar, protic


Elimination Mechanisms

What's the difference between E1 and E2 ?

 

  E2

  E1

  Reaction

  RX + base --> C=C

  same

  Mechanism

  concerted

  two steps

  Intermediate

  none

  carbocation

  Kinetics

  second-order

  first order

  Stereochemistry

  anti periplanar

  nonspecific

  Base

  important

  unimportant

  Leaving Group

  important

  important

  Alkene Produced

  Zaitsev Rule

  same


Substitution vs. Elimination

How can you make ... react in an ... mechanism ?
 

    SN1

    SN2

   E1

   E2

  CH3X

No

  good nucl.

   No

  No

 1° ( RCH2X )

No

   good nucl.,
weak base

   No

  strong base,
weak nucl.

 2° (R2CHX )

  polar protic solvent,
no base, good nucl.

    good nucl.,
weak base

polar solvent,
weak base, poor nucl.

strong base

 3° ( R3CX )

  good nucl.,
poor base

   No

  polar protic solvent,
weak base, poor nucl.

  strong base


What's most likely to happen when ... reacts with ... ?
 

 Good nucl.,
strong base,
e.g., OH-

  Good nucl.,
weak base,
e.g., I-

  Poor nucl.,
strong base,
e.g., tBuO-

  Poor nucl.,
weak base,
e.g., H2O

 CH3X

   SN2

   SN2

   SN2

  No reaction

  1° ( RCH2X )

   SN2

   SN2

   E2

  No reaction

 2° (R2CHX )

   E2

   SN2

  E2

  Slow reaction

 3° ( R3CX )

   E2

   SN1

   E2

  SN1