Chem 334 - Fall 1999
Organic Chemistry I

Dr. Carl C. Wamser

Chapter 8 Notes: Nucleophilic Substitution and 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 )

 No

   good nucl.,
weak base

 No

strong base

 3° ( R3CX )

 good nucl.,
weak base

  No

 polar solvent,
no base or 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

 No reaction

 3° ( R3CX )

  E2

  SN1

  E2

 SN1