Chapter 9: Elimination Reactions of Alkyl Halides: Competition between Substitutions and Eliminations

1
Chapter 9 Elimination Reactions of Alkyl
Halides Competition between Substitutions and
Eliminations
2
Goals

• After this chapter, you should be able to
• Predict products of E2 and E1 reactions
• Determine stereochemistry of E2/E1 Products
• Determine whether SN2, SN1, E1 or E2 will occur

3
What is an SN2 Reaction?

• SN2 mechanism S for substitution, N for
  nucleophilic and 2 because two molecules collide
  at the critical point in the reaction.

4
Review An SN2 Reaction
5
Stereochemistry of Inversion

• If the nucleophile and the leaving group are both
  high in the R/S priority order, this means that
  an R alkyl halide gives an S product, and
  vice-versa

6
Energy of Inversion
7
Energy of Inversion
8
With SN2, Size of Substituent Groups Matters
Relative Reactivity Toward SN2 tertiary lt
secondary lt primary lt methyl
9
Kinetics of Nucleophilic Substitution

• Rate kRBrNu-
• Second order kinetics

10
Effect of Bond Strength of the Leaving Group on
SN2 Reactivity

• Since the carbon-halogen bond strength increases
  up the periodic table the relative SN2 reactivity
  of the alkyl halide is
• RF lt RCl lt RBr lt RI
• TosO- is a better leaving group than I-
• OH-, NH2-, and RO- are worse than F-

11
Nucleophilicity
CH3CO2 (-) lt Cl(-) lt Br(-) lt N3(-) lt CH3O(-) lt
CN(-) lt I(-) lt SCN(-) lt CH3S(-)
12
Nucleophilicity

• Parallels basicity
• H2O lt C2H3O2- lt OH-
• Increases down the periodic table
• I- lt Cl- lt F-
• Anions are more nucleophilic than neutral
  compounds
• The solvent matters!

13
Solvent Effects

• Consider KBr as a nucleophile source
• Protic solvents with OH, -NH slow SN2 rxn
• These solvents cluster around the nucleophile
  lowering the effective nucleophilicity
• Polar aprotic solvents speed SN2
• These solvents cluster around the metal ion of
  the salt freeing the nucleophile to be
  nucleophilic.

14
Characteristics of SN2 Reactions

• Single Step Mechanism
• Inversion of configuration
• SN2 reactions are generally reliable only when
  the alkyl halide is primary
• Halogen is generally Cl or Br since
• C-F bond is too strong
• C-I bond is weak and compounds are unstable

15
An SN2 Reaction
16
SN1 Reactions

• SN1 reactions proceed by a two step mechanism
• First Leaving group leaves giving a carbocation
• Second Nucleophile attacks carbocation

17
Review An SN1 Reaction
18
SN1 Reactions
19
Leaving Groups

• OH- lt NH2 -ltRO- F - lt Cl - lt Br - lt I lt TosO-
• Susceptibility to leaving

20
Evidence for SN1 Kinetics

• The reaction rate is only dependent upon the
  concentration of the substance with the leaving
  group
• R-X ? R X- is a slow rate determining
• Racemic mixtures are usual
• Carbocation formation
• Rate kR-X where X is leaving group

21
SN1 Reaction Rates

• Depend on stability of the carbocation
• More stable carbocationfaster reaction
• -CH3 lt 1 lt
  2 lt 3
• Relative Stability of Carbocation

22
The Nucleophile and SN1

• NO EFFECT!

23
Energy for SN1
24
Solvent Effects on SN1

• Polar solvents stabilize the intermediate
  carbocation.

25
Summary SN1

• Fastest with
• Compounds that form stable carbocation
• Good leaving group
• Nucleophiles that are not basic to prevent
  competing elimination reactions
• Polar solvents

26
An SN1 Reaction
27
Elimination Reactions

• Zaitsevs Rule
• Base induced elimination reactions generally give
  the more highly substituted double bond alkene
  product

28
An E2 Reactions
29
E2 Reactions

• Single step attack of nucleophile on hydrogen on
  carbon adjacent to the carbon containing the
  leaving group.

30
E2 Kinetics

• The rate of the reaction is dependent upon the
  concentration of the compound containing the
  leaving group and the nucleophile base.
• Rate kRXBase

31
Geometry of E2

• All atoms involved are in same plane
• The hydrogen and leaving group are anti

32
Cycloalkane E2 What do you expect?
33
E2 Reaction
34
An E2 Reactions
35
Zaitsevs Rule Limitations

• Dont use for conjugated double bonds.
• You can trick the reaction into favoring the
  least substituted alkene by using a Bulky base.

36
Zaitsevs Rule Limitations

• Dont use for conjugated double bonds.
• You can trick the reaction into favoring the
  least substituted alkene by using a Bulky base.

37
E1 Reactions

• First step is identical to SN1 Elimination of
  the leaving group giving a carbocation
• First step is slow and rate determining
• Second step is the attack of a hydrogen on a
  carbon adjacent to the carbocation
• Racemic mixtures are usual

38
The E1 Reaction
39
E1 Kinetics

• Rate kR-X

E
40