SN1 and SN2 Reactions

1
SN1 and SN2 Reactions
SN1 SN2
Rate kRX kRXNuc-
Carbocation intermediate? Y N
Stereochemistry mix Inversion of configuration
Rearrangement H, CH3 possible No rearrangements
2
SN1 and SN2 Reactions
SN1 SN2
Substrate 3gt2gt1 CH3Xgt1gt2
Nucleophile Unimportant, but usually weak Strong and unhindered
Leaving group Excellent Better than nucleophile
Solvent Polar and ionizing Polar aprotic
3
SN1 and SN2 Reactions

• Do not occur with vinyl halides or aryl halides.
• Consider the carbocation formed for SN1.
• Consider the backside attack for SN2.

4
Elimination Reactions

• The substitution reaction mechanisms you have
  learned are just part of the picture.
• In the SN1 and SN2 reactions, the species that
  acts as a nucleophile may also act as a base and
  abstract a proton. This causes the elimination
  of HX and the formation of an alkene.
• An elimination reaction can occur along with the
  substitution reaction.

5
Elimination
B- is a species acting as a base. Since HX is
lost, this particular reaction is called a
dehydrohalogenation.
6
E1 Reactions

• E1 elimination, unimolecular
• Rate ksubstrate (a first order process)
• The rate-limiting step is the formation of the
  carbocation, the same as for SN1 (thats why E1
  competes).

7
E1 Reaction Mechanism

• Step 1 The substrate forms a carbocation
  intermediate (rate-limiting step).
• Step 2 Methanol acts as a base and removes
  H(fast step).

8
E1 Reaction Profile
rate k(CH3)3Br
-EA(step 1)/RT
k Ae
9
E1 Occurs with and Competes with SN1

• When bromocyclohexane is heated with methanol,
  two products are possible.

• Can you draw the mechanism that leads to each
  product?

10
SN1 E1 competing mechanisms
11
E1 Reactions

• E1 reactions are exothermic.
• E1 reactions occur in at least two steps and
  compete with SN1 reactions.
• The first step is slow. It is the formation of
  the carbocation intermediate.
• The second step is fast. It is the abstraction
  of H from the carbocation by the base.

12
Factors Affecting E1 Reactions

• Structure of the substrate
• Can a stable carbocation be formed?
• Strength of the base
• Nature of the leaving group
• The solvent in which the reaction is run.
• Must be able to stabilize the carbocation and the
  LG (which is usually an ion).

13
Factors Affecting E1 Reactions - Structure of the
Substrate

• The most important factor influencing the rate of
  E1 reactions is the stability of the carbocation
  formed.
• Stability of carbocation 3 gt 2 gt1
• Relative rates for E1 3gt21(resonance)

14
Factors Affecting E1 Reactions - Strength of the
Base

• The rate is not much affected by the strength of
  the base. Weak bases will work.

15
Factors Affecting E1 Reactions - the Leaving Group

• The LG should be good.

16
Factors Affecting E1 Reactions - Solvent Effects

• The solvent must be capable of dissolving both
  the carbocation and the leaving group.
• E1 reactions require highly polar solvents that
  strongly solvate ions.
• Typical solvents water, an alcohol, acetone (to
  help the alkyl halide to dissolve), or a mix.

17
Rearrangements in E1 Reactions

• The carbocation can undergo a structural
  rearrangement to produce a more stable species.
• hydride shift (H)
• methyl shift (CH3)
• If ionization would lead to a 1 carbocation,
  look for a rearrangement to occur.
• Ionizations can be forced by the addition of
  reagents such as AgNO3. (How?)

18
A Hydride Shift Can Occur in Either SN1 or E1
Reactions
CH3OH
H
-H
SN1

• What would the E1 products be?

19
A Methyl Shift Can Occur in Either SN1 or E1
Reactions
CH3OH
CH3
-H
SN1

• What would the E1 product be?

20
Zaitsevs Rule

• When two or more elimination products are
  possible, the product with the more substituted
  double bond will predominate.

21
Zaitsevs Rule

• When two or more elimination products are
  possible, the product with the more substituted
  double bond will predominate.
• Alkyl groups are electron-donating and contribute
  electron density to the p bond.
• Bulky groups can be more widely separated.

22
E1 Reactions - Summary

• The structure of the carbocation is the most
  important factor
• Relative rates for E1 3gt2.
• The base is typically weak or moderate in
  strength.
• The LG should be good.
• The solvent should be polar and protic to
  stabilize the carbocation and LG.
• Products can exhibit rearrangements and will
  follow Zaitsevs Rule.