Nucleophilic Substitution Reactions: Competing Nucleophiles

1
Nucleophilic Substitution Reactions Competing
Nucleophiles

• Experiment 21 (4th ed.)
• Experiment 19 (3rd ed.)

2
Read Before Class

• 4th ed.Exp. 21
• Technique 24
• 3rd ed.Exp. 19
• Technique 19 (Part B)

3
Things to Consider

• Why must the nucleophilic medium be acidic in
  this experiment?
• Are the reactions being carried out 1st order or
  2nd order?
• What effect (if any) does the nature of the
  nucleophile have on the reaction?

4
Reactions of Nucleophiles

• Nucleophilean electron-rich species that seeks
  an electron-poor site or positively-charged
  center
• SN11st order reaction Rxn rate
• favors highly substituted, more stable
  carbocation
• Tertiary gt Secondary gt Primary gt Methyl
• independent of both the nature and concentration
  of the nucleophile
• SN22nd order reaction Rxn rate
• favors less substituted, less stable carbocation
• Methyl gt Primary gt Secondary gtgt Tertiary
• dependent on both the nature and concentration of
  the nucleophile

5
Leaving Groups

• For a species to be a good leaving group, it
  needs to be able to stabilize a negative charge
• Weak bases accomplish this task effectively
• For the group VII elements (the halogens), the
  order of best leaving group to worst is
• I gt Br gt Cl gt F
• (weakest base) (strongest base)

6
Solvent Media

• The solvent chosen for the reaction medium can
  have a significant effect on the products formed
• Polar Protic Solventpolar solvent that can form
  hydrogen bonds
• Ex Water Acetic Acid
• Polar Aprotic Solventpolar solvent that cannot
  hydrogen bond to the nucleophile
• Ex Acetone DMSO

7
Nucleophilic Strength

• Ignoring solvent effects, nucleophilic strength
  parallels basicitystronger bases are better
  nucleophiles
• Polar Aprotic Solventsstrength parallels
  basicity
• F gt Cl gt Br gt I
• (Stronger Base) (Weaker Base)
• Polar Protic Solventstrength parallels ion size
• I gt Br gt Cl gt F
• (Larger) (Smaller)

8
Rxns of Alcohols

• A strong stearically hindered base such as the
  t-butoxide ion (from t-butyl alcohol) will cause
  a reaction to favor ELIMINATION over
  SUBSTITUTION. High temperatures also influence
  this reaction in the same way.
• For an alcohol to do a substitution reaction with
  an alkyl halide, it must be dissolved in strong
  acid! The acid protonates the hydroxide ion,
  making it water, which is a much weaker base than
  a hydroxide ion.