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Nucleophilic Substitution Reactions Competing Nucleophiles (PowerPoint)

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Nucleophilic Substitution Reactions Competing Nucleophiles (PowerPoint) Powered By Docstoc
					    Nucleophilic Substitution
     Reactions: Competing
         Nucleophiles


Experiment 21 (4th ed.)
Experiment 19 (3rd ed.)
Read Before Class
• 4th ed.—Exp. 21
  – Technique 24
• 3rd ed.—Exp. 19
  – Technique 19 (Part B)
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?
Reactions of Nucleophiles
• Nucleophile—an electron-rich species that seeks an electron-
  poor site or positively-charged center
• SN1—1st order reaction; Rxn rate:
   – favors highly substituted, more stable carbocation

            Tertiary > Secondary > Primary > Methyl

    – independent of both the nature and concentration of the
      nucleophile

• SN2—2nd order reaction; Rxn rate:
   – favors less substituted, less stable carbocation

           Methyl > Primary > Secondary >> Tertiary

    – dependent on both the nature and concentration of the
      nucleophile
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 > Br > Cl > F
        (weakest base)    (strongest base)
Solvent Media
• The solvent chosen for the reaction medium
  can have a significant effect on the products
  formed
• Polar Protic Solvent—polar solvent that
  can form hydrogen bonds
  – Ex: Water & Acetic Acid
• Polar Aprotic Solvent—polar solvent that
  cannot hydrogen bond to the nucleophile
  – Ex: Acetone & DMSO
Nucleophilic Strength
• Ignoring solvent effects, nucleophilic
  strength parallels basicity—stronger
  bases are better nucleophiles
• Polar Aprotic Solvents—strength parallels
  basicity:
               F > Cl > Br > I
         (Stronger Base)   (Weaker Base)

• Polar Protic Solvent—strength parallels ion
  size:
               I > Br > Cl > F
           (Larger)          (Smaller)
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.

				
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