Level 1 – Case 10
68
Perhaps the most appealing feature derived from data in Table 10.1 is that the
comparison between reaction rates with pK a
K K values does not reveal any correlation
with amine basicity. For example, the two most basic amines,
1 namely quinuclidine and Et 3 N react with rates differing by at least three orders of magnitude.
These facts are inconsistent with a base-catalyzed mechanism, which should show
a rate increase with increased basicity of the amine employed.
In Summary
The reaction between benzoyl chloride and phenol may occur through several alternate pathways involving nucleophile catalysis or base catalysis. Such pathways
are kinetically indistinguishable and the absence of KIE when phenol-d 6 is used as
substrate, is not conclusive to determinate the operative mechanism. Only the absence of a direct relationship between amine basicity and reaction rate and the lack
of phenoxide ion in the UV-vis experiments, can rule out a base catalysis mechanism, pointing to a nucleophile catalysis.
This problem is based on the work by Hubbard P, Brittain WJ (1998) J. Org.
Chem. 63:677-683.
Subjects of Revision
Nucleophile catalysis. Base catalysis. Spectroscopic detection of reaction intermediates.
1 Using aqueous pK a
K K values for reactions in aprotic,
a
nonpolar media can be incorrect. In
the discussion we have assumed that the relative amine basicity values measured in water
are a reasonable measure of their basicity trends in DCM.
68
Perhaps the most appealing feature derived from data in Table 10.1 is that the
comparison between reaction rates with pK a
K K values does not reveal any correlation
with amine basicity. For example, the two most basic amines,
1 namely quinuclidine and Et 3 N react with rates differing by at least three orders of magnitude.
These facts are inconsistent with a base-catalyzed mechanism, which should show
a rate increase with increased basicity of the amine employed.
In Summary
The reaction between benzoyl chloride and phenol may occur through several alternate pathways involving nucleophile catalysis or base catalysis. Such pathways
are kinetically indistinguishable and the absence of KIE when phenol-d 6 is used as
substrate, is not conclusive to determinate the operative mechanism. Only the absence of a direct relationship between amine basicity and reaction rate and the lack
of phenoxide ion in the UV-vis experiments, can rule out a base catalysis mechanism, pointing to a nucleophile catalysis.
This problem is based on the work by Hubbard P, Brittain WJ (1998) J. Org.
Chem. 63:677-683.
Subjects of Revision
Nucleophile catalysis. Base catalysis. Spectroscopic detection of reaction intermediates.
1 Using aqueous pK a
K K values for reactions in aprotic,
a
nonpolar media can be incorrect. In
the discussion we have assumed that the relative amine basicity values measured in water
are a reasonable measure of their basicity trends in DCM.