Chemistry Reference
In-Depth Information
1
laminopyridines are far superior to pyridine as acylation catalysts [31, 32], and were
promising candidates for attachment to modified polyethylenimines with hydrophobic
substituents. Various substituted aminopyridines of type
1
have been coupled to poly-
ethylenimines and the polymers obtained are very effective catalysts of the solvolysis of
nitrophenyl esters [28, 30].
The rate-pH curve for these polymers is S-shaped through the accessible range (Fig-
ure 3.3). The inflection point occurs at about pH 8.0 due to an average pK
a
of the poly-
mer-bound pyridine, but the small molecule dialkylaminopyridine shows a pK
a
of 9.7.
The cationic charge and apolar environment of the polymer favor the non-charged
form of the heterocycle adduct. Since binding of a non-charged substrate by the poly-
mer depends on apolar interactions, not electrostatic ones, the loss in charge on the
polymer with increasing pH does not diminish binding and, hence, no bell-shaped
curve appears.
The pK
a
of the aminopyridines attached to the polymer (Table 3.3) are invariably
lower than those of the corresponding isolated small molecules. The cationic character
of polyethylenimine produces an electrostatic field that weakens the basicity of the
nucleophile by 2-3 pK units. A lowering of an additional unit ocurrs when a long
apolar group is attached to the aminopyridine moiety. Evidently, in this molecule
the apolar environment favors further the uncharged, nonprotonated form of the pyr-
idine.
Figure 3.3 Rate (second order)-pH profile
for hydrolysis of p-nitrophenyl caproate in
aqueous solution at 25
8
Cby
(C
2
H
4
)
m
(C
12
H
25
)
0.12m
(C
9
H
11
N
2
O)
0.10m
,a
60 000 molecular weight polyethylenimine
with 10% of the nitrogens covalently atta-
ched (by an amide bond) to 3-[N-methyl-N-
(4-pyridyl)amino]propionic acid and 12%
linked to lauryl groups.
