Chemistry Reference
In-Depth Information
Table 7.7
Relative effects
of catalysts on reactivity
of phenylisocyanate
[
126
,
127
,
132
]
Relative rates of reactions
with
n
-butyl alcohol
Catalyst
None
1.0
N
-Methyl morpholine
4.0
Triethyl amine
8.6
Triethylene diamine
120.0
Tributyltin acetate
30,000
Dibutyltin diacetate
60,000
7.10.4 The Effect of Catalysts
Catalysts exert strong influence on the rates of reactions of isocyanates with active hydrogen
compounds. Most widely used ones are tertiary amines and metal salts, particularly tin compounds.
The mechanism of catalysis by tertiary amines is believed [
124
-
126
] to proceed according to the
following scheme:
R
3
R
3
N
N
+
R
3
N
R'
HOR
"
R'
catalyst
R'
NO
NO
NO
N
O
R
3
R"
R'
R'
NO
+
N
O
R
3
N
O
H
H
R"
The catalytic activity of the tertiary amines generally parallels their base strength, except when
steric hindrance is pronounced. Tin compounds exert much stronger catalytic effects on the reactions
than do tertiary amines. This is illustrated in Table
7.7
. The mechanism of catalysis by metal salts is
believed to operate as follows [
128
]:
R
R
+
R
MX
2
NO
NO
NO
MX
2
MX
2
R
R
+
NO
R'OH
NO
O
MX
2
H
MX
2
R'
OR'
OR'
OR'
R
MX
2
R
R
+
MX
2
MX
2
NO
NO
O
H
H
H
