Chemistry Reference
In-Depth Information
Phosgenation of aniline-formaldehyde condensates yields isocyanates with a functionality that
averages 2.6-2.8. The structure of the product can be shown as follows:
N
N
N
O
O
O
where, x =2 or 3
x
-
phenylene diisocyanate, 1-chloro-2,4-phenylene diisocyanate, 3,3
0
-dimethyl-4,4
0
-bisphenylene
diisocyanate, 4,4
0
-bis(2-methylisocyanophenyl)-methane, and 4,4
0
-bis(2-methoxyisocyanophenyl)
methane.
The common aliphatic diisocyanates are hexamethylene diisocyanate, hydrogenated (H
12
) 4,4
0
-
dipenylmethane diisocyanate, isophorone diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate,
and 2,4,4-trimethylhexamethylene diisocyanate. Other aliphatic diisocyanates that are commercially
used are lysine diisocyanate, methylcyclohexyl diisocyanate, isopropylidine bis-(4-cyclohexyl isocy-
anate), and tetramethylene diisocyanate. Many additional polyfunctional isocyanates are described in
the literature.
Among other aromatic diisocyanates that are in commercial use are
p
-phenylene diisocyanate,
m
7.10.3 Chemical Reactions of the Isocyanates
These reactions can be divided into two categories. They are additions to compounds with active
hydrogens and self-condensation. The reactions are well described in organic chemistry textbooks, so
there is little reason to describe them here. It is noteworthy, though, that the uncatalyzed reactions of
isocyanates with various active hydrogen compounds are probably broadly similar. Among them, the
most investigated reactions are the ones of alcohols with isocyanates [
123
]:
R
R
R
NO
NO
+
R'OH
NO
O
O
H
R'
H
R'
O
H
R'
R
O '
NO
KOH
R
+
R'OH
NO
O
H
H
R'
Electron withdrawing substituents increase the positive charge on the isocyanate carbon and move
the negative charge further away from the site for the reaction:
O
O
2
N
N
N
O
N
O
O
This makes an attack by an electron donor easier and yields a faster reaction. Electron donating
groups, therefore, have an opposite effect.
