Chemistry Reference
In-Depth Information
of resoles take place at neutral or slightly acidic conditions, both methylene and ether linkages form.
Upon heating, the ethers in turn split out formaldehyde, as shown above. The dibenzyl ethers
[
144
-
146
,
148
] also break down at elevated temperatures and form quinone methides:
The quinone methides can undergo a variety of reactions including cycloadditions with other
methides to form chroman groups:
O
O
2
O
para
The
quinone methides can couple:
2
O
HO
OH
7.12.2 Novolacs
The phenolic resins that form in acid-catalyzed condensations of phenols with formaldehyde are
different from resols. At pH below seven protonation of the carbonyl group of formaldehyde takes
place first and is followed by electrophilic aromatic substitution at the
positions of the
phenol. The initial steps of the reactions also take place in water. Here, however, a molar excess of
phenol (1.25:1) must be used, because reactions on equimolar basis under acidic conditions form
cross-linked resins. At a ratio of eight moles of formaldehyde to ten moles of phenol, novolacs of
approximate molecular weight of 850 form [
148
]. When the ratio of formaldehyde to phenol is 9:10, a
molecular weight of approximately 1,000 is reached. This appears to be near the limit, beyond which
cross-linking results. The reaction is as follows:
ortho
and
para
+
O
H
OH
OH
