stretching band could not be located and was presumably submerged in the large C-H stretching band.)
If the proposed structure for the complex is correct, it follows that methylation of either the -NH or -OH group (or both) of acetaminophen should prevent complexation with antipyrine. This was shown to be the case by melting-point determinations; neither congruently nor incongruently melting complexes were formed by the methylated derivatives of acetaminophen with antipyrine (Table 11). The literature reports of lack of complexation between antipyrine and acetanilide or phenacetin (pethoxyacetanilide) were also confirmed.
It is perhaps surprising that since phenol forms a complex with antipyrine (4), phydroxy-N-methylacetanilide does not. The reason for this, which could be confirmed unequivocally only by X-ray crystallography, probably lies in the bulky nature of the acetamido group forcing a different crystal structure on the complex with antipyrine. Thus, phydroxy-N-methylacetanilide, because of its lack of a -NH group, is unable to complex in the way acetaminophen does; and by virtue of its steric properties, it cannot complex like phenol.
REFERENCES
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