A value of
kH = 1.5 × 10
-3 M
-1 s
-1 has been determined for the generation of simple
p-quinonemethide by the acid-catalyzed cleavage of 4-hydroxybenzyl alcohol in water at 25
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C and
I = 1.0 (NaClO
4).This was combined with
ks = 5.8 × 10
6 s
-1 for the reverse addition of solvent water to the 4-hydroxybenzylcarbocation [
J. Am. Chem. Soc. 2002,
124, 6349-6356] to give p
KR = -9.6 as the Lewis acidity constantof O-protonated
p-quinone methide. Values of p
KR = 2.3 for the Lewis acidity constant of neutral
p-quinonemethide and p
Kadd = -7.6 for the overall addition of solvent water to
p-quinone methide to form4-hydroxybenzyl alcohol are also reported. The thermodynamic driving force for transfer of the elementsof water from formaldehyde hydrate to
p-quinone methide to form formaldehyde and
p-(hydroxymethyl)phenol (4-hydroxybenzyl alcohol) is determined as 6 kcal/mol. This relatively small driving force representsthe balance between the much stronger chemical bonds to oxygen at the reactant formaldehyde hydratethan at the product
p-(hydroxymethyl)phenol and the large stabilization of product arising from thearomatization that accompanies solvent addition to
p-quinone methide. The Marcus intrinsic barrier fornucleophilic addition of solvent water to the "extended" carbonyl group at
p-quinone methide is estimatedto be 4.5 kcal/mol larger than that for the addition of water to the simple carbonyl group of formaldehyde.O-Alkylation of
p-quinone methide to give the 4-methoxybenzyl carbocation and of formaldehyde to give asimple oxocarbenium ion results in very little change in the relative Marcus intrinsic barriers for the additionof solvent water to these electrophiles.