Substituent effects on carbocation stability: the pK(R) for p-quinone methide.
A value of k(H) = 1.5 x 10(-)(3) M(-)(1) s(-)(1) has been determined for the generation of simple p-quinone methide by the acid-catalyzed cleavage of 4-hydroxybenzyl alcohol in water at 25 degrees C and I = 1.0 (NaClO(4)). This was combined with k(s) = 5.8 x 10(6) s(-)(1) for the reverse addition of solvent water to the 4-hydroxybenzyl carbocation [J. Am. Chem. Soc. 2002, 124, 6349-6356] to give pK(R) = -9.6 as the Lewis acidity constant of O-protonated p-quinone methide. Values of pK(R) = 2.3 for the Lewis acidity constant of neutral p-quinone methide and pK(add) = -7.6 for the overall addition of solvent water to p-quinone methide to form 4-hydroxybenzyl alcohol are also reported. The thermodynamic driving force for transfer of the elements of 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 represents the balance between the much stronger chemical bonds to oxygen at the reactant formaldehyde hydrate than at the product p-(hydroxymethyl)phenol and the large stabilization of product arising from the aromatization that accompanies solvent addition to p-quinone methide. The Marcus intrinsic barrier for nucleophilic addition of solvent water to the "extended" carbonyl group at p-quinone methide is estimated to 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 a simple oxocarbenium ion results in very little change in the relative Marcus intrinsic barriers for the addition of solvent water to these electrophiles.
Toteva
Maria M
MM
Department of Chemistry, University at Buffalo, SUNY, Buffalo, New York 14260-3000, USA.
Moran
Michael
M
Amyes
Tina L
TL
Richard
John P
JP
D001592:Benzyl Alcohols / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D045563:Indolequinones; D007211:Indoles / Q000737:chemistry*; D007700:Kinetics; D008623:Mercaptoethanol / Q000737:chemistry; D011809:Quinones / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
D001592:Benzyl Alcohols; D002412:Cations; D045563:Indolequinones; D007211:Indoles; D011809:Quinones; D014867:Water; C068040:quinone methide; C018966:4-hydroxybenzyl alcohol; D008623:Mercaptoethanol