Comparison of pKR values of fluorenyl and anthracenyl cations

Autor: Michael C. Courtney, Aoife C. MacCormack, Rory A. More O'Ferrall
Rok vydání: 2002
Předmět:
Zdroj: Journal of Physical Organic Chemistry. 15:529-539
ISSN: 1099-1395
0894-3230
DOI: 10.1002/poc.511
Popis: A value of pKR = −5.1 for the anthracenonium ion in 50:50 (v/v) aqueous trifluorethanol is reported based on a ratio of measured rate constants kH for acid-catalysed dehydration of the 9,10-hydrate of anthracene and k for the reverse hydrolysis of the carbocation (KR = k/kH). Comparison with pKR = −15.9 for the fluorenyl cation indicates that the latter ion is less stable by more than 10 log units (15 kcal mol−1). This difference is (a) considerably larger than that between the benzhydryl (pKR = −11.7) and fluorenyl cations (ΔpK = 4.2), which has been considered too small to indicate antiaromatic character for the fluorenyl cation, and (b) comparable to that between pKas for the ionization (in DMSO) of fluorene (22.6) and diphenylmethane (32.2), which has been interpreted as implying aromatic character for the fluorenyl anion. It is shown that a difference in stability of anthracene hydrate and 9-hydroxyfluorene makes only a minor contribution to the difference in pKR values and that the fluorenyl cation is destabilized by ca 10 kcal mol−1. A smaller difference in pKas for protonation of fluorenimine (5.85) and benzhydrylimine (7.0) is consistent with the expected moderating effect of an electron-donating substituent on relative carbocation stabilities. Evidence from calculations relating to the antiaromaticity of the fluorenyl cation is reviewed in the light of these measurements. An additional comparison between equilibrium constants for the ionization of aralkylazides (Kaz) and alcohols (KR) reveals the influence of differences in geminal σ-bond interactions for the hydroxy and azido groups in their respective reactants. Copyright © 2002 John Wiley & Sons, Ltd.
Databáze: OpenAIRE
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