64666-57-5Relevant articles and documents
Experiments and calculations for determination of the stabilities of benzyl, benzhydryl, and fluorenyl carbocations: Antiaromaticity revisited
Amyes, Tina L.,Richard, John P.,Novak, Michael
, p. 8032 - 8041 (2007/10/02)
The following pKR values for the formation of benzyl, benzhydryl, and fluorenyl carbocations in 50:50 (v:v) trifluoroethanol/water at I = 0.50 (NaClO4) were determined as pKR = -log (kHOH[H2O]/kH), where kH is the second-order rate constant for acid-catalyzed reaction of the alcohol to form the carbocation and kHOH is the second-order rate constant for capture of the carbocation by water: (R+, pKR); PhCH2+, ≤-20; PhCH(Me)+, -15.4; PhC(Me)2+, -12.3; Ph2CH+, -11.7; Ph2C(Me)+, -9.3; 9-fluorenyl carbocation (9-Fl+), -15.9; 9-methyl-9-fluorenyl carbocation (9-Me-9-Fl+), -11.1. The pKR for Ph2CH+ is in fair agreement with the value estimated using acidity functions,1a but the pKR for 9-Me-9-Fl+ is ca. 4 units more positive than that from the acidity function method,1a so that the difference in the acidity of benzhydryl and fluorenyl carbocations is smaller than estimated in earlier work. The 12 π-electron cyclic fluorenyl system in 9-Fl+ and 9-Me-9-Fl+ causes only 5.7 kcal/mol and 2.4 kcal/mol, respectively, destabilization of the corresponding acyclic carbocations Ph2CH+ and Ph2C(Me)+. The pKR values show that "antiaromatic" destabilization of the 9-fluorenyl carbocations must be small. Ab initio calculations of the structures and energies of 9-Fl+ and Ph2CH+ and of the corresponding alcohols at the 3-21G//3-21G and 6-31G*//3-21G levels indicate that Ph2CH+ is ca. 8-10 kcal/mol more stable than 9-F1+, which is in good agreement with the stability difference calculated from the pKR data. This indicates that electronic factors play the major role in determining the relative energies of these carbocations. Force field calculations were performed to estimate the contribution of van der Waals and ring strains to the difference in the pKR values for Ph2CH+ and 9-Fl+. Assuming hypothetical structures for Ph2CH+ and 9-Fl+ which are free of van der Waals and ring strains, it is then estimated that there is an 8-11 kcal/mol decrease in π-electron stabilization on moving from Ph2CH+ (C2v) and 9-F1OH to 9-Fl+ and Ph2CHOH. It is concluded that 9-fluorenyl carbocations are not antiaromatic. The difference in the energy of the 9-fluorenyl and benzhydryl carbanions relative to the alcohols was calculated to be -13.2 kcal/mol at the 6-31G*//3-21G level. This difference is attributed to the difference in the energies of the HOMOs for the two carbanions.
