Hyperaromatic stabilization of arenium ions.
Benzene-cis- and trans-1,2-dihydrodiols undergo acid-catalyzed dehydration at remarkably different rates: k(cis)/k(trans) = 4500. This is explained by formation of a β-hydroxycarbocation intermediate in different initial conformations, one of which is stabilized by hyperconjugation amplified by an aromatic no-bond resonance structure (HOC(6)H(6)(+) ↔ HOC(6)H(5) H(+)). MP2 calculations and an unfavorable effect of benzoannelation on benzenium ion stability, implied by pK(R) measurements of -2.3, -8.0, and -11.9 for benzenium, 1-naphthalenium, and 9-phenanthrenium ions, respectively, support the explanation.
School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland.
Jaya S
JS
Kudavalli
Derek R
DR
Boyd
Dara
D
Coyne
James R
JR
Keeffe
David A
DA
Lawlor
Aoife C
AC
MacCormac
Rory A
RA
More O'Ferrall
S Nagaraja
SN
Rao
Narain D
ND
Sharma
D017605:Bromine Compounds / Q000737:chemistry; D053138:Cyclohexenes / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D015394:Molecular Structure; D014867:Water / Q000737:chemistry
D017605:Bromine Compounds; D053138:Cyclohexenes; D007477:Ions; D014867:Water; C048401:1,4-cyclohexadiene