Ab initiostudy of the effect of α‐substituents on the acidity of cyclopropabenzene

Abstract

The effect of substituents directly bound to the deprotonation site (C-7) on the acidity of cyclopropabenzene was studied using the MP2/6-31þG* method. The studied substituents encompass a variety ofand -accepting groups including the highly electronegative fluorine atom. It is shown that all substituents enhance the acidity of cyclopropabenzene with the effect being the smallest (0.02 kcal mol À1 ) for the methyl group and the largest (33.7 kcal mol À1 ) for the hydrosulfonyl group. It is further shown that the -subsituents employed stabilize the cyclopropabenzenyl anion less efficiently than the cyclopropenyl anion, with the effect being more pronounced for the substituents acting via inductive/field effects. This is attributed to the fact that attachment of inductively/field acting substituents to the carbanionic site predominantly stabilize the cyclopropenyl anion by increasing the s character of the lone pair, diminishing the antiaromatic character of the three-membered ring at the same time. Hence these two effects are operative in concert. The opposite occurs in related cyclopropabenzenyl anions. Here, the rehybridization at the carbanionic center does stabilize the lone pair, but decreases the anionic resonance of the whole system, because of a decrease in overlap between the lone pair and the -AO of the annelated bond. The reverse picture holds for the conjugatively acting substituents. COMPUTATIONAL METHODSThe geometries of all species considered were optimized with the second-order Møller-Plesset perturbation theory employing the 6-31þG* basis set. This model will henceforth be denoted MP2. 11 The nature of the stationary points was characterized by vibrational frequency

Interpretation of Brønsted Acidity by Triadic Paradigm: A G3 Study of Mineral Acids

“…Unlike numerous studies of the acidity of organic neutral acids, in particular with the C−H bond as a proton donor, − systematic analyses of their inorganic counterparts are scarce. − In order to fill the gap we report here on the attempt to interpret acidity of the most common inorganic acids and superacids by using triadic paradigm …”

“…32 The comparative merits of different modes of interpretation of the acidity were assessed and discussed in extenso by Deakyne. 33 Unlike numerous studies of the acidity of organic neutral acids, in particular with the C-H bond as a proton donor, [34][35][36][37][38][39][40][41][42][43][44][45] systematic analyses of their inorganic counterparts are scarce. [46][47][48][49][50] In order to fill the gap we report here on the attempt to interpret acidity of the most common inorganic acids and superacids by using triadic paradigm.…”