COMPARISON OF BRONSTED ACIDITIES OF NEUTRAL NH-ACIDS IN GAS-PHASE, DIMETHYL-SULFOXIDE AND WATER

The Bronsted acidities of several neutral NH-acids (substituted diphen ylamine, substituted anilines and imides) were measured in the gas pha se (pulsed FT ICR spectrometry), dimethyl sulfoxide and aqueous soluti on. Comparison of the Bronsted acidities of neutral NH-acids in the ga s phase, dimethyl sulfoxide and water was also carried out. It was sho wn that substituent effects on the acidity of the studied compounds ar e significantly attenuated by the transfer of the reaction series of a cidic dissociation of neutral acids from the gas phase into dimethyl s ulfoxide and water. The strongest solvent-induced attenuation of the s ubstituent effects is characteristic of the meta-substituted anilines whose sensitivity towards substituent effects decreases with transfer from the gas phase into DMSO by 2.83 times and with transfer into wate r by 4.13 times. At the same time, the reaction series of para and/or ortho-pi-acceptor substituted anilines, amides, imides and substituted diphenylamines are less sensitive to a change in gas phase for DMSO o r water. In the special case of para-acceptor substituted anilines it was demonstrated that the specific solvation induced an increase in th e acidity of the para- and/or ortho-acceptor substituted anilines as c ompared with the behavior of the corresponding meta-substituted anilin es by amounts up to 10 pK(a) units. (C) 1998 Elsevier Science B.V. All rights reserved.