σ I VALUES FOR ARYLUREIDO GROUPS Iva B. BLAGOEVA 1, *, Asen H. KOEDJIKOV 2 , Ivan G. POJARLIEFF 3 and Stefan P. STANCHEV Institute of Organic Chemistry, Bulgarian Academy of Sciences, ul. Acad. G. Bonchev block 9, Sofia 1113, Bulgaria; e-mail: 1 iva@orgchm.bas.bg, 2 vale@orgchm.bas.bg, 3 ipojarli@orgchm.bas.bg Received September 10, 2004 Accepted November 11, 2004 Dedicated to Professor Otto Exner on the occasion of his 80th birthday. Three (1-arylureido)acetic acids were prepared by means of controlled alkaline hydrolysis of the respective 1-arylimidazolidin-2,4-diones. Their p K a values and that of (3-phenylureido)- acetic acid were determined potentiometrically by titration in 0.01 M solutions at 25.0 °C. Four new σ I constants were calculated from the acidity constants. Ke y w o rd s : Hydantoin; N -Arylhydantoic acids; Acidity constants; Sigma values; N -Arylureido groups; Hammett equation; LFER. There is a continued interest in σ constants related to the application of QSAR in drug design 1 . In our study 2 of the mechanism of acyl transfer reac- tions by means of cyclizations of strained hydantoic acids, we encountered the need to estimate the p K a values of the unstable tetrahedral intermedi- ates using Hammett LFER 3 . These required as parameters σ I values for sub- stituted ureido groups. Exner and Lakomý 4 have determined the σ p and σ m for the unsubstituted ureido group from the acidity constants of the re- spective benzoic acids. These have been used to calculate 5 a field value for NHCONH 2 , F = 0.09. Charton’s 6 σ I value of 0.23 for the ureido group is based on the hydantoic acid acidity constant. For substituted ureido groups we were forced to use estimates based on various assumptions 2 . This prompted us to determine the σ I values of some aryl substituted ureido groups which should presumably be of more general interest. The approach we chose was to use the acidity constants of (arylureido)acetic acids. Charton 6 has shown that these are not affected significantly by steric ef- fects. Collect. Czech. Chem. Commun. (Vol. 69) (2004) 2276 Blagoeva, Koedjikov, Pojarlieff, Stanchev: doi:10.1135/cccc20042276