$ Lev Moissejevitch * Professor Vlasov # Agnes % Ivo, Ivari ^ Ilmar #eq numbritel punktid ära. Näitan. Extension of the Self-Consistent Spectrophotometric Acidity Scale in Acetonitrile, Unification of Different Acidity Scales Agnes Kütt †, Ivo Leito *,†, Ivari Kaljurand †, Lilli Sooväli †, Vladislav M. Vlasov ‡, Lev M. Yagupolskii § and Ilmar A. Koppel † † Institute of Chemical Physics, Department of Chemistry, University of Tartu, Jakobi 2 Str., 51014 Tartu, Estonia. E-mail: [email protected]; Phone: +372 7 375 259; Fax: +372 7 375 264. ‡ Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Science, Novosibirsk 630090, Russia § Institute of Organic Chemistry, Ukrainian National Academy of Science, Murmanskaya Str. 5, Kiev 02094, Ukraine. Abstract F F F F F F F F F O CN CN F F F O F CN F F O O O O ∆pK (MeCN) a 4.74 5.10 pK (MeCN) a 13.01 17.75 22.85 The previously composed self#Agnes, siin sõnastasin veidi umber, et jääks rohkem uue asja mulje, mitte vana edasiarendamise mulje. JACS-I jaoks oluline. Self-consistent spectrophotometric acidity scales in acetonitrile (AN) was expanded to rangeranging from 3.7 – to 28.1 pKa units, which is over 24 orders of magnitude. 112 newreported. Altogether 203 acidity measurements werehave been carried out and 49 new compounds were93 acids are included to in the scale. With previous scales altogether 95 compounds are included to the present scale. The relative acidity (∆pKa value) of any two acids on the scale can be obtained by combainingcombining at least two independent sets of measurements. This multiple overlapping measurements makes results more reliableof measurements is essential to achieve high reliability of the pKa values. The overall consistency of the measurements is s = 0.04 pKa units. (#tekstis 0.03). #Selle lause siiapanemises ma natuke kahtlen#In the area of weaker acids, the scale ishas been built up with well-behaved compounds in AN, which have important matter about correctness of the scale and are excellent basis for further studies. #Siia mõtleme veel, milliseid olulisemaid tulemusi panna… #%^Me peame mõtlema, mis on meie cutting edge selles artiklis. Minu meelesst see võiks olla eeskätt see, et on skaala, mis on väga kvaliteetne, igapidi kontrollitud ja seeläbi hea tööriist teistele. Me ei saa eriti rõhutada superhappeid, sest selels vallas pole siin kuigi palju uut võrreldes 1998 aastaga. Peaksime proovida kuidagi sisse panna, et kogu "normaalse" happelisuse ala atsetonitriilis on nüüd kaetud. Mõtted? Introduction In previous publications 1, 2 have been established self-consistent spectrophotometric acidiy scales acidity scale in acetonitrile (AN). These scales ) has been established. That scale spans over 12 pKa units and covers significant part of strong acids. The primary goal of the present work is to significantly extend the self-consistent that acidity scale toward weaker acidic direction for several pKa unitsacidities in order to embrace the range of "conventional" acidities (#%^Kahtlane termin, aga asja mote on näidata, et see siin on mingil määral lõplik asi – jällegi vajalik JACSile). The extension of the scale adds many new interesting compounds (eg perfluorophenol, fluorenes etc), ) for which pKa data in acetonitrile have not been available to date. Included are also useful indicator compounds for further studies (eg the families of disulfonimides, phenylmalononitriles and diarylacetonitriles) and also some commoncommonly encountered acids (4- toluenesulfonic acid, picric acid, acetic acid, benzoic acid). Considerable amountnumber of compounds in the scale are different groupsfamilies of stable CH-acids, which prove are excellent reference compounds in building blocks for the scale in very wide area of pKa values. and useful reference compounds for future investigators. #Sellest jutust osa on justkui resultide jutt, aga las olla praegu siin. Considerable amount of acidity data for various compounds acids in AN has been accumulated. Lately no, however during the recent years little experimental (#tähtis sõna! Massiliselt on arvutuslikke artikleid, mida tuleks siin ka veidi mainida) data of pKa values of acids have has been published. Analysis of the literature 3 , 4 , 5 , 6 , 7 shows that an acidity 2 scale in the pKa range of 14-27 exists in AN, whereat majority of pKa values still holds over 19 units. Great bulk of measurements are carried out using potentiometry and conductometry as the methods. In present work the spectrophotometric acidity scale of pKa range of 4-28 are reported. We will also compare our data with the data reported in the literature to make conclusions about the contractoin/expansion or deviation of the acidity scales. AN have many properties that make it a suitable medium for acid-base studies. It has low basicity and very low ability to solvate anions 4 .The low basicity give AN an advantage over the other very popular solvent for acid-base studies – DMSO, which is considerably more basic (stronger acceptor of hydrogen bonds). As AN has high dielectric constant (D = 36.0 4 ) it favours the dissociation of ion pairs into free ions. The autoprotolysis 8 constant of AN is very low – pKauto ≥ 33 , which makes it a good differeating solvent. Additionally, the advantages of AN are its transparency down to 190 nm and relative ease of purification. Acidity of an acid HA in solvent S refers to the equilibrium HA + S →← A- + SH+ (1.) and is expressed as dissociation constant Ka or more commonly its negative logarithm pKa. a(SH + ) ⋅ a(A- ) K = (2.) a a(HA) Because of the complications of measuring the acidity of the medium, a(SH+ ), in nonaqueous solvents, we used a method that eliminates the need for its determination. Our method of acidity measurements is based on measuring relative acidities of the acids HA1 and HA2 according to the following equilibrium: - - → A + HA (3.) HA 2 + A1 ← 2 1 The relative acidity of the two acids HA1 and HA2 (∆pKa) is defined: - a(A1 ) ⋅ a(HA2 ) ∆pKa = pKa (HA2 ) − pKa (HA1 ) = − log K = log - (4.) a(HA1 ) ⋅ a(A2 ) The method consist of UV-Vis spectrophotometric titration of a solution, where both of the acids are present, with a transparent acid or base. There is no need for the determination of a(SH+ ) any more. #Arvutused, mis on avaldatud. Vähemasti natuke peab neist juttu olema. Suurem osa neist on DMSO kohta. Tuleb püüda jõuda tõdemuseni, et arvutused arenevad hoolega, aga vähemasti praegu eksperimendi vastu veel ei saa. 3 3 , 4 , 5 , 6 , 7 Analysis of the literature shows that an acidity scale in the pKa range of 14-27 exists in AN, whereas the majority of pKa values still holds over 19 units#tagumisest osast ma ei saa hästi aru #meie oma varasem skaala tuleb siia ka sisse tuua, aga on oluline rõhutada, et praegusel on eeliseid: 1. rohkem aineid; 2. kuivades tingimustes tehtud. Mingil määral tuleks ilmselt diskuteerida (tagapool muidugi) ka kokkulangevust, niipalju, kui seda on. The majority of measurements have been carried out using potentiometric and conductometric methods. In the present work the spectrophotometric acidity scale of pKa range of 4-28 is reported. We will also compare our data with the data reported in the literature to make conclusions about the contraction/expansion or deviation of the acidity scales. #%^seda eespool olevat osa tuleb meil kõigil veel täiendada-kohendada. AN has many properties that make it a suitable medium for acid-base studies. It has low basicity and very low ability to solvate anions 4 . The low basicity gives AN an advantage in studies of strong acids over the other very popular solvent for acid-base studies – DMSO, which is considerably more basic (stronger acceptor of hydrogen bonds). As AN has high dielectric constant (D = 36.0 4 ) it favors the dissociation of ion pairs into free 8 ions. The autoprotolysis constant of AN is very low, pKauto ≥ 33 , which makes it a good differentiating solvent. Additionally, the advantages of AN are its transparency for Uv radiation down to 190 nm and relative ease of purification. Acidity of an acid HA in a solvent S refers to the equilibrium + HA + S →← A- + SH (5.) and is expressed as dissociation constant Ka or more commonly its negative logarithm pKa. a(SH + ) ⋅ a(A- ) K = (6.) a a(HA) Because of the complications of measuring the acidity of the medium, a(SH+ ), in nonaqueous solvents, we use (#see annab lausele selle sisu, et me üleüldiselt kogu aeg kasutame sellist meetodit, mitte ainult seekord) a method that eliminates the need for its determination. Our method of acidity measurements is based on measuring relative acidities of the acids HA1 and HA2 according to the following equilibrium: - → A- + HA (7.) HA 2 + A1 ← 2 1 The relative acidity of the two acids HA1 and HA2 (∆pKa) is defined: - a(A1 ) ⋅ a(HA2 ) ∆pKa = pKa (HA2 ) − pKa (HA1 ) = − log K = log - (8.) a(HA1 ) ⋅ a(A2 ) 4 The method consist of UV-Vis spectrophotometric titration of a solution, where both of the acids are present, with a transparent acid or base. As can be seen from eq (8.) There is no need for the determination of a(SH+ ) any more. The acid-base equilibria (5.) in weakly solvating solvents such as AN are more complex than those in water. In addition to the eqilibrium 5., there are other equilibria present in the system 4 . In AN, the poorly solvated anions vigorously form hydrogen-bonded complexes with hydrogen-bond donors present in the solution. When the donor is the conjugate acid of the anion, then the homoassociation process takes plase.place: K − AHA A- + AH →← A- ⋅⋅⋅ HA (9.) K − is the homoassociation constant. AHA a(A − ⋅⋅⋅ HA) K − = (10.) AHA a(A − ) ⋅ a(HA) If the donor is some other acid, then the heteroassociation process is present. ThisThese side of -reactions have to be supressedsuppressed or taken into account if the accurate acidity data are to be obtained.