Effects of solvent and structure on the reactivity of 6-substituted nicotinic acids with diazodiphenylmethane in aprotic solvents

• Published in: Journal of the Serbian Chemical Society Vol. 74; no. 12; pp. 1359 - 1370
• Main Authors: Drmanic, Sasa, Marinkovic, Aleksandar, Jovanovic, Bratislav
• Format: Journal Article
• Language: English
• Published: Serbian Chemical Society 01.01.2009
• Subjects: aprotic solvents; diazodiphenylmethane; pyridine carboxylic acids; rate constants; solvatochromic parameters
• ISSN: 0352-5139; 1820-7421
• DOI: 10.2298/JSC0912359D

The rate constants for the reactions of diazodiphenylmethane (DDM) with 6-substituted nicotinic acids in aprotic solvents at 30?C were determined. The obtained second order rate constants in aprotic solvents, together with literature data for benzoic and nicotinic acids in protic solvents, were used for the calculation of solvent effects, employing the Kamlet-Taft solvatochromic equation (linear solvation energy relationship - LSER) in the form: log k = log k0 + + s?*+ a? + b?. The correlations of the kinetic data were performed by means of multiple linear regression analysis taking appropriate solvent parameters. The sign of the equation coefficients (s, a and b) were in agreement with the postulated reaction mechanism, and the mode of the solvent influences on the reaction rate is discussed based on the correlation results. A similar contribution of the non-specific solvent effect and electrophilic solvation was observed for all acids, while the highest contribution of nucleophilic solvation was influenced by their high acidity. Correlation analysis of the rate data with substituent ?p parameters in an appropriate solvent using the Hammett equation was also performed. The substituent effect on the acid reactivity was higher in aprotic solvents of higher dipolarity/polarizability. The mode of the transmission of the substituent effect is discussed in light of the contribution of solute-solvent interaction on the acid reactivity. Konstante brzina 6-supstituisanih nikotinskih kiselina sa diazodifenilmetanom (DDM) su odredjene u razlicitim proticnim i aproticnim rastvaracima na 30?C. Izracunate konstante brzina, kao i literaturni podaci, korisceni su za izracunavanje efekata rastvaraca koriscenjem Kamlet-Taft-ove solvatohromne jednacine. Konstante brzina su korelisane sa parametrima rastvaraca koriscenjem Kamlet-Taft-ove jednacine oblika: log k = log k0 + + s?*+ a? + b?. Korelacije dobijenih kinetickih rezultata sa odgovarajucim parametrima rastvaraca su izvedene primenom metode visestruke linearne regresione analize. Znak koeficijenata (s, a i b) u dobijenim korelacijama je u saglasnosti sa reakcionim mehanizmom. Slican doprinos nespecificnih efekata i elektrofilne solvatacije rastvaraca je uocen za sve ispitivane kiseline, a najveci doprinos nukleofilne solvatacije polaznog stanja je posledica visoke kiselosti ispitivanih kiselina. Uticaj rastvaraca na vrednosti reakcionih konstanti je diskutovan na osnovu dobijenih korelacionih rezultata. Korelaciona analiza konstanti brzina sa ?p konstantama supstituenata, u ispitivanom rastvaracu, izvrsena je primenom Hammett-ove jednacine. Efekti supstituenata na reaktivnost ispitivanih kiselina su znacajniji u aproticnim rastvaracima visoke dipolarnosti/polarizabilnosti. Nacin prenosa efekata supstituenata je diskutovan u svetlu doprinosa interakcija rastvorak-rastvarac na reaktivnost ispitivanih kiselina.