Kinetic Study of the Alkaline Hydrolysis of 1,n‐Bis(4‐cyanopyridinium)alkanes: Charge Density and New Conformational Effects on the Reactivity of 1,3‐Bis(4‐cyanopyridinium)propane

Abstract

The alkaline hydrolysis reaction rates of 1,n-bis(4-cyanopyridinium)alkane derivatives C n bis(CP) 2+ with n = 3, 6, and 8 were studied and compared to the reaction rate of the N-methyl-4-cyanopyridinium (MCP + ). C 6 bis(CP) 2+ and C 8 bis(CP) 2+ obeyed the first-order kinetic law. However for C 3 bis(CP) 2+ data fitted to a consecutive two-step model reaction, the observed rate constants (k obs ) of C 8 bis(CP) 2+ and C 6 bis(CP) 2+ are approximately 50% and 100%, respectively, higher than those for MCP + , an effect mainly assigned to the higher charge density of these two derivatives. For C 3 bis(CP) 2+ , the k obs of the second (slow) step is almost twofold the value observed for C 6 bis(CP) 2+ , whereas the first (fast) step is approximately six times higher. As for MCP, the hydrolysis of C n bis(CP) 2+ generates pyridone (P o ) and carbamidopyridinium (A + ) units. For C 3 bis(CP) 2+ , however at pHs above 11.5, one additional product is formed. From the existence of the new product and the kinetic evidence, a "sandwichedtype" complex with the OH − inserted between the rings is proposed. This structural effect in the C 3 bis(CP) 2+ due to the conformational effect justifies the (i) two kinetic steps, (ii) high rate constants, (iii) high P o /A + ratios, (iv) observed temperature and salt effects, and (v) the formation of the new product. C 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 478-486, 2013

Microenvironment Effects on the Kinetics of the Alkaline Hydrolysis of Bispyridinium Conformers

“…6), resulting in a first fast step ( F ) represented by k 1 followed by a slow step ( S ) k 2 . This time‐dependence profile was exactly the same as obtained with C n bis(2BP)/OH − and C n bis(4CP)/OH − . In fact, all trends of this reaction were in accordance with our previous results.…”

“…Indeed, from the plots k 1obs and k 2obs versus [OH − ], the second‐order rate constants were obtained ( k 1 and k 2 ; Table ). The k 1 follows the sequence C 8 bis(2AP) 2+ < C 6 bis(2AP) 2+ < C 5 bis(2AP) 2+ < C 4 bis(2AP) 2+ ≪ C 3 bis(2AP) 2+ , that is, it increases with increasing charge density of the reactant, as previously observed for other C n bis reactants . In addition, it is clear from Table that k 2 undergoes a small decrease as the methylene spacer length increases, becoming practically invariable for n = 5, 6, and 8 derivatives.…”

“…The C n bis(2AP) 2+ /OH − reaction also exhibits the isosbestic point, as observed for reactions, as C n bis(2BP) 2+ /OH − , C n bis(2BP)/N 3 − , and C n bis(4CP)/OH − with a stable reaction intermediate. These results are due to the intermediate, which is half of the start material and half of the end product (as depicted in Scheme ).…”

“…Another investigated hydrolysis involved bis‐(4‐cyanopyridinium)alkanes homologous, C n bis(4CP) 2+ against OH − . Again the reactivity followed MCP + < C 8 bis(4CP) 2+ < C 6 bis(4CP) 2+ ≪ C 3 bis(4CP) 2+ , and the product composition [P]/[A] ratios were higher for the short‐bridged reactants . During this investigation, an unusual product was obtained in the reaction of C 3 bis(4CP) 2+ at pH higher than 11.5, which was identified as 1,3‐(4‐cyano‐2‐pyridone)‐(4‐cyano‐1,2‐dihydropyridine)propane .…”

Estudo teórico de complexos de transferência de carga em solução

“…O C 4 (4CP ) + (Figura 1.1a)é formado por apenas um anel piridínio e experimentalmente tem sido amplamente estudado desde a década de 1950 [21]. Já o C 3 bis(4CP ) 2+ (Figura 1.1b)é formado por dois anéis conectados por uma cadeia carbônica (derivado bispiridínico), tendo sido estudado recentemente por Hioka e colaboradores [34,35,[55][56][57][58].…”