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Supramolecular catalysis induced by polysaccharides. Homogeneous hydrolysis of p ‐nitrobenzyl amylose xanthate
Author(s) -
Humeres Eduardo,
Gerônimo Vanilde F.,
Zwirtes Inês R.,
Mazera Edio,
Aliev Abil E.
Publication year - 2003
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.626
Subject(s) - chemistry , hydrolysis , amylose , xanthate , cellulose , catalysis , reaction rate constant , iodide , absorbance , supramolecular chemistry , inorganic chemistry , entropy of activation , medicinal chemistry , organic chemistry , molecule , kinetics , chromatography , physics , quantum mechanics , starch
p ‐Nitrobenzyl amylose xanthate (AmXNB) was synthesized and characterized by 13 C NMR spectroscopy in solution and the solid state. The degree of substitution (DS), calculated from the sulfur content, was 7.0, and this value was similar to that obtained from solid‐state 13 C NMR using the signal of C‐1 as internal standard. The hydrolysis of AmXNB was studied in 10% (v/v) DMSO with µ = 0.5 (KCl) at 25 °C. The basic hydrolysis was pseudo‐first order, but the water‐catalyzed hydrolysis in the pH range 7–9 showed a biphasic plot of ln (ΔAbsorbance) vs time, as has been observed for cellulose xanthate esters, occurring through two parallel reactions with rate constants k ′ H 2 O (fast) = 5.3 × 10 −5 s −1 and k ″ H 2 O (slow) = 3.3 × 10 −6 s −1 . The fast hydrolysis was more than three orders of magnitude faster than that of the O ‐ethyl analog. The activation parameters were Δ H ≠ = 20.5 kcal mol −1 and Δ S ≠ = +10 cal K −1 mol −1 . They showed that the acceleration of the fast hydrolysis of AmXNB and cellulose analogs is due to an entropy of activation effect. There is a linear increase of log k ′ H 2O (fast) with increase in the concentration of the small Li + ion that produces an increase of the 3‐D hydrogen‐bond network of water while the large singly charged iodide ion has a considerable inverse effect. These results are strongly consistent with the theory that the supramolecular catalysis induced by modified polysaccharide esters is due to the 3‐D hydrogen‐bond network of the water in the solvation shell. Copyright © 2003 John Wiley & Sons, Ltd.