摘要
糖淀粉对不同链长的对位取代苯酚羧酸酯在1:1(V/V)MeSO-HO中的水解有显著的加速作用,乙酸酯的水解是二级反应;十二酸酯和十六酸酯则均呈Michaelis-Menten型饱和动力学.讨论了受物的链长和取代基对络合物解离常数K和催化速度常数k的影响.乙酸酯及十二酸酯水解反应的k,k,k和k均符合Hammett方程,而十六酸酯则不符合.包结过程以不利的熵变、有利的焓变为特征.比较(?)专和自发水解的(?)表明,糖淀粉加速长链酯水解的主要原因是形成了螺旋包结络合物.还讨论了由于十六酸酯分子间簇集和自身绕曲对包结和催化过程的影响.
Hydrolytic rate constants of p-substituted phenol esters of carboxylic acids with various chain lengths were measured in the 1:1(V/V)MeSO-HO.Amylose accelerates the hydrolytic rates of all substrates,but the catalytic patterns are different for long and short chain sudstrates,i.e.,acetates(2-X)show 2nd order kinetics,dodecanoates,(12-X)and hexadecanoates(16-X)follow Michaelis-Menten saturation kinetics.The dissociation constants Kof inclusion complexes are dependent on the chain length of substrates,but no correlation can be found between Kand the electronic effects of substituents.The rate constants k,k,kand kof 12-X and 2-X conform to the Hammett relation,theρvalues are almost the same,whether in the presence or absence of amylose.But k,kand kvalues of 16-X all cannot be correlated by the Hammett equation,presumably because of the aggregation and self-coiling of 16-X in this poor solvent.Thermodynamic parameters⊿Hand⊿Sof the inclusion process and activation parameters⊿H(?)and⊿S(?)were obtained from the temperature dependence of Kand k.The results indicate that the formation of inclusion complexes between amylose and substrates is an entropy disfavored and enthapy favored process.Comparison of⊿H(?),⊿S(?)with⊿Hand⊿Sof the spontaneous hydrolysis shows that the acceleration of hydrolysis of long chain substrates by amylose is caused by the formation of helical inclusion complexes.The influence of intermolecular aggregation and self-coiling of 16-X on the inclusion and catalytic processes is also discussed.
出处
《化学学报》
SCIE
CAS
1985年第1期57-63,共7页
Acta Chimica Sinica
