Influence of β-cyclodextrin on kinetics and mechanism of ethyl acetate saponification reaction

Ciklodekstrini (CD) so v zadnjih letih pritegnili pozornost raziskovalcev zaradi svoje potencialne uporabe kot »molekularni reaktorji«, ki omogočajo izvedbo organskih reakcij v vodnih raztopinah (zelene tehnologije). So ciklični oligosaharidi, ki nastanejo pri encimski razgradnji škroba. Imajo obliko prisekanega stožca s hidrofobno notranjostjo in primarnimi ter sekundarnimi OH skupinami nameščenimi na zunanjih straneh odprtine. Zunanja površina CD je hidrofilna, zaradi česar so relativno dobro topni v vodi. Hidrofobna notranjost pa omogoča vključitev hidrofobnih molekul v notranjost CD, nastane vključitveni kompleks gostitelj-gost. Gostujoče molekule so v kompleks vezane samo na osnovi van der Waalsovih sil, brez tvorbe kovalentnih vezi, pri čemer se spremenijo fizikalne lastnosti in kemijska reaktivnost. Poznavanje mehanizma nastanka CD kompleksa in njegovega vpliva na potek reakcije je tako ključnega pomena. V doktorski disertaciji smo na modelnem sistemu alkiltrimetilamonijevih bromidov raziskali vpliv van der Waalsovih sil in hidrofobnega efekta. Proces micelizacije smo proučevali z merjenjem specifične prevodnosti in iz na lasten način izpeljanega pseudo-faznega modela določili termodinamske parametre: standardno prosto Gibbsovo energijo (∆G0mic), entalpijo (∆H0mic) in entropijo (∆S0mic) micelizacije ter jih analizirali kot funkcijo povečanja dolžine alkilne verige in temperature. Rezultati kažejo, da je micelizacija termodinamsko ugoden proces (∆G0micCyclodextrins (CD) have attracted the attention of researchers in recent years because they can be used as "molecular reactors" that anable organic reactions in aqueous solutions (green technologies). They represent cyclic oligosaccharides formed during the enzymatic degradation of starch. They have the shape of a truncated cone with a hydrophobic interior and primary and secondary OH groups attached to the outside of the opening. The outer surface of CD is hydrophilic, which makes them relatively soluble in water. The hydrophobic interior allows the inclusion of hydrophobic molecules in the cavity of CD and creates a host-guest inclusion complex. The guest molecules are bound to the complex only by van der Waals forces without forming covalent bonds, which changes the physical properties and chemical reactivity. Therefore, knowledge of the mechanism of CD complex formation and its influence on the course of the reaction is crucial. In this dissertation, we have studied the influence of van der Waals forces and hydrophobic effect on a model system of alkyltrimethylammonium bromides. The mycelization process was studied by measuring the specific conductance and the thermodynamic parameters were determined using a pseudo-phase model derived by us: the standard Gibbs free energy (∆G0mic), enthalpy (∆H0mic) and entropy (∆S0mic) of mycelization and analysed with the increase in alkyl chain and temperature. The results show that mycelization is a thermodynamically favorable process (∆G0mic < 0). In all cases, ∆G0mic is only slightly dependent on temperature and the value of ∆G0mic is more negative for systems with a longer alkyl chain. We found that ∆S0mic is always positive and its variation with temperature shows that the mycelization process is an entropy-driven process. Subsequently, we investigated the effect of β-cyclodextrin on the mycelization process. The obtained results showed that the inclusion complex (CD:S+) was formed and the apparent critical micelle concentration (CMC*) was equal to the sum of the concentration of CD-complexed surfactant monomer and the concentration of free dissolved monomers in equilibrium with CD-free mycelized surfactant. The binding constant K11 was determined for the inclusion complex, and its value increased with increasing alkyl chain lenght and thus hydrophobicity. We developed a mathematical model to calculate the concentrations of uncomplexed cyclodextrin, uncomplexed surfactant ion (S+) and inclusion complex (CD:S+) in both submicellar and micellar regions. Saponification of ethyl acetate (EtAc) was performed in a batch reactor. By measuring the specific conductance, we followed the concentration change of the reactants and determined the reaction rate constant (k) based on a mathematical model. Arrhenius parameters (A, Ea) were obtained from the temperature dependence of k. According to the transition state theory (TST), we also determined the activation enthalpy (〖∆H〗^#) and activation entropy (〖∆S〗^#) from the Eyring equation. Low 〖∆H〗^# and negative 〖∆S〗^# are consistent with the reaction mechanism forming the tetrahedral intermediate. According to the proposed mechanism, the OH- ions behave like a base. The nucleophilic part of water attacks the electrophilic C atom of the ester, breaking the π-bond and forming an intermediate (CH3(C2H5O)OH(CO-)), which later decomposes into products (C2H5OH and CH3COO-). The saponification reaction of ethyl acetate was then carried out at different concentrations of added ß-CD. We derived a new mathematical model to determine the reaction kinetics and the relationship between conductivity and concentration. The observed reaction rate constant (kobs) decreased with the increasing β-CD concentration at all temperatures. We found that cyclodextrin does not affect the formation of products, but only acts as an inhibitor due to the inclusion complex formed (CD:EtAc). Alkaline hydrolysis occurs only with the free ester in solution. From the variation