Activation energy unsaturated esters

The hydrolysis of esters is of technical interest therefore many different esters such as acetates [18], phthalates [19], natural fats [20] and others were investigated. A detailed investigation of the hydrolysis of ethylacetate (tubular reactor, 23-30 MPa, 250-450 °C, 4-230 s) [7] without the addition of a catalyst shows a lower activation energy at subcritical conditions than at supercritical conditions, indicating two different reaction mechanisms. Under subcritical conditions nucleophilic attack on a protonated ester is assumed to be the rate-determining step of the hydrolysis process. The formation of a protonated ester is favored in the subcritical region because here the self-dissociation of water and the dissociation of the acid, formed via hydrolysis, increase. At 350 °C, 30 MPa, 170 s reaction time, and without additional acid, the conversion to acid and alcohol was 96 %, which is the equilibrium value. In other cases, mostly with unsaturated esters, the acids formed undergo decarboxylation, which leads to poorer yields [12]. 

Since the activation energy of the reaction of R02 with the C—H bond is equal to 25 50 kJ/mol, the contribution of the multidipole interaction to the activation barrier is considerable and can either increase or decrease this barrier. Below we present the results of studying the addition reactions of R02-, ROOH, and O2 at the double bond of mono- and polyfunctional unsaturated (acrylate) esters. 

Since the Es value is determined by the relative activation free energy from the unsaturated initial state to the saturated tetrahedral intermediate state of the ester hydrolysis, Hancock and his coworkers considered that a hyperconjugation effect of a-hydrogen may contribute to the estimate of Es values 19). To separate the hyperconjugation effect from the true steric effect , they defined the parameter E° (corrected steric) as Eq. 20, assuming that the hyperconjugation effect is proportional to the number of a-hydrogen atoms, nH. By definition, E (Me) = 0. 

---