Acidity constant esters

Schmid et al. studied in detail the sulfonation reaction of fatty acid methyl esters with sulfur trioxide [37]. They measured the time dependency of the products formed during ester sulfonation. These measurements together with a mass balance confirmed the existence of an intermediate with two S03 groups in the molecule. To decide the way in which the intermediate is formed the measured time dependency of the products was compared with the complex kinetics of different mechanisms. Only the following two-step mechanism allowed a calculation of the measured data with a variation of the velocity constants in the kinetic differential equations. 

FIG. 1 Critical micelle concentration as a function of the number of carbon atoms in the hydrophobic rest of sodium a-sulfo fatty acid methyl esters. Methods O, surface tension +, conductivity A, solubilization of a dye x, solubility (all without electrolyte) , surface tension with a constant electrolyte concentration of 5 x 10"2 mol/L. (From Ref. 57.)... 

Fig. 9 Correlation of (A) the second order rate constants (k2 = kcatIKM) and (B) the transition stabilization (pATS) with the hydrophobicity (it) of the substituent of the amino acid residue for the cleavage of /V-acetylamino acid methyl esters by a-chymotrypsin. The open symbols are for the points for two branched residues (valine and isoleucine). Data from Table A6.8.

The three saturated long-chain tert-butyl peresters are members of a homologons series, and as such, the weighted least-squares regression analysis of the enthalpies of formation V5. number of carbons yields a methylene increment of —26.7 kJmol , a typical valne for liquids. The methylene increment for the terf-butyl esters of the Cg, Cjo, Cn and C14 acids is —28.0 kJmol. The closeness of these two values ensures that the enthalpies of formal reaction 16 will be nearly constant. For the three pairs from Table 3, the value is —70.3 8.1 kJmol. The standard deviation from the mean is quite large because the arithmetic difference for the C12 ester and perester, —79.5 kJmol, is quite a bit more negative than the differences for the Cio and C14 pairs, —64.4 and —66.9 kJmol, respectively. Unfortunately, the acids and esters are in different phases and so we are reluctant to attempt any comparison between them, such as a formal hydrolysis reaction or disproportionation with hydrogen peroxide. 

A novel method for production of paraffinic hydrocarbons, suitable as diesel fuel, from renewable resources was illustrated. The fatty acid ethyl ester, ethyl stearate, was successfully converted with high catalyst activity and high selectivity towards formation of the desired product, heptadecane. Investigation of the impact of catalyst reduction showed that the reduction pretreatment had a beneficial effect on the formation of desired diesel compound. The non-pretreated catalyst dehydrogenated ethyl stearate to ethyl oleate. The experiments at different reaction temperatures, depicted that conversion of ethyl stearate was strongly dependent on reaction temperature with Eact=69 kj/mole, while product selectivities were almost constant. Complete conversion of ethyl stearate and very high selectivity towards desired product (95%) were achieved at 360°C. 

Table 12.1 Reaction Rate Constants for the Base-Catalyzed Hydrolysis of Benzoic Acid Ethyl Ester (Ethyl Benzoate) in Various Organic Solvent-Water Mixturesa...

Acid-Catalyzed Hydrolysis. In acid-catalyzed ester hydrolysis the species that undergoes the rate-determining step is the protonated ester (Fig. 13.10). When the molecule is in this protonated form, the enhanced depletion of electrons near the central carbon promotes the approach of an electron-rich oxygen of a water molecule. Hence, the hydrolysis rate depends on the fraction of compound molecules that are protonated. This fraction, in turn, depends on how strong a base the ester function is. If we define an acidity constant (see Chapter 8) for the protonated species... 

Figure 13.15 Effects of substituents on the base-catalyzed hydrolysis of benzoic acid ethyl esters in ethanoliwater (85 15) at 25°C. Relative reaction rates are correlated with Hammet Oj constants (data from Tinsley, 1979).

Some of these tests cover very broadly the general chemical makeup of waxes. Constants such as acid, ester, saponification and iodine numbers can be used to identify, differentiate or classify waxes chemically (Ref 59). Specific tests are used to determine the values of these numbers for waxes. The saponification number generally indicates the total quantity of both esters and free acids in a wax. The acid value is an indication of the quantity of free fatty acids. The ester value is the relative quantity of esters... 

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