Esterification determination

A review on the regioselective manipulation of hydroxy groups using organotin derivatives includes a section on esterification. Determination of the position of ester groups in glycosphingolypids by 2D H n.m.r. spectroscopy has been described. 

The Coloqmthoides variety of CitruIIus lanatus (an ancestor of the watermelon) is mainly grown in Eg) ) . The economic importance of this crop is owing to its seeds, and the rest of the fruit is discarded. For this reason, the waste of CitruUus lanatus var. Colocynthoides as a new source of pectin was proposed [42]. For the extraction, HCl was used at different pH values and temperatures, and it was found that at pH 2 and 85°C the highest yield of 19.75% (w/w) was reached. The degree of esterification determined was -55%. With these results, it is stated that this residue may be a potential source of pectin, but more research is needed to fully understand this source of production. 

Sulfonate Esters. Sucrose sulfonates are valuable intermediates for the synthesis of epoxides and derivatives containing halogens, nitrogen, and sulfur. In addition, the sulfonation reaction has been used to determine the relative reactivity of the hydroxyl groups in sucrose. The general order of reactivity in sucrose toward the esterification reaction is OH-6 OH-6 > OH-1 > HO-2. 

Determining the degree of substitution using standard proton nmr refles on the integral ratio between the ceUulosic ring protons ( i 5.0-2.96) and the ester alkyl protons ( i 1.26 for butyryl and propionyl and i 2.06 for acetyl methyl groups). This simple procedure is used extensively to determine the extent of esterification and is currently the fastest, easiest way for determining the DS of mixed cellulose esters. 

We have seen this theme before in Section 19.14 when we presented the mechanism of the Fischer esterification. As was the case then, formation of the tetrahedral intermediate is rate-determining. 

Oxidation of ecgonine (2) by means of chromium trioxide was found to afford a keto acid (3). This was formulated as shown based on the fact that the compound undergoes ready themnal decarboxylation to tropinone (4)The latter had been obtained earlier from degradative studies in connection with the structural determination of atropine (5) and its structure established independently. Confirmation for the structure came from the finding that carbonation of the enolate of tropinone does in fact lead back to ecgonine. Reduction, esterification with methanol followed by benzoylation then affords cocaine. 

It was of interest to determine whether glycosides of 6-deoxy-D-xylo-hex-5-enopyranose were susceptible to enzyme hydrolysis by / -glucosi-dase. Since aromatic glucosides are hydrolyzed by this enzyme at a much faster rate than aliphatic glycosides, phenyl 6-deoxy-/ -D-rt/Zo-hex-5-enopyranoside (18) was prepared (20). Phenyl / -D-glucopyranoside was converted to the 6-tosylate by selective esterification and then, by conventional procedures, transformed to phenyl 2,3,4-tri-0-acetyl-6-deoxy-... 

Determination of carboxy groups. Esterification or polyesterification kinetics is usually followed by this titration which is both easy and accurate. Each sample is dissolved in a solvent or a mixture of solvents (CHCI3, QH /EtOH or MeOH, toluene/EtOH or MeOH...) and then titrated with alcoholic KOH. The end point is determined either with an indicator (in most cases phenolphthalein) or with a pH-meter. An accuracy of about 0.1-1% can generally be achieved. 

Determination of hydroxy groups. Hydroxy groups are rarely titrated. Indeed, this titration is more complex and less accurate than carboxy-group determination. However, it is very helpful to know the hydroxy group content, for example in the case of reactions carried out with a great excess of add. In this case, the variations of the add concentration during the esterification are very small and are determined only with poor accuracy. 

Determination of water removal rate. This technique is usually applied to the continuous following of esterification kinetics. 

The volume of the water removed in esterifications through azeotropic distillation287-289 and also in polyesterifications is determined6,19,264,292). This method is difficult to carry out because it is necessary to quantitatively collect the water which is released and to determine its volume or its weight. It is important to estimate as accurately as possible the amount of water remaining in the distillation column. 

Fiery1 252-254) studied only the last stage of the reactions, i.e. when the concentration of reactive end groups has been greatly decreased and when the dielectric properties of the medium (ester or polyester) no longer change with conversion. Under these conditions, he showed that the overall reaction order relative to various model esterifications and polyesterifications is 3. As a general rule, it is accepted that the order with respect to acid is two which means that the add behaves both as reactant and as catalyst. However, the only way to determine experimentally reaction orders with respect to add and alcohol would be to carry out kinetic studies on non-stoichiometric systems. 

Since the catalytical behaviour of titanium derivatives depends on their degree of condensation, these catalysts are used under conditions where the water concentration is very low, for instance in transesterifications or in the last steps of esterifications. However, the amounts of water required to hydrolyse these compounds are so low (less than 0.5 ppm for Ti(OBu)4) that hydrolysis is probably the determining phenomenon in most studies. It seems that before Fradet and Marshal230 the contribution of this side effect has not been taken into consideration. 

Several authors suggested mechanisms for esterifications catalyzed by titanium tetraalk-oxides. Bolotina et al.16,221,2221 who studied the polyesterification of 2-ethylhexyl phtha-late with 2-ethylhexanol found the same reaction order with respect to catalyst, acid and alcohol, namely 1 they suggested the following rate-determining step ... 

As sulfonic acid cannot be vaporized, its determination by the direct gas chromatography (GC) method is not possible. To enable determination by GC, the reactivity of the S03H group is used the esterification of the S03H group with diazomethane via acid chloride is one way to transfer the sulfonic acid to volatile compounds. By conversion of the sulfonic acid with phosphoric acid at 200-210°C, the S03H group is cleaved and the hydrocarbons are obtained [184-186]. 

The yields were determined by gas chromatographic analysis after esterification of aliquots with sulfuric acid and methanol in 1,2-dichloroethane. 

These results do not prove that the ester is an essential intermediate in aqueous solution even though it is present, but the result with the hindred triterpene is convincing In this case the esterification step, which is normally fast, has become rate-determining and the disappearance of the isotope effect must mean that C-H cleavage occurs after the formation of the ester and not independently of it. The generality of this result is apparent from the stopped-flow investigation of isopropanol oxidation ... 

The sol-gel transition has been determined visually, with calcium and copper, for different pectins under different external conditions. As shown in Figure 5 for sample C44 the homogeneous gel phase is situated between the two transition lines. The extension of this phase was found to depend mainly on the DE, temperature and nature of the cation. With calcium the amount of cation required to get a gel increased with the degree of esterification and above 50% it became impossible to get a gel [8]. 

Mort, A.J., Qiu, F., and Maness, N.O. (1993) Determination of the pattern of methyl esterification in pectin. Distribution of contiguous nonesterified residues. CarbohydrJies. 247 21-35. 

Analytical methods. The anhydrouronic acid content (AUAC), degree of esterification (DE), degree of amidation (DA) and ash content were determined according to the methods described in Food Chemical Codex II ed [5]. 

The importance of the degree of esterification (%DE) to the gelation properties of pectins makes it desirable to obtain a fast and robust method to determine (predict) the %DE in pectin powders. Vibrational spectroscopy is a good candidate for the development of such fast methods as spectrometers and quantitative software algorithms (chemometric methods) becomes more reliable and sophisticated. Present poster is a preliminary report on the quantitative performance of different instrumentations, spectral regions, sampling techniques and software algorithms developed within the area of chemometrics. 

The degree of esterification of the methyl ester of PGA was determined acidimetrically after hydrolysing the ester with sodium hydroxide [6]. 

Methods of analysis. The polyuronic acid content (PUAC) and the degree of esterification (DE) were determined according to Owens et al (10). 

After Michaelic constant (KJ determination pectin concentration with known esterification degree was expressed in mol/M according to the content of anhydrogalacturonic acid. 

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