Saponification equivalent of an ester

The determination of the saponification equivalent of an ester by the alcohohc potassium hydroxide method is described in Section 111,106 an alternative procedure using diethylene glycol is given below. This constant should be determined if possible in the prehminary examination, since a knowledge of its value together with the boihng point provides a basis for a fairly good approximation of the size of the ester molecule. 

Determination of the Saponification Equivalent of an Ester BY THE Diethylene Glycol Method... 

Saponification see Hydrolysis Saponification equivalent of an ester, determination of. 392, 1065 Saturated aliphatic hydrocarbons, 233 reactions and characterisation of 234, 1058 table of, 235 ... 

It is frequently advisable in the routine examination of an ester, and before any derivatives are considered, to determine the saponification equivalent of the ester. In order to ensure that complete hydrolysis takes place in a comparatively short time, the quantitative saponi fication is conducted with a standardised alcoholic solution of caustic alkali—preferably potassium hydroxide since the potassium salts of organic acids are usuaUy more soluble than the sodium salts. A knowledge of the b.p. and the saponification equivalent of the unknown ester would provide the basis for a fairly accurate approximation of the size of the ester molecule. It must, however, be borne in mind that certain structures may effect the values of the equivalent thus aliphatic halo genated esters may consume alkali because of hydrolysis of part of the halogen during the determination, nitro esters may be reduced by the alkaline hydrolysis medium, etc. 

The saponiflcatlon equivalent or the equivalent weight of an ester is that weight in grams of the ester from which one equivalent weight of acid is obtainable by hydrolysis, or that quantity which reacts with one equivalent of alkali. The saponification equivalent is determined in practice by treating a known weight of the ester with a known quantity of caustic alkali used in excess. The residual alkali is then readily determined by titration of the reaction mixture with a standard acid. The amount of alkafi that has reacted with the ester is thus obtained the equivalent can then be readily calculated. 

The Hantzsch pyridine synthesis involves the condensation of two equivalents of a 3-dicarbonyl compound, one equivalent of an aldehyde and one equivalent of ammonia. The immediate result from this three-component coupling, 1,4-dihydropyridine 1, is easily oxidized to fully substituted pyridine 2. Saponification and decarboxylation of the 3,5-ester substituents leads to 2,4,6-trisubstituted pyridine 3. 

Wiesenberger determined the saponification equivalent of esters by a method in which the ester is first saponified with an excess of base. The excess is then neutralized on passage through a bed of acidic resin, and the organic acid formed during saponification is titrated with standard base. In this way a relatively large excess of base can be used to ensure quantitative saponification. The determination is a direct titration therefore only one standard solution is required, rather than the two that are customary in indirect determination. 

If an ester is hydrolyzed in a known amount of base (taken in excess), the amount of base used up can be measured and used to give the saponification equivalent the equivalent weight of the ester, which is similar to the neutralization equivalent of an acid (sec Sec. 18.21). 

The saponification equivalent (S E) measures the number of equivalents of base required for complete hydrolysis of an ester It is defined as... 

It is possible to carry out the hydrolysis of an ester with alkali in a quantitative manner so that the saponification equivalent, SE, results. This value is analogous to the equivalent mass of an acid (Sec. 25.13) in that it is the molar mass of the ester divided by the number of ester functions in the molecule. Therefore the SE is the number of grams of ester required to react with one gram-equivalent of alkali. 

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