CH,COOH = + Ethyl Acetic acid Ethyl acetic Water alcohol ester The reaction evidently involves the simultane ous formation of water and of ethyl acetic ester. On the other hand, it was also stated above that water decomposes ethyl acetic ester into its com ponents. Therefore, even while the ester is being formed from its components, it is broken up again by the action of the water formed along with it. In other words, two opposite reactions take place simultaneously, one being a process of estrification, the other a process of saponifica tion. If the two processes took place with equal rapidity from the very beginning, neither could evidently make any progress: so that, whether we would mix alcohol and acetic acid, or water and ethyl acetic ester, no change at all would ensue.
In reality, however, this is not the ease, one of the reasons being as follows: All chemical re actions take place according to the law of ma SR action. By this law, rapidity given substances react with each other at a given temperature is proportional to the amounts of those substances contained in nnit volume. The greater the amounts present, the inure rapid the reaction. When alcohol and acetic acid are mixed together, a reaction starts in with con siderable rapidity. During the reaction both sub stances gradually disappear as such. Their amounts present in every unit of volume, there fore, gradually diminish, and hence the reaction (i.e. the estrifiention) becomes gradually slower and slower. tin the other hand, since the reaction produces ester and water, the amounts of these gradually increase, and hence the reaction be tween them (i.e. the saponification) gradually becomes more and more rapid. The velocities with which the two opposite reactions take place, therefore, gradually become equal, and when this equilibrium is finally reached the composition of the mixture ceases to change. Not that all re action has then entirely ceased. On the contrary, both of the opposite reactions continue to take place as before. Only for every amount estritied, an exactly equivalent amount is now saponified, and hence no change can be observed. In other words, a 'dynamic' (not a 'static') equilibrium is established in the mixture, which is now com posed of four substances—acid. alcohol, ester, and water. This equilibrium can be reached in two ways: (1) By starting with a mixture of alcohol and acid, or (2) by starting with a mixture of ester and water. Thus, when 46 grams of alco hol are mixed with 60 grams of acetic acid (46 and 60 are the relative reacting weights of alco hol and acetic acid), a process of estrification en sues, and continues until the composition of the mixture becomes as follows: grains of alco hol, 20 grams of acetic acid, 58% grams of ethyl acetic ester, and 12 grams of water. In this mix ture no further change can take place. But a mixture of precisely the same composition is final ly obtained if. to start with, SS grams of ethyl acetic ester and IS grams of water (SS and 18 are the relative reacting weights of the ester and of water) have been allowed to react upon each other.
All this holds good, of course, only in case none of the products of the reaction is eliminated. For
if, for example, we were to remove the water produced by the estrification, the counteracting process (i.e. the saponification) could not take place, and hence the estrification would proceed unchecked until all the alcohol and acid had combined. As a matter of fact, this is the case when some dehydrating agent (such as sulphuric acid, zinc chloride, etc.) is added to a mixture of alcohol and acid, the estrification being then complete.
The decomposition of esters, when effected by water alone, is a very slow process. It takes place much more rapidly under the influence of mineral acids and bases. The action of acids is termed 'catalytic action.' but its nature is really unknown. All we know is that the acid itself remains unchanged; that after the process is over we find the same amount of it as we originally added to the ester: and that the stronger tile acid employed the more rapid is the decomposi tion effected by it. (See CATALYTIC ACTION.) On the contrary, the saponifying action of bases is at present quite well understood: at least, it is explained partly by the law of mass-action al ready mentioned in this sketch, and partly by the theory of electrolytic dissociation. (See Disso ciAnox.) The exact manner in which the facts in question are explained by theory cannot be discussed here. The resulting principles, how ever, may he briefly summed up as follows: The saponifying action of a base is due to the pres .ence of electronegative hydroxyl ions (OH) in its aqueous solution. Since, according to the law of mass-action, the rapidity of any reaction in general depends on the amounts of the active substances contained in unit volume, the rapidity of a saponification must depend on the amount of ester and on the amount of the basic hydroxyl ions present in every unit of volume. The strong er the base the greater the number of hydroxyl ions in its solution, and hence the' greater its saponifying power. If the base is weak (like ammonium hydroxide), its small number of hy droxyl ions is still further (and very consider ably) diminished by the presence of one of its salts; hence the presence of such salts has a re tarding effect on the process of saponification, especially in case the base is weak. Further, since a salt necessarily forms during the saponi fication (see, for example, the equation repre senting, above, the saponification of ethyl acetic ester by caustic potash), the rate of saponifica tion must be diminished not only by the disap pearance of the ester and base as such, but also by the formation of the salt and free alcohol, the products of the reaction. The mathematical ap plication of these principles leads to a method of calculating the rapidity with which a saponifi cation may take place, if the amounts of ester and base and the strength of the latter are given. The results thus obtained on a purely theoretical basis have, in a large number of cases, been veri fied by actual experiment, and the agreement of the theoretical and experimental figures has been found remarkably good throughout.