ESTERS. 223 ESTERS. CHjCOCl, which is the chloride of acetic acid. The read ion is as follows: CH,< (i( I + (,ll,<tll = (M COOCA 4 I K I Acetyl Alcohol Ethyl acetic Hydi Uorlc chloride ester acid The esters of organic acids, such as ethyl acetic ester, arc. as a rule, colorless, pleasant-smelling, more or less volatile Liquids. Some occur ready formed in the vegetable world, imparting their odor to fruits and flowers. Artificially prepared esters, therefore, serve to flavor candy, pastry, and perfumes, and are sold under the names of pear oil. apple-oil, pineapple-oil, etc. Other esters occur ready formed, both in the vegetable and in the animal worlds, and are known as oils and fats (qq.v.). By the action of water, esters are broken up into their components (i.e. into alcohols and acids). Thus, ethyl acetic ester is broken up into ordinary acetic acid and ethyl alcohol accord- ing to the following equation: CH,COOC 2 H, -f- H.O = CH3COOH + C : H,OH Ethyl acetic Water Acetic- acid Ethyl ester alcohol Mineral acids and metallic hydroxides have the same effect on esters, even more pronounced. Thus, potassium hydroxide (caustic potash) de- composes ethyl acetic ester as follows: CH 3 COOC = H 5 " + KOH = CH,COOK + C.H.OH Potassium acetate Estrification AND Saponification. The eon- version of an acid into an ester is termed estrificar Hon. The decomposition of an ester into its con- stituents is termed saponification (the reason for using this term is stated in the article Fats). When the saponification of an ester is effected by water alone, without the aid of acids or metallic hydroxides, the ester may be said to be 'hydro- lyzed.' for the term hydrolysis is applied in gen- eral to any process in which water effects the de- composition of a given substance. The processes of estrification and saponifica- tion have furnished a considerable portion of the material upon which certain very important the- ories of modern chemistry have been tested and verified. The theories themselves are explained in some detail in the article Reaction, Chemical. A brief account of their bearing on the processes of estrification and saponification may, however, not be out-of place in the present sketch. It was stated above that ethyl acetic ester may be formed by the action of ethyl alcohol upon acetic acid. This reaction takes place according to the following equation: C4XOH + CH 3 COOH = CH 3 COOC.H B + HX> 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 case, one of the reasons being as follows: All chemical aciimis take place according to the law of ma action. B5 this law, the rapidity with which two given Bubstances react with each other at a given temperature is proportional to the amounts of those substances contained in unit volume. The greater the amounts present, the more rapid the reaction. When alcohol and acetic acid arc mixed together, a reaction starts in with coi 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 estrification) becomes gradually slower and slower. On the other hand, since the reaction produces ester and water, the amounts if 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- ad ion has then entirely ceased. On the contrary, both of the opposite reactions continue to take place as before. Only for every amount estrified, 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: 15% grams 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, 88 grams of ethyl acetic ester and 18 grams of water (88 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 i- over we find the same amount of it as we originally added to the ester: and that the stronger the acid employed the more rapid is the decomposi- tion effected by it. (See Catalytic Action.)
