THE PRINCIPLES OF ORGANIC CHEMISTRY 55 



It should be noted that these reactions in which salts are 

 formed from alcohols with elimination of water are typical 

 examples of what are known as reversible reactions ; that is, 

 when a certain anjount of water and salt is formed, an equili- 

 brium is attained, and the reverse action tends to take place, 

 resulting in the formation of acid and alcohol. Such reactions 

 are generally written thus : — 



EOH + ECOOH t- ECOOK + H^O 



If it is desired that the reaction should become complete 

 it is necessary to add some substance such as strong sulphuric 

 acid or chloride of zinc which will take up water as it is 

 formed. 



It is probable that under specific conditions nearly all 

 chemical reactions are reversible. The case of the esters 

 is interesting as a simple one, which has been carefully 

 studied. 



Ethers. — Esters should not be confused with ethers, which 



■p 

 are bodies of the general formula t>,^0, E. in this case 



representing a hydrocarbon residue ; thus, in ordinary ether 



E = the group CgHj or ethyl, and its formula is p^Tj'/ 0. 



Phenols. — ^When the group OH is connected directly 

 with a benzene ring, substances known generally as phenolic 

 compounds are produced, the simplest of which is ordinary 

 OH 



carbolic acid or phenol, | ]. On oxidation these substances 



yield somewhat complicated mixtures and are thus distin- 

 guished from ordinary alcohols. 



