280 CONSIDERATION OF CARBON COMPOUNDS. 



also homologous series of organic acids running parallel with 

 the corresponding series of hydrocarbons or alcohols. 



Occurrence in nature. Organic acids are found and formed 

 both in vegetables and animals, and are present either in the 

 free state, or (and more generally) in combination with bases as 

 salts, or with alcohols as compound ethers. Uncombined or as 

 salts are found, for instance, citric, tartaric, and^ oxalic acids in 

 plants, formic acid in some insects, uric acid in urine, etc. ; as 

 compound ethers, are found many of the fatty acids in the various 

 fats. 



Some organic acids are also found as products of the decom- 

 position of organic matter in nature. 



Formation of acids. Many acids are produced by oxidation of 

 alcohols. As intermediate products, are formed aldehydes, 

 which may be looked upon (as stated in the last chapter) as 

 alcohols from which two atoms of hydrogen have been removed. 

 For instance : 



C 2 H 5 HO + O = C 2 H 3 HO + H 2 0. 



Ethyl Acetic 



alcohol. aljehyde. 



C 2 H 3 HO + O = C 2 H 3 O.HO. 



Acetic Acetic 



aldehyde. acid. 



Acids are obtained from compound ethers by boiling them 

 with alkalies, when salts are formed, which may be decomposed 

 by sulphuric or other acids. For instance: 



C2 + KHO = ^O + CH.HO. 



Ethyl acetate. Potassium Potassium Ethyl alcohol. 



hydrate. acetate. 



2(C 2 H 3 K0 2 ) + H 2 S0 4 = : 2(C 2 H 4 2 ) + K 2 SO 4 . 



Potassium acetate. Sulphuric Acetic acid. Potassium 



acid. sulphate. 



Acids are also formed by destructive distillation (acetic acid); 

 by fermentation (lactic acid) ; by putrefaction (butyric acid) ; by 

 oxidation of many organic substances (formic acid by oxidation 

 of starch), etc. 



