MONOBASIC FA TTY ACIDS. 323 



In a similar manner, as there are homologous series of alcohols 

 corresponding to the various series of hydrocarbons, there are also 

 homologous series of organic acids running parallel with the corre- 

 sponding 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 decomposition 

 of organic matters 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 OH + O = C 2 H 3 OH -f H 2 O. 

 Ethyl alcohol. Acetic aldehyde. 



C 2 H 3 OH -f O = C 2 H 3 O.OH. 

 Acetic aldehyde. Acetic acid. 



Acids are obtained from compound ethers by boiling them with 

 alkalies, when salts are formed, which may be decomposed by sul- 

 phuric or other acids. For instance : 



KOH = C2 



Ethyl acetate. Potassium Potassium Ethyl alcohol. 

 hydroxide. acetate. 



2C 2 H 3 KO 8 -f H 2 SO 4 = 2C 2 H 4 O 2 -f K 2 SO 4 . 

 Potassium Sulphuric Acetic acid. Potassium 

 acetate. acid. sulphate. 



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

 fermentation (lactic acid) , by putrefaction (butyric acid) ; by oxida- 

 tion of many organic substances (oxalic acid by oxidation of 

 starch), etc. 



Properties. Organic acids show the characteristics mentioned of 

 inorganic acids, viz., when soluble, have an acid or sour taste, redden 

 litmus, and contain hydrogen replaceable by metals, with the forma- 

 tion of salts. 



Most organic acids, and especially the higher members, show these 

 acid properties in a less marked degree than inorganic acids ; in fact, 



