ioo PRACTICAL ORGANIC AND BIO-CHEMISTRY 



HALOGEN SUBSTITUTION DERIVATIVES OF THE 

 FATTY ACIDS. 



The fatty acids behave like a saturated hydrocarbon towards the 

 halogens, especially chlorine and bromine, substitution of hydrogen 

 atoms in the chain of carbon atoms (not the COOH group) taking 

 place. The most typical compounds are mono-, di- and tri-chloracetic 

 acids. 



In the case of the higher fatty acids containing three and more 

 carbon atoms several isomers can be formed : 



CHg.CHCl.COOH CH 2 C1 . CH 2 . COOH 



a-chloropropion'c acid. -chloropropionic acid. 



These acids are distinguished by using the Greek letters, that carbon 

 atom next to the carboxyl group being called the a-carbon atom, the 

 next /8, the next 7, and so on. 



Preparation. 



The chloro-substituted fatty acids are prepared (a] by the action of 

 chlorine upon the fatty acid in direct sunlight, or in the presence of 

 iodine, or (b) by the action of halogen upon the acid chloride (p. 101), or 

 (c) by indirect methods from malonic ester. 



Monochloracetic Acid. 



Chlorine is passed into boiling acetic acid, to which a little sulphur 

 or iodine has been added : 



CH,COOH + IC1 3 = CH 2 C1 . COOH + IC1 + HC1 

 IC1 + C1 2 = IC1 3 . 



Monochloracetic acid is a colourless solid melting at 62 and boil- 

 ing at 185-187. It closely resembles acetic acid in its reactions. 



Dichloracetic Acid. 



Dichloracetic acid is usually prepared by heating chloral hydrate 

 with potassium cyanide or ferrocyanide : 



CC1 3 . CH(OH) 2 + KCN = CHCl a . COOH + HCN 4- KC1. 

 It is a liquid which boils at 190-191. 



Trichloracetic Acid. 



Trichloracetic acid is prepared by oxidising chloral with concen- 

 trated nitric acid : 



CC1 3 . CHO + O = CC1 3 . COOH. 



It is a colourless solid melting at 55 and boiling at 195. On boiling 

 with alkalies it is converted into chloroform and carbonate : 



CC1 3 . COOH + NaOH = CHC1 8 4- NaHCO 8 . 



It forms salts with bases and yields an acid chloride (p. 101) like acetic 

 acid. 



The acidity of these acids increases with the number of chlorine 

 atoms ; trichloracetic acid is a strong acid almost equal to mineral acids. 



