II. CIIKMISTKY 3 



monia,' or sodium hydrosulfide.^ Catalytic hydrogenation^ has been carried 

 out with platinum oxide as catalyst. More recently, Kavenscroft and his 

 associates" employed electrolytic reduction of the nitro compound to pre- 

 pare P.VBA in very high yields. 



The oxidation of p-acetotoluide offers an alternative route for the prepa- 

 ration of PABA. Potassium^ or calcium^ permanganate has been used for 

 this purpose. Hydrolysis of the p-acetamidobenzoic acid with mineral acid 

 yields the amino compound. 



Of special interest are the syntheses leading to isotope-labeled PABA. 

 Murray and his collaborators' ° have described a reaction whereby C'^ is 

 introduced in the carboxyl group: 



1. C»02 



2. H2O 



> 



H^N— ^~^— Br _!5:CiHLLU [(Li).,N— <^ ^"^Q 



H2N— <. ^— Ci^OOH 



PABA labeled with N'" has been synthesized by Lustig et al}^ by heating 

 a solution of p-bromobenzoic acid in a sealed tube with N'^-labeled am- 

 monia. 



Br— <^^ ^— COOH _5^^ H2Ni5</ \— COOH 



B. PHYSICAL PROPERTIES 



PABA crystallizes in the form of monoclinic prisms which have a melting 

 point of 186 to \K1°}- ^- '^ Andrews and his coworkers''* reported a melting 

 point of 188 to 188.5°. The molal heat capacity of the solid was determined 

 to be 42.0 calories at 25° and 62.2 calories at the melting point, with a heat 

 of crystallization of 5000 calories per mole.''^ 



PABA is soluble in water, aqueous alcohol, methanol, isopropanol, bu- 

 tanol, ether, glacial acetic acid, chloroform, and ethyl acetate; sparingly 



' W. L. Lewis and H. C. Cheetham, J. Am. Cheni. Soc. 43, 2117 (1921). 



* J. Hirata, Japanese Pat. 109,708 (Feb. 21, 1935) \C.A. 29, 4776 (1935)]. 



« R. Adams, F. L. Cohen, and O. W. Rees, /. Am. Chem. Soc. 49, 1093 (1927). 



^ P. H. Ravenscroft, R. W. Lesis, and O. W. Brown, Trans. Electrochcm. Soc. 84, 

 145 (1943). 



8.\. W. Hofniaiin, Bcr. 9, 1299 (1876). 



9 F. inimann and J. B. Uzbachian, Ber. 36, 1797 (1903). 

 '« \. Murray, 111, W. W. Foreman, ,tii«l W. L;uif>;ham, ./. .\iii. Chcni. Soc. 70, 1037 



(1948). 

 " 1^. Lustier, A. R. (ioldfarb, and G. Gersti, Arch. Hiochcm. 5, 59 (1944). 

 '2 C. L. Lazzell and J. Johnston, J. Phijs. Chem. 32, 1331 (1928). 

 '3 D. H. Andrews, G. Lynn, and J. Johnston, J. Am. Chem. Soc. 48, 1274 (1926). 



