6 P-AMINOBENZOIC ACID 



This theory can be extended to account for the low absorption of the 

 hydrochloride of PABA, since in the salt form the amino group can no 

 longer resonate with the benzene nucleus or with any group on the ring, 

 so that only the resonance within the ring itself, or between the ring and 

 the carboxyl group can function in absorption. 



The high degree of absorption exhibited by the N-alkyl analogs of 

 PABA and their esters occurs at those wavelengths of light which are 

 responsible for sunburn. The investigation of these compounds as potential 

 sun-screening agents^""^^ has received considerable attention in recent years. 



C. CHEMICAL PROPERTIES 



PABx\ is decomposed by prolonged boiling with water to give aniline 

 and carbon dioxide."^ Concentrated hydriodic acid at elevated tempera- 

 tures degrades PABA to benzoic acid.^^ Potassium chlorate and hydro- 

 chloric acid convert the amino acid to chloranil.^ Mild oxidation of PABA 

 with sodium hypobromite yields p-azobenzoic acid.^^ 



In those reactions involving the amino or carboxyl group PABA exhibits 

 the behavior usually associated with arylamines or benzoic acid-type mole- 

 cules. Alkylation of the amine function has been carried out with methyl, 

 ethyl, and allyl iodides and a base^^ • ^^ as well as with dimethyl sulfate and 

 base.^^ By the proper choice of conditions the reaction can be controlled 

 to yield the mono- or dialkylated amine^^ or the trialkyl-p-benzobetaine.^'^ • ^^ 



PABA shows no exception to the ease with which aromatic amines and 

 phenols, which are not sterically hindered, can be brominated. Beilstein 

 and Geitner*'' used bromine water to produce a mixture of 3 , 5-dibromo-4- 

 aminobenzoic acid and 2,4,6-tribromoaniline. Francis and Hill,'*^ using a 

 mixture of potassium bromide and potassium bromate in acid medium to 

 generate bromine, found that at 0° formation of the dibromo compound is 

 favored, whereas above 40° the tribromoaniline is obtained. The use of 

 iodine monochloride in cold acid solution leads to the formation of 3-iodo- 



^" S. Rothman and J. Rubin, /. Invest. Dermatol. 5, 445 (1942). 



31 W. D. Kumler and T. C. Daniels, J. Am. Pharm. Assoc. Set. Ed. 37, 474 (1948). 



^^ A. C. Giese, E. Christensen, and J. Jeppson, J. Am. Pharm. Assoc. 39, 30 (1950). 



33 E. Christensen and A. C. Giese, J. Am. Pharm. Assoc. 39, 223 (1950). 



3* L. McMaster and R. L. Shriner, J. Am. Chem. Soc. 45, 751 (1923). 



35 A. Kwisda, Monatsh. 12, 419 (1891). 



36 W. Meigen and E. Nottebohm, Ber. 39, 744 (1906). 



37 A. Michael and J. F. Wing, Am. Chem. J. 7, 195 (1885-1886). 



38 M. Jaff^, Ber. 38, 1208 (1905). 



39 J. Johnston, Proc. Roy. Soc. (London) A78, 82 (1906). 

 «F. Beilstein and P. Geitner, Ann. 139, 1 (1866). 



« A. W. Francis and A. J. Hill, J. Am. Chem. Soc. 46, 2498 (1924). 



