CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 1509 



Attempts were made to substitute allylthiourea for phenylhydrazine 

 as reductaiit in chlorophyll-sensitized reduction of methyl red, using ace- 

 tone as solvent ; but no bleaching of methyl red was noted under these con- 

 ditions. 



To check w^hether phenylhydrazine formed a complex with chlorophyll, 

 the absorption spectra of chlorophyll a and h were measured in the presence 

 of 0.05 M phenylhydrazine (in methanol). No change was noted in the 

 spectrum of chlorophyll a; in chlorophyll h, the red band was somewhat 

 broadened toward the longer waves. (C/. Watson's data, given below and 

 in chapter 37C.) 



The views on the mechanism of the phenylhydrazine-methyl red reac- 

 tion expressed in this paper were changed by Livingston and Pariser 

 (1948), who studied more closely the dependence of the quantum yield 

 on the concentrations of the reactants (phenylhydrazine and methyl red). 

 Methyl red is known to occur in three colored forms, depending on acidity; 

 it was concluded that only one of them — the intermediate one — takes di- 

 rect part in the reaction. To determine the concentration of this form, ex- 

 tinction curves were obtained for methyl red solutions in methanol (2 X 

 10-* M, containing from 0.4 to 10"* M HCl; and from 10 -« to 10 -» M 

 NaOCHs). These curves were interpreted as due to the superposition, with 

 varying relative intensities, of three absorption bands, with peaks at 406 

 (I), 491 (II), and 521 (III) van, respectively. The forms giving the bands 

 (I) and (III), can be obtained in the pure state in strongly alkaline or 

 strongly acid solution, respectively; but the intermediate form, which 

 gives band (II), usually is present only in mixture with one of the other 

 two forms. This intermediate form was interpreted by Thiel as a "zwit- 

 terion": 



-OOC-C6H4 - N+=N— C6H4N(CH3)2 

 H 



However, this formula, with two opposite charges, may be correct for aque- 

 ous solutions only; in methanolic solution, Livingston and Pariser con- 

 sidered more likely tne existence of a neutral molecule, with a hydrogen 

 bond between the hydroxyl and nitrogen : 









H 



<^ \>_N=N-<^ ^(CH3),N 



The absorption peaks of the acidic form (III) are situated not too far 

 apart in aqueous and methanolic solution (517 and 521 m/i, respectively) ; 



