EMIL H. WHITE 185 



to be as clliciciu as kiminol (10), whereas the N-methyl and the 

 O-niethyl analogs (IV and V) with blocked hydrazide rings were not 

 cheniiluniinescent (7) . Two isomers ol limiinol, VI with a five- 

 nienibered ring (15) and VII with exchanged nitrogen and carbonyl 

 groups (17), were also not chemiluminescent. Maleic hydrazide 

 (\^III) and the linear hydrazides (IX) proved to be chemiluminescent, 

 but only weakly so. In general, therefore, a diacylhydrazine bearing 

 two hydrogen atoms (insining the ready formation of nitrogen) is re- 

 quired in the heterocyclic ring, and for efficient chemiluminescence 

 this system should be attached to an aromatic ring (presumably to in- 

 crease the efficiency of the light emission) . Although a few hydrazides 

 have been prepared in which this attached ring is heterocyclic (X and 

 XI; both only weakly chemiluminescent) (31) or naphthalenic (XII; 

 almost as efficient as luminol) (9) , most of the cyclic hydrazides 

 that have been synthesized correspond in type to general formulas 

 XIII and XIV. 



Two generalizations correlating structure and efficiency of chemi- 

 luminescence have been made by Drew (10). He noted that the 

 5-substituted phthalazinediones (XIV) were superior to the 6-substi- 

 tuted isomers (XIII) , and that within each group the chemilumines- 

 cence efficienciy was related to the ability of the substituent R to release 

 electrons into the conjugated system. Selected compounds illustrating 

 these conclusions are given in Table I. The increase in light emission 

 caused by the halogens, the relative order of the halogens, and the 

 effect of the nitro group suggest that the resonance effect, not the 

 inductive effect, of the substituents is involved (Fig. 3) . The same 



:Br^ < i=0-^ 



Fig. 3. 



order of substituents has been observed for organic reactions in which 

 the substituent is called upon to stabilize a partial positive charge in 

 the transition state (3) . The generalization is too simple to account 

 for the effect of all of the substituents, however, since the order of the 

 5-methyl and the 6,7-dichloro derivatives is anomalous. 



^Vhatever the detailed explanation for the substituent effect in 

 chemiluminescence may be, it is significant that a similar effect has 

 been found for the fluorescence of these compounds (23) as well as 

 for the fluorescence of the corresponding phthalic acids. This ob- 

 servation strongly suggests that the effect of the substituent is exerted 



