120 SPECTROSCOPIC INVESTIGATIONS 



are very similar to those of the analogous naphthoquinones, Fig. 13. 

 On the other hand, if we compare with 2-hydroxynaphthoquinone the 

 series: 



O O 



/OH 1 /OH 



O 



s 



o 







5-Hydroxybenzofuranquinone 



o 



5-Hydroxythianaphthenequinone 



.OH 



Se 







5-Hydroxyselenanaphthenequinone 



we have such a series of compounds with decreasing electron affinity 

 of the hetero atom. This leads to interesting changes in the absorption 

 spectra, Fig. 14. The bands marked with a, a', and a" appear to form 

 a group related to each other and to the short-wavelength bands of 

 the naphthoquinones. The same holds for the groups marked b and 

 c respectively which compare with the 330-m/x and long-wavelength 

 bands of the naphthoquinones. The less the electron affinity of the 

 hetero atom, the more "aromatic" is the character of the nucleus and 

 the more are the bands in the absorption spectra shifted toward 

 longer wavelengths. This holds least for the long-wavelength bands 

 but we will not go into the reason for this here. As is well known, 

 furan has only very weak aromatic properties, and this agrees with 

 the observation that the absorption spectrum of 5-hydroxybenzo- 

 furanquinone is very similar to that of dihydroxybenzoquinones. So 

 far we have not yet had the opportunity to investigate the absorption 

 spectra of the corresponding hydroquinones in detail, owing to their 

 unstability. There is no reason to expect a deviation from what has 

 been discussed above, however. 



