COLOR AND CHEMICAL CONSTITUTION 133 



upon dilution of its solutions, a fact confirming the spectroscopic evi- 

 dence of oscillations due to benzol structure in solutions of small con- 

 centration. The presence of isorropesis, here brought about by two 

 carbonyl groups in juxtaposition and indicated, as we have seen, by 

 vibratory frequencies in the ether of longer wave-lengths than those 

 due to keto-enol tautomerism, stands out at once as the source of color 

 in chemical compounds. Again we note that the more pronounced this 

 isorropesis the more active chemically are the groups undergoing the 

 disturbance. The additive capacity of benzil is markedly less than 

 that of diacetyl. 



Among the compounds which furnish us with examples of this 

 nature, or substances in which two carbonyl groups can come under 

 the influence of each other, we may mention the most important of 

 all, that of para-benzoquinone. This quinone is a derivative of benzol 

 in which two oxygen atoms are located in the para-position to each 

 other. In order to satisfy the bivalence of the oxygen atoms, the para 

 carbon atoms are regarded as having their free affinities absorbed in 

 these oxygen atoms, leaving the remaining carbon atoms to arrange 

 their free affinities in two pairs of double linkings (according to the 

 Kekule hypothesis). It may be, however, that the second affinity of 

 each oxygen atom will assert itself in a linking between these two 

 atoms and therefore leave the characteristic benzol nucleus undis- 

 turbed. The two forms may be graphically shown as follows: 



o 



II 



c 



/\ 



HC CH 

 HC CH 



C 



O 



Now it has been proved chemically that para-benzoquinone can 

 exist in each of these two forms. We have then just such an example 

 of making-and-breaking as has been indicated in isorropesis, but in 

 addition a change in the manner of linkings in the molecule accom- 

 panies this process. The absorption spectrum of para-quinone gives a 

 band with its head at the oscillation frequency 2,150, one almost iden- 

 tical with that obtained from other ring compounds, e. g., camphor- 

 quinone, where two carbonyl groups are adjacent. From a study of the 

 pulsations of the benzol ring the para-positions have been shown to 

 be closely related, and in fact so well brought under the influence 

 of each other as to be considered as practically adjacent. Since the 

 position of the absorption band is in the blue region color must, of 

 course, be present in this substance. Simple quinones usually show a 

 yellow to an orange color. This color is undoubtedly due to isorropesis 

 and whenever we have this class of substances — known as quinones — 



