ON ABSOKPTION SPECTRA OF ORGANIC COMPOUNDS. 



223 



carbostyril^ QH^v 



.CH = CH 



and o-oxycarbanil' C,H4' 



NH— CO 







NH' 



\ 



CO, was determined 



by comparison of their absorption curves with those of their nitrogen and oxygen 

 methyl derivatives. 



It may readily be understood that high hopes were engendered that this 

 method might prove to be of immense value to the chemist as independent evi- 

 dence in the determination of the constitution of compounds, but it may be 

 said at once that these high hopes have not been realised. A very brief account 

 may be given of the various attempts that have been made to co-ordinate consti- 

 tution and absorption of light, because all of these attempts have some importance 

 in relation to more recent developments. Following on Hartley's successful 

 work an attempt was made to determine the constitution of ethyl acetoacetate 

 and its metallic derivatives by comparison with its two ethyl derivatives, ethyl 

 /3-ethoxycrotonate and ethyl ethylacetoacetate.* It was found, however, that 

 the parent ester and its metallic derivatives differ in absorptive power very 

 materially from the two isomeric ethyl derivatives. The two latter do not show 

 selective absorption, whilst the metallic derivatives show well-marked absorption 

 bands. The deduction was made from this that the origin of the absorption 

 bands is to be found not in any specific structure but in a tautomeric equilibrium 

 between the two forms, that is to say, the selective absorption of light is due to 



O OM 



II I 



the change of linking involved in the process — C — CHM— ;^— G = CH — , where 



M stands for hydrogen or a metal. 



This theory was extended to aromatic compounds where the selective absorp- 

 tion was considered to be due to the oscillation of linking supposed to be present 

 in the benzene ring. The absence of selective absorption observed with some 

 benzenoid compounds was considered to be due to the restraint on the oscillation 

 exercised by certain strongly electro-negative substituent groups such as N0„, 



&c.= ■ u • 



Without question one of the most important theories connoting absorption and 

 structure is that known as the quinonoid theory which connected visible colour 

 with a structure analogous to that of either para- or or</io-benzoquinone. This 

 theory has found great favour on account of the undoubted fact that when a 

 quinonoid structure is possible the substance in the majority of cases is visibly 

 coloiired, whilst in the case of an isomeric substance in which a quinonoid struc- 

 ture is not possible the colour is in general less intense or indeed very slight. 

 It was a simple matter to apply the oscillation theory in explaining the visible 

 • colour of the quinonoid compounds. The oscillation was suggested as that 

 between the two forms 



O 







I 



o 



< 



o 



Similarly the visible colour of the a-diketones was explained by the oscillation 



O O 0-0 



I 



* — C = C - , which after all is only a slight varia- 

 This particular type of oscillating linking was 



between the two forms — C — C — 

 tion of the quinonoid conception, 

 named isorropesis.' 



It was soon pointed out, however, that this theory was open to serious 

 objection because certain compounds in which no oscillation seemed possible 



