652 



SCIENCE PROGRESS 



Chromophores 



The principal chromophoric groups are — C = O, carbonyl, 

 _ C = S, - C = N, nitrile, - N =N, azo, N 2 0, azoxy, N = O, 

 nitroso, and N0 2 , nitro, but other chromophores, some containing 

 sulphur and tellurium, are also known. The most important of 

 the groups just mentioned are the carbonyl, azo, and nitro 

 groups ; and it may be as well to describe them more closely. 



The Carbonyl group by itself does not produce colour, but a 

 second — C = O group in juxtaposition strengthens the action to 

 such an extent that coloured compounds almost invariably 

 result. In the /3 and 7 positions x this influence is not felt. Thus 

 we have — 



CH 3 -CO-CH 3 CH 3 



Diacetyl. N 



Yellow. 



CO -CIV CO 



Acetyl acetone. 



CH 3 CH S • CO ' (CH 2 ) 2 • CO • CH S 



Acetonyl acetone. 



Colourless. 



In the aromatic series many cases are known where a single 

 carbonyl group apparently acts as a chromogenic centre, but it 

 must be remembered that, accepting Kekule's formula, many 

 double bonds are present, and these might well act as additional 

 " chromophores." Thus in fluorenone, besides the CO group 

 we have a distribution of double bonds similar to that in 

 fulvene. 



c = o 



c< 



CI 

 CI 



Coloured. Colourless. 



It is noteworthy that on replacing the carbonyl oxygen with two 

 chlorine atoms the chromophore is destroyed. Fluorenone 

 dichloride is colourless. 2 



In both the aliphatic and aromatic series it has invariably 

 been found that compounds containing the azo group, — N = N — 

 are highly coloured. The simplest members of the series, 



1 Reckoned from the first carbonyl, e.g. — 



a /3 V 



CH 3 • CO • CH 2 • CH, • CO • CH 3 . 



2 Smedley, Journ. Chem. Soc, 1905, 1249. 



