10 THE ABSORPTION SPECTRA OF SOLUTIONS. 



The Theory of Chromophores. 



In considering absorption spectra it is often quite sufficient to speak qual- 

 itatively of the color of different compounds. The introduction of certain 

 groups into colorless compounds often results in a colored compound. Any- 

 such group is a chromophore. Sometimes the chromophore may be weak, and 

 it may require the addition of several chromophores to produce a colored com- 

 pound. Ultimately the color is due to absorbers existing in the chromophore, 

 probably the valency electrons. Among the better known chromophores are 

 the groups: 



>C = C< =CO >C = NH, CH = N N = N- 



>-N~N- \ .N- ,Q X> 



?C Nf N=0 Nf NC i 



O V> V) 



= N = =C = S C S 2 C . 



The ethylene group, >C = C<, is a weak chromophore, and several 

 chromophores must be present before the absorption is sufficient to produce 

 color. An example is that of benzene and its isomer : 



Benzene is colorless and exhibits absorption only in the ultra-violet, 

 whereas the isomeric compound is orange yellow. 



The carbonyl group, =C = O, is also weak, and several of the groups 

 must be present in a compound to produce color. The following examples 

 indicate how an increasing number of the carbonyl groups produces a greater 

 and greater absorption of the shorter wave-lengths: 



R CO R, colorless. 

 CH 3 CO CO CH 3 , yellow. 

 C 6 H 5 CO CO C 6 H 5 , yellow. 

 CH 3 CO CO CO CH 3 , orange. 

 C fl H 5 CO CO CO CO C 6 H 5 , red. 



The combination of two phenyl groups causes absorption only in the 

 ultra-violet. The introduction of the CH = N chromophore is sufficient 

 to produce color, as is shown in the yellow benzylidenaniline, C 6 H 5 CH = 

 N C 6 H 5 . The azochromophore, N = N , produces the same effect, as 

 shown in the orange azobenzene, C 6 H 5 N = N C 6 H 5 . The chromophores 



-^C N N and C Nf , on the other hand, are very weak. 



/ \/ / X) 





 The nitroso group, N = 0, is a very strong chromophore when it is 

 joined to a carbon atom directly. The effect of this chromophore is shown in 



