130 POPULAR SCIENCE MONTHLY 



might be anticipated, gives an absorption spectrum resembling very 

 closely that of the mono-alkyl derivatives of benzol just mentioned. 



The absorption band of phenol, C H 5 OH, differs from that of the 

 mono-alkyl derivatives in that one pronounced band has replaced the 

 two prominent bands in the spectra of the latter. In the case of 

 anisol, C 6 H 5 'OCH 3 , the methyl derivative of phenol, known as an 

 ether, the two prominent bands are again in appearance. In other 

 words, the substituting group methoxyl (OCH 3 ) partakes more of the 

 nature of a saturated alkyl group, whereas the hydroxyl group (OH) 

 acts very differently. By a close examination of the two bands from 

 anisol and the one from phenol we see that the transmitted portion, 

 or that portion which serves to divide the one band into two, lies be- 

 tween the oscillation frequencies, 3,640-3,655. This is exactly the 

 region where the absorption bands due to keto-enol tautomerism make 

 their head. In other words, the presence of just such dynamical 

 isomerism as may be caused by the wandering of the labile hydrogen 

 atom of phenol will account for this absorption band and its position 

 in overlying the regular bands due to phenolic structure, as shown in 

 the case of anisol, etc. That a condition of dynamical isomerism is 

 really present in a free phenol is further proved by the shifting of the 

 absorption band to the left upon the addition of sodium hydroxide 

 to its solutions; a result always observed in keto-enol tautomerism. 

 Upon the bands formed by anisol the addition of alkali has no effect. 

 On the other hand, the addition of hydrochloric acid to a phenol re- 

 tards this tautomerism and when large excess of the acid is used the 

 transmitted portion of the spectrum or that which is due to the free 

 benzol nucleus begins again to make its appearance. The spectrum 

 observed in the case of nitrobenzol, C 6 H 5 • N0 2 , and other derivatives, 

 where the substituent possesses marked residual affinity (due here to 

 the oxygen atoms) shows only a general absorption. This condition, 

 therefore, is brought about when the active residual affinity of the new 

 groups restrains or locks up the free affinities of the benzol ring and 

 thus retards its internal motions. 



As with the mono-derivatives of benzol, so also with the disubsti- 

 tuted derivatives, the general rule holds true; wherever the substituents 

 are groups well saturated, they will exert scarcely any retarding action 

 upon the pulsations of the original molecule. The disubstituted de- 

 rivatives are classified as ortho, meta and para, according as the groups 

 are adjacent, once removed, or twice removed (diametrically across the 

 ring) respectively, from each other. The para compounds give a 

 spectrum more closely resembling that of the parent substance, benzol, 

 and hence may be said to be the more symmetrical arrangement, or 

 that which accords best with the even or symmetrical pulsations of the 

 benzol molecule. With the ortho- and meta-compounds we may say 

 that the unsymmetrical loading of the ring operates against the even 



