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, C6H5OH, 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, C6H5 • OCH3, the methyl derivative of phenol, known as an ether, the two prominent bands are again in appearance. In other words, the substituting group methoxyl (OCH3) 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 between 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 retards 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, C6H5 • NO2, 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 disubstituted 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 derivatives 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