COLOR AND CHEMICAL CONSTITUTION 131 



pulsations and tends to produce irregular vibrations which give rise 

 to less distinctive absorption bands. 



In aliphatic, as well as in aromatic, compounds, we often observe 

 that a certain amount of residual affinity lurking in the oxygen atoms 

 can exert a strong influence upon an entire group of atoms. One of 

 the simplest and most reactive combinations in which oxygen may be 

 found is that known as the carbonyl group (CO), which, when 

 occurring between two carbon radicals, constitutes a ketone as we have 

 already noted. The simplest ketone is acetone, CH 3 — CO — CH 3 . 

 The additive capacity of this carbonyl group for various reagents is 

 well known, but this capacity very often decreases in power with an 

 increase of the molecular aggregation in the near vicinity. For ex- 

 ample, the additive capacity of the carbonyl group in the compound 

 methyl-ethyl ketone, CH 3 — CO — C 2 H 5 , is usually less than that in 

 acetone. These and similar facts have been explained upon the 

 hypothesis of " steric hindrance " for lack of a better phrase. Though 

 at times this hypothesis may best explain some of the intricate prob- 

 lems, still it hardly dare be supposed that the paths of intra-molecular 

 vibration of the atoms is other than large in comparison with the size 

 of the atoms themselves; consequently, slight increase in the mass of 

 the substituents should have no appreciable effect upon the activity of 

 a neighboring group. Oftentimes it was found that very large sub- 

 stituents increased the additive capacity of a carbonyl group. Thus 

 when one of the hydrogen atoms of acetone is replaced by a carbethoxyl 

 group (COOC 2 H 5 ), a group formed by the replacement with ethyl 

 of the hydrogen atom in the regular organic acid group, carboxyl 

 (COOH), we get a great increase in the activity of the original 

 carbon group. The compound so formed would have the formula, 

 CH 3 — CO — CH 2 — COOC,H 5 , i. e., ethyl aceto-acetate — the very 

 same compound as. was studied with reference to keto-enol tautomerism. 

 An explanation of the increased activity in this case from the stand- 

 point of dynamical isomerism which may be present seems to be most 

 adequate. The oxygen atom exists temporarily in the enolic (OH) 

 stage and the hydrogen atom, at the moment of departing, must leave 

 the oxygen atom and consequently the carbonyl group nascent, i. e., in 

 an exceedingly active form, similar here, no doubt, to the state acquired 

 by ionization in solution. Again the hydrogen atom itself at the 

 moment of separation would be most susceptible also to chemical action. 



In order to get an idea of the relation of this carbonyl group to 

 the carboxyl group, one of the simplest compounds which exhibits this 

 arrangement was studied. The example taken was the ethyl ester of 

 pyruvic acid, CH 3 — CO — COOC 2 H 5 . Here there was observed an 

 absorption band lying much nearer the red end of the spectrum than 

 that obtained in the case of ethyl aceto-acetate. The band had a head 

 at about the oscillation frequency 3,100, whereas the band of the latter 



