Open main menu

Page:Popular Science Monthly Volume 72.djvu/135

This page has been validated.

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, CH3—CO—CH3. 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 example, the additive capacity of the carbonyl group in the compound methyl-ethyl ketone, CH3—CO—C2H5, 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 problems, 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 substituents increased the additive capacity of a carbonyl group. Thus when one of the hydrogen atoms of acetone is replaced by a carbethoxyl group (COOC2H5), 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, CH3—CO—CH2—COOC2H5, 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 standpoint 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, CH3—CO—COOC2H5. 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