ALDEHYDES
When alcohols are treated with certain oxidizing agents two hydrogen atoms are removed from each molecule of the alcohol. The resulting compounds are known as aldehydes. The relation of the aldehydes derived from methyl and ethyl alcohol to the alcohols themselves may be shown as follows:
| Alcohols | {CH3OH | Corresponding aldehydes | {CH2O |
| {C2H5OH | {C2H4O |
The first of these (CH2O) is a gas known as formaldehyde. Its aqueous solution is largely used as an antiseptic and disinfectant under the name of formalin. Acetaldehyde (C2H4O) is a liquid boiling at 21°.
ACIDS
Like the other classes of organic compounds, the organic acids may be arranged in homologous series. One of the most important of these series is the fatty-acid series, the name having been given to it because the derivatives of certain of its members are constituents of the fats. Some of the most important members of the series are given in the following table. They are all monobasic, and this fact is expressed in the formulas by separating the replaceable hydrogen atom from the rest of the molecule:
| H·CHO2 | formic acid, a liquid boiling at 100°. |
| H·C2H3O | acetic acid, a liquid boiling at 118°. |
| H·C3H5O2 | propionic acid, a liquid boiling at 140°. |
| H·C4H7O2 | butyric acid, a liquid boiling at 163°. |
| H·C16H31O2 | palmitic acid, a solid melting at 62°. |
| H·C18H35O2 | stearic acid, a solid melting at 69°. |
Formic acid (H·CHO2). The name "formic" is derived from the Latin formica, signifying ant. This name was given to the acid because it was formerly obtained from a certain kind of ants. It is a colorless liquid and occurs in many plants such as the stinging nettles. The inflammation caused by the sting of the bee is due to formic acid.
Acetic acid (H·C2H3O2). Acetic acid is the acid present in vinegar, the sour taste being due to it. It can be prepared by either of the following methods.
1. Acetic fermentation. This consists in the change of alcohol into acetic acid through the agency of a minute organism commonly called mother of vinegar. The change is represented by the following equation: