Alcoholic potash

In organic chemistry, alcoholic potassium is actually an antiquated term for a solution of potassium hydroxide in ethanol . Solutions in methanol were / are called "methyl alcoholic potash" or "methanolic potash".

properties

Potassium hydroxide dissolves in methanol and ethanol much more easily than sodium hydroxide and is therefore preferred over it. 40.3 grams per 100 ml (7.19 mol / liter) dissolve in methanol at 28 ° C. The solubility in ethanol at 28 ° C is 29.0 g / 100 ml (5.17 mol / liter).

The solutions conduct electricity and are "conductive". This proves that the potassium hydroxide is dissociated; When the solid potassium hydroxide is dissolved, the potassium ions are solvated by alcohol molecules and hydroxide ions are released. The latter are in an acid / base equilibrium with the alcohol:

${\displaystyle \mathrm {KOH_{(fest)}+ROH\rightleftharpoons \ K_{(ROH)}^{\,+}+HO^{\,-}} }$

${\displaystyle \mathrm {HO^{\,-}+ROH\rightleftharpoons \ H_{2}O+RO^{\,-}} }$

Alcoholic potash thus contains both hydroxide and alkoxide ions. The equilibrium can be changed by adding water. The commercially available 96 percent ethanol contains 4% water anyway. In practice, solid KOH is often dissolved in a little water (caution! Strong heat, protective goggles!), Allowed to cool and then the desired amount of alcohol is added.

Ethanolic potash soon turns yellow due to autoxidation when standing in air , which is not the case with methanolic potash. Therefore the latter is often preferred.

use

The strongly basic solutions are important reagents in organic analysis and synthesis. The alkoxide ions present in equilibrium are nucleophiles . Therefore, alcoholic potash is used in many nucleophilic substitution reactions. Carboxylic acid esters are " saponified ", i. H. converted into potassium carboxylate solutions if they are left to stand or heated with at least 1 equivalent of alcoholic potash. Triglycerides are broken down into glycerine and fatty acids (as potassium salts ), which is important for their analysis ( saponification number ).

Individual evidence

1. ^ Gmelin's Handbook of Inorganic Chemistry. 8th edition. Band potassium, system no. 22, Verlag Chemie, Berlin 1938, p. 236 ff.