Question

In order to extract isobutyric acid from a solution of diethyl ether, one should wash the solution with:
A. one 30mL wash of water
B. three 10mL washes of water
C. one 30mL wash of hexane
D. three 10mL washes of hexane

Answer 1

To solve this question, let’s first understand the properties of carboxylic acids. In case of simple aliphatic carboxylic acids, with four carbon atoms it is miscible in water due to the formation of hydrogen bonds with water. Solubility decreases by increasing the number of carbon atoms due to the increased hydrophobic interaction of the hydrocarbon part. Also, carboxylic acids are also soluble in less polar organic solvents.

Complete step by step answer:
As we know, isobutyric acid has a carboxylic acid group, which is polar and can readily hydrogen bond. It can be extracted from the organic solvent diethyl ether by washing with a polar solution like water. Multiple small volume washes extract better than one large volume wash. Use of a second organic solvent, hexane, will not extract the isobutyric acid from the diethyl ether. Now, we will consider all the four options.
A. One 30mL wash of water is incorrect as multiple smaller volume washes will yield higher extraction results.
B. Three 10mL washes of water is correct (option B) as isobutyric acid will enter the aqueous layer and wash off of the organic layer of diethyl ether.
C. One 30mL wash of hexane is also incorrect because hexane is another organic solvent like diethyl ether.
D. Three 10mL washes of hexane is also incorrect.

So, the correct answer is Option B.

Note:
We need to know that isobutyric acid is also known as 2-methylpropanoic acid or isobutyric acid. It is a carboxylic acid with structural formula ${(C{H_3})_2}CHCOOH$. It is an isomer of n-butyric acid. By deprotonation or esterification iso butyrates are formed.