Question

Lab: Extraction and Isolation of Caffeine and Aspirin

Explain, using polarity and intermolecular forces, why you expect that aspirin and the caffeine will dissolve in methylene chloride, but acetaminophen will not.

Answer 1

Aspirin has two functional groups attached to the aromatic ring: an ester group and a carboxylic acid group. It is an acidic compound and dissolves in basic solutions (in which aspirin is converted into a water-soluble salt) but not in neutral or acidic aqueous solutions. It is also soluble in some organic solvents such as dichloromethane (CH2Cl2).
On the other hand, acetaminophen is a phenol with an acetamido group attached to the aromatic ring. It is a very weak acid and dissolves in strongly alkaline solutions but not in mildly basic, neutral, or acidic aqueous solutions. In addition, it is
insoluble in dichloromethane.
Contrary to the above two compounds, caffeine is polar and soluble in aqueous solutions. Being a base (with a lone pair available at one of the nitrogen atoms), it is highly soluble in strong acids because caffeine can be protonated and converted to a water-soluble salt. Like aspirin, caffeine is also soluble in dichloromethane.
They can be easily separated by making use of their different solubility in different solvents. For example, aspirin and caffeine are soluble in dichloromethane while acetaminophen does not.