Question

Question: Now that you have learned about solubility, explain why aspirin (Section 2.7) crosses a cell membrane as a neutral carboxylic acid rather than an ionic conjugate base.

Short answer

Answer

To pass through the non-polar interior of the cell membrane, aspirin crosses the cell membrane in its neutral carboxylic acid form.

Step-by-step solution

Aspirin

Aspirin, also known as acetylsalicylic acid, remains in its neutral acidic form when surrounded by an acidic medium and converts to its ionic form in a basic medium.

Proton transfer reaction

Aspirin undergoes a proton transfer reaction in a basic medium to form its ionic form. In this reaction, the most acidic hydrogen of the aspirin is removed. The most acidic hydrogen in aspirin is the one that is directly connected to oxygen.

Conversion of the neutral acid form of aspirin to its ionic form in basic medium

Behavior of aspirin in the cell membrane

Traveling to the various targeted locations in the body, aspirin dissolves in the aqueous bloodstream and reaches the different body parts through blood flow. However, as the non-polar interior of the cell membrane is impermeable to ionic compounds, the ionic form (or deprotonated form) of aspirin must be re-protonated to pass through the non-polar interior of the cell membrane. Thus, aspirin converts to its neutral acid form to pass through the cell membrane at the target site.