Aspirin is a weak acld with a pKa of 3.5. the ionizable proton is highlighted in

Question

Aspirin is a weak acld with a pKa of 3.5. the ionizable proton is highlighted in red In the figure. Aspirin is absorbed into the blood through the cells lining the stomach and the small intestine. Absorption requires assage through the plasma membrane, the rate at which is determined by the polarity of the molecule. harged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly. What fraction of aspirin is uncharged (protonated) in the stomach (pH 1.5)? Number 0.01 CH, O at fraction of asprin is uncharged protonated) in the small intestine (pH 6)? Number 316 Aspirin is more efficiently absorbed in the small intestine at pH Oform predominates. 6, because the deprotonated n in the stomach at pH 1.5, because the deprotonated form predominates. in the small intestine at pH 6, because the protonated form Previous NextSave And

Explanation / Answer

Aspirin is a weak acid (pka=3,4), so in the acidic conditions of the stomach (pH=1,4) very little of it is ionized in the stomach after oral administration, so the undissociated form (more lipophilic and easier to absorb) prevails. With a pKa of 3.5, aspirin is in its protonated (neutral) form at pH below 2.5. At higher pH, it becomes increasingly deprotonated (anionic). Thus, aspirin is better absorbed in the more acidic environment of the stomach,as absorption requires passage through the plasma membrane, the rate of which is determined by the polarity of the molecule: charged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly.

Very little of it is ionized in the stomach after oral administration as using the Henderson-Hasselbach equation we obtain: [A-]/[AH] = 10(pH - pKa) = 10(1.5 - 3.5) = 10(-2) = 1/100 = 0.01

fraction of aspirin uncharged in small intestine at pH 6 = [A-]/[AH] = 10(pH - pKa) = 10(6 - 3.5) = 10(2.5)  

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