6. Pentothal (or thiopental) is a potent anesthetic that is administered intrave

Question

6. Pentothal (or thiopental) is a potent anesthetic that is administered intravenously prior to surgeries; it reaches the brain within one minute of injection and puts the patient to sleep. The structure of pentothal is given below:

The dissociable proton is on the sulfur, and this dissociation has a pKa of 7.6. If pentothal were administered orally instead of intravenously, would it be absorbed via the stomach (pH = 2)? Provide a quantitative explanation. (Hint: Ionized molecules cannot cross the plasma membrane of the cells of the stomach lining, but neutrally charged molecules can. Is pentothal ionized in the stomach?)

Explanation / Answer

Let the pentothal be represented as RSH since we are concerned with SH proton acidic nature.

The ionized form will be RS- when it loses H+

The combination of RSH and RS- is called a buffer ( a weak acid RSH and its conjugate base RS- make buffer)

pH of buffer = pka of acid + log [ conjugate base] / [ acid]

2 = 7.6 + log [ RS-] /[RSH]

[RS-]/[RSH] = 10 ^ ( 2-7.6) = 2.5 x 10^-6

Thus when we have pH = 2 conditions we will have RS- 2.5 x 10^-6 times that of RSH amount. This means very less amount of RSH is ionized. Hence very negligible amount is ionized and alsmost all RSH stays neutral

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