6. Pentothal (or thiopental) is a potent anesthetic that is administered intrave
ID: 1077182 • Letter: 6
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