Question

Hypnotics and sedative barbiturate pK$_a$ value

• A. 4–6
• B. 5–8
• C. 9–11
• D. 7–9

Hint:

Barbiturate activity is influenced by lipid solubility and ionization, which are pH-dependent due to their pK$_a$.

Answer 1

The Correct Option is D

- Barbiturates are derivatives of barbituric acid and act as central nervous system depressants.
- They are weak acids due to the presence of acidic hydrogen at the 5-position of the barbituric acid ring.
- The pK$_a$ value of hypnotic and sedative barbiturates generally lies in the range of 7–9, making them weak acids that are mostly unionized at acidic pH and ionized at basic pH.
- This property affects their solubility and ability to cross the blood-brain barrier, thus influencing onset and duration of action.
- Examples include phenobarbital (pK$_a$ ~7.3) and pentobarbital (pK$_a$ ~8.1).