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).

Answer 2

The question asks for the typical pK_a range of hypnotic and sedative barbiturates. Let's check each range against what is known about barbiturate acid-base behavior and the pharmacokinetic role this value plays.

1. 4-6: A pK_a this low would mean the drug is almost fully ionized at physiological pH (7.4). Barbiturates are weak acids, but not strong enough to sit fully ionized under normal blood pH, so this range is too acidic to describe them.
2. 5-8: This band overlaps the true range but starts too low. If the pK_a were near 5, the drug would again be mostly ionized in plasma, which does not match how barbiturates are known to distribute and cross the blood-brain barrier in their unionized form.
3. 9-11: This range is too basic. A pK_a of 9-11 belongs to weakly basic groups such as aliphatic amines, not to the acidic C5 proton of the barbituric acid ring that governs barbiturate ionization.
4. 7-9: This range sits close to physiological pH, which is exactly what allows barbiturates to exist as a mix of ionized and unionized species in the blood. Reported values for individual barbiturates (phenobarbital around 7.3, amobarbital and pentobarbital near 7.9-8.0) all fall inside this band.

Checking real drug values against each band confirms that hypnotic and sedative barbiturates cluster in the 7-9 pK_a range, since this is the range that keeps a workable unionized fraction available for CNS penetration.

Therefore, the correct answer is 7-9.