Sun21November0420PM 3

Barbiturates are derived from barbituric acid. Molecular substitutions at key positions of the barbituric ring confer specific pharmacodynamic and pharmacokinetic properties.

Which of the following substitutions of the barbituric ring most influences the lipid solubility of the molecule?

(Please select 1 option)

Methylation at position 1

Aromatic group at position 5 Incorrect answer selected

Sulphur atom at position 2 This is the correct answer

Length of carbon chain at position 5

Branched side chains at position 5

Explanation

Hypnotic activity is introduced into the barbituric acid molecule by the addition of side chains, especially if at least one of them is branched, in position 5.

The length of the side chains in the 5 position influences both the potency and the duration of action of the barbituric acid derivatives.

Replacing the oxygen atom with a sulphur atom at position 2 of an active barbiturate produces a barbiturate with a more rapid onset and a shorter duration of action; the thiobarbiturates.

Methylation of an active barbiturate in position 1 produces a drug such as methohexitone with not only a rapid onset and short duration of action but also an increased incidence of excitatory side effects.

The substitution by an aromatic or heterocyclic group at position 5 is often associated with anticonvulsant effects, that is, phenobarbitone.

Answer Statistics

1

10%

2

31%

3

36%

4

13%

5

12%

Times answered: 262