Barbital is a barbiturate sedative-hypnotic.
|S. NO.||PHYSICAL AND CHEMICAL PROPERTIES|
|1||Molecular weight||184.19 g/mol|
|2||Physical appearance||White crystalline powder|
|4||Octanol/water partition coefficient||0.65|
|5||Solubility||Water solubility is 0.04M|
|6||Presence of ring||Pyrimidine|
|7||Number of chiral centers||Not present|
Mechanism of Action
i. Barbital potentiates GABA receptor subunits α1, α2, α3, α4, α5 and α6.
ii. Barbital acts as an antagonist for neuronal acetylcholine receptor subunit α4 and α7; glutamate receptor 2 ; glutamate receptor ionotropic, kainate 2.
Structure Activity Relationship
- Tri-keto form is most stable in aqueous solution.
- 4,6-dialcoholic tautomeric forms are least stable in aqueous solution.
- 5,5-disubstituted barbituric acid is the prime requirement for the barbiturates to be sedative hypnotics.
- Esterification of either of the 1,3-diazine nitrogens decreases hypnotic activity.
- Substitution of either of the 1,3-diazine nitrogens with aliphatic carbons retains the anticonvulsive properties.
- Esterification of the 5th-position substituents yields agents with analgesic activity but with weak hypnotic properties.
- Introduction of the polar functional group at the 5th– position yields compounds which are fully devoid of sedative-hypnotic or anticonvulsive activity.
- As the number of carbons at R2 carbon increases, the lipophillicity of the drug increases.
- Modification of the 2nd-position oxygen of the barbiturate backbone with sulfur atom yields thiobarbiturate derivatives with increased lipophillicity, shorter duration of action, faster time of onset compared to oxy-derivative. 
Method of synthesis
Barbital can be synthesized by the condensation of diethylmalonic ester with urea in presence of sodium ethoxide.
Barbital is used for:
- Central nervous system depression
Side effects of barbital are:
- Respiratory problems
- Lack of coordination
- Memory weakness
Q.1 Barbital potentiates which receptor?
a) GABA receptor
b) Neuronal acetylcholine receptor
c) Glutamate receptor
d) Kainate 2
Q.2 Therapeutic use of drug barbital is/are?
a) CNS depression
b) Cancer treatment
d) All of the above
Q.3 Which amongst the following are the correct statements with respect to the SAR of drug Barbital?
I. Substitution of either of the 1,3-diazine nitrogens with aliphatic carbons retains the anticonvulsive properties.
II. Esterification of the 5th-position substituents yields agents with analgesic activity and strong hypnotic properties.
III. Introduction of the polar functional group at the 5th– position yields compounds which are fully devoid of sedative-hypnotic or anticonvulsive activity.
IV. As the number of carbons at R2 carbon increases, the lipophillicity of the drug decreases.
a) I, III
b) II, IV
c) I, II
d) III, IV
Q.4 The starting chemicals required for the synthesis of drug Barbital?
a) Benzyl chloride
c) Diethylmalonic ester
d) None of the above
Q.5 Correct sequence for the True/False for the physiochemical properties of the drug barbital is?
I. Molecular weight is 184. 19 g/mol
II. It is present in Red amorphous solid form
III. Melting point is 100oC
IV. 3 chiral carbons are present in the structure of barbital.
Q.6 Correct statements for the IUPAC nomenclatures of the drugs are?
I. Barbital: 5,5-diethylpyrimidine-2,4,6(1H,3H,5H)-trione.
II. Phenobarbital: 5-ethyl-5-(3-methylbutyl)-1,3-diazinane-2,4,6-trione
III. Butabarbital: 5-butan-2-yl-5-ethyl-1,3-diazinane-2,4,6-trione
IV. Amobarbital: 5-Ethyl-5-phenyl-1,3-diazinane-2,4,6-trione
a) II, III
b) I, IV
c) I, II, IV
d) I, III
Q.7 Match the following drugs with their correct classifications-
|i. Barbital||A. Barbiturate sedative-hypnotic|
|ii. Apraclonidine||B. Mixed-acting sympathomimetics|
|iii. Tolazoline||C. Nonselective adrenergic antagonist|
|iv. Methamphetamine||D. Selective α2-adrenergic agonist|
a) i-A, ii-B, iii-C, iv-D
b) i-B, ii-D, iii-A, iv-C
c) i-A, ii-D, iii-C, iv-B
d) i-C, ii-A, iii-B, iv-D
REFERENCES Lemke TL, Zito SW, Roche VF, Williams DA. Essentials of Foye’s principles of medicinal chemistry. Wolters Kluwer; 2017, 490-91