Question

Urea is [CPMT $1984$]
A.Monoacidic base
B.Diacidic base
C.Neutral
D.Amphoteric

Answer 1

Hint: Urea is an organic compound with chemical formula $CO{{(N{{H}_{2}})}_{2}}$. It is the major nitrogen-containing agent in mammalian urine. The chemical properties such as acidity, basicity, or amphoteric can be understood when urea is dissolved in an aqueous solution. From the ${{P}^{H}}$value, we can understand whether urea is neutral or not. When the solution ${{P}^{H}}=7$ is called neutral.

Complete answer:In urea two $-N{{H}_{2}}$ groups are attached by a carbonyl group ($C=O$). It is a white crystalline solid soluble in aqueous solution and alcoholic solution such as ethanol but insoluble in ether. Under investigation, it is found that the $C-N$bond length $1.37\overset{o}{\mathop{A}}\,$but in aliphatic amines the $C-N$bond length $1.47\overset{o}{\mathop{A}}\,$. The difference in bond length value indicates that urea has some double bond character. This can be explained by the following resonance structures:


There are three resonance structures and both $C-N$bonds are identical and the negatively charged oxygen atom is capable of coordinating with one proton. Thus, urea acts as a monoacidic base.
This can be explained in another way when urea is dissolved in water, abstracts a proton from water, and leaves $H{{O}^{-}}$an ion in the solution. Thereby urea acts as a monoacidic base. A monoacidic base is a base that forms one $H{{O}^{-}}$ion when one molecule of it undergoes complete ionization. As it is a monobasic acid the
${{P}^{H}}$value is always greater than $7$, hence it is not a neutral compound.

Thus, option (A) is correct.

Note: Although urea is a monobasic acid, it is a weaker base than amines and a stronger base than amides The strong basicity of amines is due to the better availability of lone pairs on nitrogen for donation. In amide, the lower basicity is due to the poor availability of lone pair electrons as lone pairs on nitrogen participate in resonance.