Urea is the diamide of carbonic acid : Recall that urea was the first compound to be synthesised in the laboratory (Wohler in 1828). It is the normal end product of protein metabolism and excreted along with urine. Under normal conditions, an adult man excretes about 30 gm of urea in urine in 24 hours.
Preparation
1. From urine : Urine is concentrated and treated with concentrated nitric acid when crystals of urea nitrate are obtained. The crystals are separated, dissolved in water and decomposed with barium carbonate. From the reaction mixture, urea is extracted with alcohol and the extract is finally evaporated to give crystals of urea.
\(\underset {Urea\,nitrate}{2CO(NH_2)_2.}NNO_3+BaCO_3 \longrightarrow\)
\(\underset {Urea}{2CO(NH_2)_2}+Ba(NO_3)_2+CO_2+H_2O\)
2. Wohler's synthesis : A solution of potassium cyanate* and ammonium sulphate is evaporated to dryness when ammonium cyanate is formed which then undergoes rearrangement to form urea.
3. Laboratory method : In laboratory, urea may be prepared by treating carbonyl chloride (phosgene) or ethyl carbonate with ammonia.
This reaction suggests the diamide structure to urea.
4. Manufacture :
(i) Liquid carbon dioxide and liquid ammonia are reacted in an autoclave to form ammonium carbamate. The latter is now heated at 130-150°C under a pressure of about 35 atmopsheres to give urea.
(ii) By the partial hydrolysis of cyanamide in feebly acidic solution.
Physical properties. Urea is a colourless, odourless solid (m.p. 132°C), readily soluble in water and alcohol, but insoluble in ether. Its aqueous solution is neutral to litmus.
Chemical properties. Look at the structural formula of urea and observe the presence of an amide and amino group. In practice, urea gives properties of both these functions.
Some of the important reactions are listed below :
1. Basic nature : Although an aqueous solution of urea is neutral to litmus, it behaves as a very weak monoacid base. Thus a concentrated solution of urea reacts with strong nitric acid or oxalic acid solution to form sparingly soluble urea nitrate and urea oxalate respectively.
Urea reacts with fuming H2SO4 (H2SO4 + SO3) to form sulphamic acid.
H2N.CO.NH2 + H2SO4 + SO3 → H2NSO3H + CO2
It is important to note that urea is weaker base than amines but stronger than ordinary amides. The strong basic character of amines is due to absence of resonance and hence easy availability of the lone pair of electrons on nitrogen for protonation. On the other hand, ordinary amides are weaker bases due to incomplete (poor) availability of the electron pair of nitrogen due to resonance.
Further, the basic (although weak) character of ordinary amides can also be explained in terms of the resonance stabilisation of the protonated amides, i.e., once the protonated amide is formed it is stabilised by resonance.
On the same ground i.e., stabilisation of protonated species, we can explain the stronger b nature of urea than ordinary amide i.e. the protonated urea undergoes resonance to a greater extent than ordinary amides and hence it is more stable and more basic than ordinary amides.
2. Hydrolysis : Urea is hydrolysed in the presence of acids, bases or enzyme urease (present in soil and soyabeans) forming carbon dioxide and ammonia.
Enzymatic hydrolysis of urea¹ is a quantitative reaction and hence used for estimating urea by measuring the amount of ammonia liberated.
3. Action of heat (Biuret reaction) : When heated, urea melts at 132°C. On further heating it loses ammonia at 150°C to form a solid white residue known as biuret.
\(\underset {Urea\,(2\,molecules)}{H_2N.CONH_2+HNH.CONH_2} \)
\(\xrightarrow {heat} \underset {Biuret}{H_2N.CONH.CO.NH_2}+NH_3\)
When an aqueous solution of biuret is treated with sodium hydroxide and a drop of very dilute copper sulphate solution, violet colour is produced. This reaction is used as a test under the name of biuret test for detecting the presence of peptide grouping-CONH- (cf. the peptide linkage in protein) in a given compound.
4. Reaction with alkaline solution of hypohalite.
H2N.CO.NH2 + 3NaQBr + 2NaOH → 2NaBr + Na2CO3 + N2 + 2H2O
This reaction is used for the clinical estimation of urea in the urine by measuring the volume of N2 evolved.
5. Reaction with formaldehyde. (Formation of urea formaldehyde resins).
Formaldehyde reacts with urea in presence of a base or acid to form methylol urea and dimethylol urea.
\(\underset{Formaldehyde}{HCHO +H_2N}.\underset{Urea}{CO.NH_2}\longrightarrow \underset {Methylol \,urea}{HOCH_2.NH.CO.NH_2}\)
\(HOCH_2.NH.CO.NH_2+HCHO \longrightarrow \)
\(\underset {Dimethylol\,urea}{HOCH_2.NH.CO.NH.}CH_2OH\)
Dimethylol urea formed above may undergo following three types of reactions.
(i) It may condense with more urea molecules to form linear polymers.
(ii) It may condense with more formaldehyde molecules to form cross-linked polymers.
(iii) Two molecules of dimethylol urea condense with each other to form linear polymers.
The complex mixture of linear and cross-linked condensation products is known as urea formaldehyde resin. Different types of resins may be produced by changing the conditions of reaction. Urea formaldehyde resins are of great industrial importance. These are used (a) for gluing and impregnating timber, (b) in the manufacture of paints, varnishes and lacquers, (c) in the manufacture of electrical fittings, telephones, toys, table toys and radio cabinets.
6. Reaction with hydrazine forms semicarbazide.
\(H_2NCONH_2+ H_2NCONH_2 \xrightarrow {100°C} \underset {Semicarbazide}{H_2NCONHNH_2}+NH_3\)
