Urea

urea-

source : ch.ic.ac.uk

Urea or NH₂CONH₂

Urea is di amide of carbonic acid therefore, it is also known as carbamide.

Methods of Preparation:

Laboratory Preparation-

a) Wohler Synthesis-

When potassium cyanate reacts with ammonium sulphate then Ammonium isocyanate is formed which on intermolecular rearrangement gives urea.

2KCNO + (NH₄)₂SO₄ —->2NH₄CNO +H₂SO₄

b) From phosgene or Ethyl Carbonate:

When carbonyl chloride or ethyl carbonate reacts with NH₃ , urea is formed.

Manufacture-

a) From  CaC₂ or Partial hydrolysis of CaCN₂–

When CaC₂ reacts with N₂ at 800 ⁰ C , Calcium cyanamide is formed which on hydrolysis with dil. H₂SO₄ gives urea.

b) From  CO₂ & NH₃–

When NH₃ & CO₂ are heated at 150-200 ⁰ C & high pressure , ammonium Carbamate is formed, which on decomposition gives urea.

Physical Properties :

1. It is colourless, odourless, crystalline solid
2. Its melting point is 132⁰ C.
3. It is soluble in water due to formation of  intermolecular  hydrogen bonding with water

Chemical Properties :

1. Basic Nature –

It is mono acidic base but its aqueous solution is neutral. It forms salt with strong acid.

NH₂CONH₂ +HNO₃ —–> NH₂CONH₂.HNO₃

(urea nitrate)

2NH₂CONH₂ +H₂C₂O₄ ——-> (NH₂CONH₂)₂.H₂C₂O₄

(urea oxalate)

2. Hydrolysis-

Urea on hydrolysis gives NH₃ & CO₂.

a) On hydrolysis with acid,  NH₄Cl & CO₂ is formed

b) on hydrolysis with base, NH₃ & Na₂ CO₃ is formed.

c) on hydrolysis in presence of urease enzyme, NH₃ & CO₂ is formed.

3. Effect of heat-

a) On heating at 132⁰ C- urea melts & on further slow heating, NH₃ is evolved & a solid residue biuret is formed.

b) When urea is heated rapidly above 170⁰ C-

cyanic acid is formed which on polymerisation gives cyanuric acid.

3HNCO—->(HNCO)₃

Cyanuric acid

4. Reaction with alkaline Sodium hypobromite-

we get N₂ gas

NH₂CONH₂ +NaOBr + NaOH —->N₂ +Na₂CO₃ +NaBr +H₂O

5. Reaction  with NaNO₂ + HCl or HNO₂–  We get   N₂ & CO₂ .

NH₂CONH₂ + 2 HNO₂ —–>2N₂ +3H₂O + 2CO₂

6. Reaction with Acetyl Chloride / Acetic Anhydride  –

We get Acetyl urea ,  which is also known as ureides.

NH₂CONH₂ +CH₃COCl —-> NH₂CONHCOCH₃ +HCl

NH₂CONH₂ +(CH₃CO)₂O —-> NH₂CONHCOCH₃ + CH₃COOH

7. Reaction  with Oxalic Acid-

In presence of phosphoryl Chloride, Oxalyl urea is formed.

8. Reaction  with ethyl alcohol-

-As a result of reaction we get ethyl carbamate or urethane

NH₂CONH₂ +C₂H₅OH —–> NH₂COOC₂H₅ +NH₃

9. Reaction  with hydrazene-

consequently, we get Semicarbazide .

10. Formation of cyclic ureides-

When it reacts with di ethyl malonate or di ethyl oxalate  in presence of PCl₃ , cyclic ureides are formed.

Uses-

-1 It is a good Nitrogenous fertilizer.
2. In the formation of Semi Carbazide.
3. In preparing urea-formaldehyde plastic.

Test-

1. Biuret Test-

When urea is heated gently then it gives solid residue biuret which gives violet colour with NaOH & CuSO₄ solution.

2. It gives white precipitate of urea nitrate with Conc. HNO₃

3. It on heating with NaOH gives NH₃ gas.

4. When it reacts with oxalic acid , white precipitate of urea -oxalate is formed.

 

 

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