Ureas

In chemistry, ureas are a class of organic compounds with the formula (R₂N)₂CO where R = H, alkyl, aryl, etc. Thus, in addition to describing a specific chemical compound (H₂N)₂CO), urea is the name of a functional group that is found in many compounds and materials of both practical and theoretical interest. Generally ureas are colorless crystalline solids, which, owing to the presence of fewer hydrogen bonds, exhibit melting points lower than that of urea itself.

Structure of N,N'-diphenylurea showing intermolecular hydrogen bonding.

Biotin, a water-soluble B vitamin, is a urea.

Synthesis

Ureas can be prepared many methods, but rarely by direct carbonation, which is the route to urea itself. Instead, methods can be classified according those that assemble the urea functionality and those that start with preformed urea.[1]

Assembly of N-substituted urea functionality

Phosgenation entails the reaction of amines with phosgene, proceeding via the isocyanate (or carbamoyl chloride) as an intermediate:

COCl₂ + R₂NH → R₂NC(O)Cl + HCl

COCl₂ + RNH₂ → RNCO + 2 HCl

Overall reaction:

COCl₂ + 2 R₂NH → (R₂N)₂CO + 2 HCl

Tetramethylurea is prepared in this way and in general secondary amines give reliable results. Reactions using primary amines must be carefully controlled as the isocyanate intermediate can react with the urea to form a biuret:

R₂NC(O)N(H)R' + R"NCO → R₂NC(O)NR'C(O)NHR"

Unsymmetrical ureas are generated by condensation of isocyanates with amines:

RNCO + R'₂NH → (R'₂N)(R(H)N)CO

Analogously, unsymmetrical primary ureas are generated by condensation of ammonium salts and alkali metal cyanates:[2]

Na⁺NCO⁻ + [R₂NH₂]Cl → (R’₂N)(H₂N)CO + KCl

The artificial sweetener dulcin is produced by the condensation of ethoxyaniline with potassium cyanate. Si(NCO)₄ is also used as a precursor to such unsymmetrical ureas.[3]

bis(hydroxymethyl)urea and related compounds are additives for permanent press clothing.

Isobutylidenediurea.

The very high toxicities of compounds such as phosgene and isocyanates makes them unappealing to work with and there has been a drive towards safer reagents. These have traditionally been more expensive and hence mostly been limited to laboratory-scale work.[4]

From urea

Urea undergoes transamidation with alkyl and aryl amines:

(H₂N)₂CO + R₂NH → (R₂N)(H₂N)CO + NH₃

(R₂N)(H₂N)CO + R₂NH → (R₂N)₂CO + NH₃

These reactions are used to prepare cyclic ureas. Monomethylurea, precursor to theobromine, is produced from methylamine and urea. Phenylurea is produced similarly but from anilinium chloride:[5]

(H₂N)₂CO + [R₂NH₂]Cl → (R₂N)(H₂N)CO + NH₄Cl

Ureas containing N-H bonds, including urea itself, are readily alkylated by aldehydes. The products are α-hydroxyalkylureas. Formaldehyde gives the bis(hydroxymethyl)urea, an intermediate in the formation of urea-formaldehyde resins. Cyclic ureas result from glyoxal ((CHO)₂):

(H₂N)₂CO + (CHO)₂ → (CH(OH)NH)₂CO

Two equivalents of urea condense with isobutyraldehyde giving the alkylidene derivative:

2(H₂N)₂CO + OC(H)CHMe₂ → (H₂N)C(O)NH]₂CHCHMe₂ + H₂O

This derivative, isobutylidenediurea, is used as a slow-release fertilizer because in the soil it slowly hydrolyzes, reverting to urea, an excellent source of fixed nitrogen.

References

1. C. Nitschke; G. Scherr (2012). "Urea Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.o27_o04.
2. Frederick Kurzer (1951). "Arylureas I. Cyanate Method p-Bromophenylurea". Org. Synth. 31: 8. doi:10.15227/orgsyn.031.0008.
3. Roy G. Neville and John J. McGee (1965). "N-Mono- and N,N-Disubstituted Ureas and Thioureas". Org. Synth. 45: 69. doi:10.15227/orgsyn.045.0069.
4. Bigi, Franca; Maggi, Raimondo; Sartori, Giovanni (2000). "Selected syntheses of ureas through phosgene substitutes". Green Chemistry. 2 (4): 140–148. doi:10.1039/B002127J.
5. "Arylureas II. Urea Method p-Ethoxyphenylurea". Org. Synth. 31: 11. 1951. doi:10.15227/orgsyn.031.0011.