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In chemistry, ureas are a class of organic compounds with the formula (R
2N)
2CO where R = H, alkyl, aryl, etc. Thus, in addition to describing the specific chemical compound urea ((H
2N)
2CO), urea is the name of a
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...
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 bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
s, exhibit melting points lower than that of urea itself.
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.
Assembly of N-substituted urea functionality
Phosgenation entails the reaction of
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...
s with
phosgene
Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, esp ...
, proceeding via the
isocyanate
In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...
(or
carbamoyl chloride
A carbamoyl chloride is the functional group with the formula R2NC(O)Cl. The parent carbamoyl chloride, H2NCOCl is unstable, but many N-substituted analogues are known. Most examples are moisture sensitive, colourless, and soluble in nonpolar org ...
) as an intermediate:
:COCl
2 + R
2NH → R
2NC(O)Cl + HCl
:COCl
2 + RNH
2 → RNCO + 2 HCl
Overall reaction:
:COCl
2 + 2 R
2NH → (R
2N)
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
Biuret is a chemical compound with the chemical formula . It is a white solid that is soluble in hot water. A variety of organic derivatives are known. The term "biuret" also describes a family of organic compounds with the chemical formula , wher ...
:
:R
2NC(O)N(H)R' + R"NCO → R
2NC(O)NR'C(O)NHR"
Unsymmetrical ureas are generated by condensation of
isocyanate
In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...
s 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:
:Na
+NCO
− +
2NH2">2NH2l → (R’
2N)(H
2N)CO + NaCl
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.
180px, and related compounds are additives for permanent press">bis(hydroxymethyl)urea and related compounds are additives for permanent press clothing.
image:IBDU.png, 220px, 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.
From urea
Urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important ...
undergoes
transamidation with alkyl and aryl amines:
:(H
2N)
2CO + R
2NH → (R
2N)(H
2N)CO + NH
3
:(R
2N)(H
2N)CO + R
2NH → (R
2N)
2CO + NH
3
These reactions are used to prepare cyclic ureas. Monomethylurea, precursor to
theobromine
Theobromine, also known as xantheose, is the principal alkaloid of '' Theobroma cacao'' (cacao plant). Theobromine is slightly water- soluble (330 mg/L) with a bitter taste. In industry, theobromine is used as an additive and precursor to ...
, is produced from
methylamine
Methylamine is an organic compound with a formula of . This colorless gas is a derivative of ammonia, but with one hydrogen atom being replaced by a methyl group. It is the simplest primary amine.
Methylamine is sold as a solution in methanol, ...
and urea. Phenylurea is produced similarly but from anilinium chloride:
[{{cite journal
, authors=
, title=Arylureas II. Urea Method p-Ethoxyphenylurea
, journal=Org. Synth.
, year=1951
, volume=31
, page=11
, doi=10.15227/orgsyn.031.0011]
:(H
2N)
2CO +
2NH2">2NH2l → (R
2N)(H
2N)CO + NH
4Cl
Ureas containing N-H bonds, including urea itself, are readily alkylated by aldehydes. The products are α-hydroxyalkylureas. Formaldehyde gives the
bis(hydroxymethyl)urea
Bis(hydroxymethyl)urea is an organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms) ...
, an intermediate in the formation of
urea-formaldehyde resin
Urea-formaldehyde (UF), also known as urea-methanal, so named for its common synthesis pathway and overall structure, is a nontransparent thermosetting resin or polymer. It is produced from urea and formaldehyde. These resins are used in adhesive ...
s. Cyclic ureas result from glyoxal ((CHO)
2):
:(H
2N)
2CO + (CHO)
2 → (CH(OH)NH)
2CO
Two equivalents of urea condense with
isobutyraldehyde
Isobutyraldehyde is the chemical compound with the formula (CH3)2CHCHO. It is an aldehyde, isomeric with n-butyraldehyde (butanal). Isobutyraldehyde is made, often as a side-product, by the hydroformylation of propene. Its odour is described as ...
giving the alkylidene derivative:
:2(H
2N)
2CO + OC(H)CHMe
2 → (H
2N)C(O)NH]
2CHCHMe
2 + H
2O
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