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 the specific chemical compound urea ((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.
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]
Phosgenation entails the reaction of amines with phosgene, proceeding via the isocyanate (or carbamoyl chloride) as an intermediate:
Overall reaction:
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:
Unsymmetrical ureas are generated by condensation of isocyanates with amines:
Analogously, unsymmetrical primary ureas are generated by condensation of ammonium salts and alkali metal cyanates: [2]
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]
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]
Urea undergoes transamidation with alkyl and aryl amines:
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]
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):
Two equivalents of urea condense with isobutyraldehyde giving the alkylidene derivative:
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.
Phosgene is an 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. It can be thought of chemically as the double acyl chloride analog of carbonic acid, or structurally as formaldehyde with the hydrogen atoms replaced by chlorine atoms. Phosgene is a valued and important industrial building block, especially for the production of precursors of polyurethanes and polycarbonate plastics.
In organic chemistry, isocyanate is the functional group with the formula R−N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for the production of polyurethanes, a class of polymers.
Diphosgene is an organic chemical compound with the formula ClCO2CCl3. This colorless liquid is a valuable reagent in the synthesis of organic compounds. Diphosgene is related to phosgene and has comparable toxicity, but is more conveniently handled because it is a liquid, whereas phosgene is a gas.
Nitrous acid is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.
In organic chemistry, an acyl chloride is an organic compound with the functional group −C(=O)Cl. Their formula is usually written R−COCl, where R is a side chain. They are reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride, CH3COCl. Acyl chlorides are the most important subset of acyl halides.
In organic chemistry, an acyl halide is a chemical compound derived from an oxoacid by replacing a hydroxyl group with a halide group.
In organic chemistry, a carbamate is a category of organic compounds with the general formula R2NC(O)OR and structure >N−C(=O)−O−, which are formally derived from carbamic acid. The term includes organic compounds, formally obtained by replacing one or more of the hydrogen atoms by other organic functional groups; as well as salts with the carbamate anion H2NCOO−.
Hydantoin, or glycolylurea, is a heterocyclic organic compound with the formula CH2C(O)NHC(O)NH. It is a colorless solid that arises from the reaction of glycolic acid and urea. It is an oxidized derivative of imidazolidine. In a more general sense, hydantoins can refer to groups or a class of compounds with the same ring structure as the parent compound. For example, phenytoin (mentioned below) has two phenyl groups substituted onto the number 5 carbon in a hydantoin molecule.
Triphosgene (bis(trichloromethyl) carbonate (BTC) is a chemical compound with the formula OC(OCCl3)2. It is used as a solid substitute for phosgene, which is a gas and diphosgene, which is a liquid. Triphosgene is stable up to 200 °C. Triphosgene is used in a variety of halogenation reactions.
The cyanate ion is an anion with the chemical formula OCN−. It is a resonance of three forms: [O−−C≡N] (61%) ↔ [O=C=N−] (30%) ↔ [O+≡C−N2−] (4%).
In organic chemistry, an iminium cation is a polyatomic ion with the general structure [R1R2C=NR3R4]+. They are common in synthetic chemistry and biology.
Chloramines refer to derivatives of ammonia and organic amines wherein one or more N−H bonds have been replaced by N−Cl bonds. Two classes of compounds are considered: inorganic chloramines and organic chloramines. Chloramines are the most widely used members of the halamines.
In organic chemistry, thiocarbamates (thiourethanes) are a family of organosulfur compounds. As the prefix thio- suggests, they are sulfur analogues of carbamates. There are two isomeric forms of thiocarbamates: O-thiocarbamates, ROC(=S)NR2 (esters), and S-thiocarbamates, RSC(=O)NR2 (thioesters).
Michler's ketone is an organic compound with the formula of [(CH3)2NC6H4]2CO. This electron-rich derivative of benzophenone is an intermediate in the production of dyes and pigments, for example Methyl violet. It is also used as a photosensitizer. It is named after the German chemist Wilhelm Michler.
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 organic solvents. An example is dimethylcarbamoyl chloride (m.p. −90 °C and b.p. 93 °C). Carbamoyl chlorides are used to prepare a number of pesticides, e.g. carbofuran and aldicarb.
Transamidation is a chemical reaction in which an amide reacts with an amine to generate a new amide:
Imidoyl chlorides are organic compounds that contain the functional group RC(NR')Cl. A double bond exist between the R'N and the carbon centre. These compounds are analogues of acyl chloride. Imidoyl chlorides tend to be highly reactive and are more commonly found as intermediates in a wide variety of synthetic procedures. Such procedures include Gattermann aldehyde synthesis, Houben-Hoesch ketone synthesis, and the Beckmann rearrangement. Their chemistry is related to that of enamines and their tautomers when the α hydrogen is next to the C=N bond. Many chlorinated N-heterocycles are formally imidoyl chlorides, e.g. 2-chloropyridine, 2, 4, and 6-chloropyrimidines.
Sodium cyanate is the inorganic compound with the formula NaOCN. A white solid, it is the sodium salt of the cyanate anion.
In organophosphorus chemistry, an aminophosphine is a compound with the formula R3−nP(NR2)n where R = H or an organic substituent, and n = 0, 1, 2. At one extreme, the parents H2PNH2 and P(NH2)3 are lightly studied and fragile, but at the other extreme tris(dimethylamino)phosphine (P(NMe2)3) is commonly available. Intermediate members are known, such as Ph2PN(H)Ph. These compounds are typically colorless and reactive toward oxygen. They have pyramidal geometry at phosphorus.
Hydroxymethylation is a chemical reaction that installs the CH2OH group. The transformation can be implemented in many ways and applies to both industrial and biochemical processes.