Cinnoline is an

aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, proper ...

with the formula C₈H₆N₂. It is

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

ic with other

naphthyridine Diazanaphthalenes are a class of aromaticity, aromatic heterocyclic compound, heterocyclic chemical compounds that have the formula C8H6N2. They consist of a naphthalene double ring in which two of the carbon atoms have been replaced with nitrogen a ...

s including

quinoxaline A quinoxaline, also called a benzopyrazine, in organic chemistry, is a heterocyclic compound containing a ring complex made up of a benzene ring and a pyrazine ring. It is isomeric with other naphthyridines including quinazoline, phthalazine and c ...

phthalazine Phthalazine, also called benzo-orthodiazine or benzopyridazine, is a heterocyclic organic compound with the molecular formula C8H6N2. It is isomeric with other naphthyridines including quinoxaline, cinnoline and quinazoline. Synthesis Phthalazine ...

and

quinazoline Quinazoline is an organic compound with the formula C8H6N2. It is an aromatic heterocycle with a bicyclic structure consisting of two fused six-membered aromatic rings, a benzene ring and a pyrimidine ring. It is a light yellow crystalline so ...

Properties

The free base can be obtained as an oil by treatment of the

hydrochloride In chemistry, a hydrochloride is an acid salt resulting, or regarded as resulting, from the reaction of hydrochloric acid with an organic base (e.g. an amine). An alternative name is chlorhydrate, which comes from French. An archaic alternati ...

with base. It co-crystallizes with one molecule of ether as white silky needles, (m.p. 24–25 °C) upon cooling ethereal solutions. The free base melts at 39 °C. It has a taste resembling that of

chloral hydrate Chloral hydrate is a geminal diol with the formula . It was first used as a sedative and hypnotic in Germany in the 1870s. Over time it was replaced by safer and more effective alternatives but it remained in use in the United States until at ...

and leaves a sharp irritation for some time.

Discovery and synthesis

The compound was first obtained in impure form by cyclization of the

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

''o''-C₆H₄(N₂Cl)C≡CCO₂H in water to give 4-hydroxycinnoline-3-carboxylic acid. This material could be

decarboxylated Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...

and the hydroxyl group reductively removed to give the parent heterocycle. This reaction is called the Richter cinnoline synthesis. Improved methods exist for its synthesis. It can be prepared by

dehydrogenation In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At ...

of dihydrocinnoline with freshly precipitated

mercuric oxide Mercury(II) oxide, also called mercuric oxide or simply mercury oxide, is the inorganic compound with the formula Hg O. It has a red or orange color. Mercury(II) oxide is a solid at room temperature and pressure. The mineral form montroydite is ...

. It can be isolated as the

. Cinnolines are cinnoline derivatives. A classic

organic reaction Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, mechanistic organ ...

for synthesizing cinnolines is the Widman–Stoermer synthesis, a

ring-closing reaction A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where ...

of an α-vinyl-

aniline Aniline (From , meaning ' indigo shrub', and ''-ine'' indicating a derived substance) is an organic compound with the formula . Consisting of a phenyl group () attached to an amino group (), aniline is the simplest aromatic amine. It is an in ...

with

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

and

sodium nitrite Sodium nitrite is an inorganic compound with the chemical formula . It is a white to slightly yellowish crystalline powder that is very soluble in water and is hygroscopic. From an industrial perspective, it is the most important nitrite sa ...

A conceptually related reaction is the Bamberger triazine synthesis towards triazines. Another cinnoline method is the Borsche cinnoline synthesis.

Safety

Cinnoline is toxic.

References

{{reflist Aromatic bases

Properties

Discovery and synthesis

Safety

See also

References