Cinnoline is an aromatic

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 chemistry is the branch of organic chemistry dealing with the synthesis, properties, and ...

with the formula C₈H₆N₂. It is

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

ic with other naphthyridines including quinoxaline,

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 Phthalazi ...

and quinazoline.

Properties

The free base can be obtained as an oil by treatment of the hydrochloride 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 and leaves a sharp irritation for some time. Cinnoline derivatives are obtained from oxycinnoline carboxylic acid, which is formed by digesting orthophenyl propiolic acid diazo chloride with water. Oxycinnoline carboxylic acid on heating gives oxycinnoline, melting at 225 °C, which with phosphorus pentachloride gives chlorcinnoline. This substance is reduced by

iron filing Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

s and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

to dihydrocinnoline.

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(NH₂)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 t ...

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 of dihydrocinnoline with freshly precipitated mercuric oxide. It can be isolated as the hydrochloride. 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 al ...

of an α-vinyl- aniline with hydrochloric acid and sodium nitrite: Widman-stoermer-reaction

The sodium nitrite is first converted to nitrous acid which then forms the

electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...

intermediate dinitrogen trioxide. The next intermediate is the stable

nitrosamine In organic chemistry, nitrosamines (or more formally ''N''-Nitrosamines) are organic compounds with the chemical structure , where R is usually an alkyl group. They feature a nitroso group () bonded to a deprotonated amine. Most nitrosamines are ...

with goes on to lose water forming the diazonium salt which then reacts with the vinyl group in the ring-closing step. A conceptually related reaction is the

Bamberger triazine synthesis The Bamberger triazine synthesis in organic chemistry is a classic organic synthesis of a triazine first reported by Eugen Bamberger in 1892.Hassner, A., Stumer, C., ''Organic Synthesis Based on Name Reactions'': 2nd. Ed. Tetrahedron Organic Chemi ...

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