The Pschorr cyclization is a

name reaction A name reaction is a chemical reaction named after its discoverers or developers. Among the tens of thousands of organic reactions that are known, hundreds of such reactions are well-known enough to be named after people. Well-known examples include ...

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...

, which was named after its discoverer, the

German German(s) may refer to: * Germany (of or related to) ** Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...

chemist Robert Pschorr (1868-1930). It describes the intramolecular substitution of aromatic compounds via aryldiazonium salts as intermediates and is

catalyzed Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

. The reaction is a variant of the Gomberg-Bachmann reaction. The following reaction scheme shows the Pschorr cyclization for the example of

phenanthrene Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) with formula C14H10, consisting of three fused benzene rings. It is a colorless, crystal-like solid, but can also appear yellow. Phenanthrene is used to make dyes, plastics and pesticides, e ...

Reaction mechanism

In the course of the Pschorr cyclization, a diazotization of the starting compound occurs, so that an aryldiazonium salt is formed as intermediate. For this, sodium nitrite is added to

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

to obtain

nitrous acid Nitrous acid (molecular formula ) is a weak and monoprotic acid known only in Solution (chemistry), solution, in the gas phase and in the form of nitrite () salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazoni ...

. The nitrous acid is protonated and reacts with another equivalent of nitrous acid to the intermediate 1 which is later used for the diazotization of the

aromatic amine In organic chemistry, an aromatic amine is an organic compound consisting of an aromatic ring attached to an amine. It is a broad class of compounds that encompasses anilines, but also many more complex aromatic rings and many amine substituents ...

The intermediate 1 reacts in the following way with the starting compound: Pschorr cyclization part 2 MV1

Intermediate 1 replaces a hydrogen atom from the amino group of the starting compound. A nitroso group is introduced as new substituent, producing under the release of nitrous acid intermediate 2. Intermediate 2 then reacts via a

tautomer Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydr ...

ism and

dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...

to the aryldiazonium cation 3. Pschorr cyclization part 3 MV1

Nitrogen is then cleaved from the aryldiazonium cation 3 by the use of the copper catalyst. The aryl radical thus formed reacts via ring closure to the intermediate stage 4. Finally, rearomatization takes place using again the copper catalyst and phenanthrene is formed.

Atom economy

The Pschorr cyclization has a relatively good

atom economy Atom economy (atom efficiency/percentage) is the conversion efficiency of a chemical process in terms of all atoms involved and the desired products produced. The simplest definition was introduced by Barry Trost in 1991 and is equal to the ratio ...

, since essentially only

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

is produced as a waste material. For the

diazotization Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. General propert ...

, two equivalents of nitrous acid are used, of which one equivalent is being re-formed in the course of the reaction. The copper is used in catalytic amounts only and does therefore not affect the atomic efficiency of the reaction. However, when considering the atom economy it has to be mentioned that the Pschorr cyclization has often only low yields.

References

{{Reflist Name reactions