Hydroquinone

Colorless and odorless, crystalline solid

Fixed

1.36 g · cm -3

170 ° C

286 ° C

15 MPa (20 ° C)

Soluble in water: 72 g · l-1 (20 ° C)

Risk

Not set

Template: Infobox chemical / molecular formula search available

Hydroquinone (1,4- dihydroxybenzene ) is a phenol and catechol addition (1,2- dihydroxybenzene ) and resorcinol (1,3- dihydroxybenzene ) the third possible dihydroxybenzene. Here are the two hydroxy groups in the para position.

Discovery

Friedrich Wöhler received in 1844 by the dry distillation ( > 280 ° C) of quinic acid, a product mixture containing as main component contained a new colorless compound next to benzene, benzoic acid and salicylic acid. After dissolving the distillate in water, filtering off the insoluble material and distilling off the more volatile components, crystallized from the remaining solution initially benzoic acid and from their mother liquor ultimately hydroquinone from which he obtained by repeated recrystallization in the form of colorless, six-sided prisms pure.

Occurrence

Hydroquinone comes as about 10 % solution together with 28% hydrogen peroxide in the defense glands of bombardier beetles before. In the defense case, the mixture catalase is added and the attacker than 100 ° C and ' splashed hot, corrosive repellent.

Representation

Hydroquinone can be synthesized by the oxidation of phenol Elbs.

Properties

Hydroquinone is a stronger reducing agent than catechol because the resultant of catechol o -benzoquinone energy and therefore is a stronger oxidising agent. The latter is the cause for electrostatic repulsion of the adjacent carbonyl groups.

Hydroquinone is a colorless, moderately water-soluble solid. It can be converted by oxidation to benzoquinone ( Chinon ):

In this reaction as an intermediate product of the deeply colored, poorly water-soluble charge-transfer complex quinhydrone ( not shown).

Use

In the photo lab technology hydroquinone is used as a reducing agent for developing films and pictures. Because of the dangers to the environment and health, efforts are made to replace the substance for those applications where possible by less risky substances. In skin creams using hydroquinone despite possible health risks will be tolerated since the early 1990s. It is also used as an inhibitor of radical reactions, in order to prevent the formation of ether peroxides.

Toxicology

In animal experiments, a carcinogenic effect could be detected. The carcinogenicity in humans and exposure within the permissible limits is controversial.

Reactions

The simple bromination of hydroquinone with potassium bromide and bromine in carbon tetrachloride leads to bromohydroquinone.

Full bromination for the analytical detection provides 2.5 dibromohydroquinone whose melting point is 186 ° C. Also known is the tetrabromohydroquinone, which is, however, prepared from benzoquinone.

Methylation with dimethyl sulfate yields the 1,4- dimethoxybenzene, whose melting point is 56 ° C.

A direct nitration of hydroquinone is not possible, as it decomposes. The hydroxy groups must be protected by reaction with acetic anhydride and sulfuric acid as a catalyst with acetyl, then takes place nitration at positions 2 and 6. Saponification of the resulting Dinitrodiacetylhydrochinons eventually leads to 2,6- Dinitrohydrochinon (melting point 135-136 ° C).