Nitrophenol

The nitrophenols form a group of substances derived from both the phenol and the nitrobenzene in chemistry. The structure consists of a benzene ring with an added hydroxy (-OH) and nitro ( - NO2) as a substituent. Due to their different arrangement of three structural isomers arise with the empirical formula C6H5NO3. They are accessible mostly by nitration of phenol with ( concentrated ) nitric acid. You fall on, among others, by their yellow color.

Representation

2 - and 4 -nitrophenol in a mixture arise in the nitration of the phenol with dilute nitric acid. The - I effect and the M effect of the hydroxy group of the phenol ortho- para- directing effect in the second substitution. The separation is achieved by means of steam distillation, with only the o -nitrophenol passes. New separation methods are based on the fact that only o -nitro phenol is soluble in n- pentane.

Both isomers can be weiternitriert to 2,4-dinitrophenol and picric acid to.

The preparation of 3-nitro phenol is achieved in two stages, first by halogenation (e.g. Cl2/AlCl3 ) nitrobenzene (meta- directing ); then the replacement of the halogen atom takes place in a nucleophilic aromatic substitution by OH:

Another path runs via the diazotization of 3- nitroaniline and subsequent boiling down of the diazonium salt.

Properties

Nitrophenols are crystalline solids; They are sparingly soluble in water, soluble in ethanol, ether and chloroform. 4-nitrophenol has a weak phenol -like odor.

PKa values

The weakly acidic nature of phenol must be justified by the resonance stabilization of the phenoxide ion. The nitro groups act electron withdrawing; the phenolic OH bond is increasingly polarized. 2 - and 4 -nitrophenol have compared to the 3 -nitrophenol a lower pKa value; so are their acidities greater. In the ortho -and para- form the phenolate ion can move a double bond at the electron-withdrawing nitro ( -M effect). The second O there can form a negative charge center. At the meta- form is not possible.

Melting points

The melting points show significant differences. The 2- nitrophenol has the lowest melting point, since it can form an intramolecular hydrogen bond. The other two isomers form, in contrast, from intermolecular hydrogen bonds. In the 2- nitrophenol energy is required to break these bonds. The 4 -nitrophenol having the highest melting point, due to its symmetry.

Solubility in water

The nitrophenols are sparingly soluble in water, but the values ​​differ within this group. The significantly poorer solubility of 2- nitrophenol in water can also be well explained by the intramolecular hydrogen bond. The molecule is characterized markedly apolar outwardly. In contrast, the solubility of the 3 - and 4- nitrophenol is approximately equal to and in comparison much better. Here now form more intermolecular hydrogen bonds between the phenolic hydroxyl group and water.

Proof

For qualitative analytical detection arises in the bromination with bromine and potassium bromide for 2- nitrophenol 4,6 - dibromo derivative, which has a melting point of 117 ° C. For 4- nitrophenol, 2,6 - dibromo derivative produced with a melting point of 142 ° C. 3 -nitrophenol is also brominated twice with KBr/Br2, the product melts at 91 ° C.

Use

Nitrophenols fall in the chemical, pharmaceutical and defense industries as intermediates in the production of paints, leather, rubber, pesticides, fungicides, pesticides and ammunition.

Safety

Nitrophenols are toxic by inhalation, in contact with skin. It may cause irritation of the eyes, digestive system, blood poisoning, liver damage, dizziness, nausea, headache and respiratory irritation may occur. Maybe they are carcinogenic and sensitizing. Alcohol-related effect is amplified. After contact with skin should be washed off immediately with plenty of water. You have toxic effects on the nervous system of living beings.

A big problem is the removal of nitro phenols and related compounds from the groundwater; whereas this contamination are sometimes found in former explosives or paint factories and military installations. A new development of complete removal of nitrophenols of groundwater is a catalyst composed of iron with a tetra- amido macrocyclic ligand ( TAML Fe ) and allows for the oxidation of the toxins by hydrogen peroxide. It will be no additional toxic substances. The catalyst was developed at Carnegie Mellon University.