Nitro compounds

The structure of the nitro group

Nitro compounds are organic compounds that contain one or more nitro functional groups (-N O 2). They are often highly explosive, especially when the compound contains more than one nitro group. The presence of impurities or improper handling can trigger a violent exothermic decomposition.

Aromatic nitro compounds are typically synthesized by the action of a mixture of nitric and sulfuric acids on a suitable organic molecule. Some examples of such compounds are trinitrophenol (picric acid), trinitrotoluene (TNT), and trinitroresorcinol (styphnic acid). Chloramphenicol is a rare example of a naturally occurring nitro compound.

1. Preparation

In organic synthesis various methods exists to prepare nitro compounds.

1.1. Aliphatic nitro compounds

Nitromethane adds to aldehydes in 1,2-addition in the nitroaldol reaction Nitromethane adds to alpha-beta unsaturated carbonyl compounds as a 1,4-

addition in the Michael reaction as a Michael donor Nitroethylene is a Michael acceptor in a Michael reaction with enolate

compounds In nucleophilic aliphatic substitution sodium nitrite (NaNO2) replaces an alkyl

halide. In the so-called ter Meer reaction (1876) named after Edmund ter Meer.

the reactant is a 1,1-halonitroalkane:

In one study, a reaction mechanism is proposed in which in the first slow step a proton is abstracted from nitroalkane 1 to a carbanion 2 followed by isomerization to a sodium nitronate 3 and finally nucleophilic displacement of chlorine based on an experimentally observed kinetic isotope effect of 3.3. When the same reactant is reacted with potassium hydroxide the reaction product is the 1,2-dinitro dimer

1.2. Aromatic nitro compounds

In electrophilic substitution, nitric acid reacts with aromatic compounds in nitration.

A classic method starting from halogenated phenols is the Zinke nitration.

2. Reactions

Nitro compounds participate in several organic reactions.

2.1. Aliphatic nitro compounds

Aliphatic nitro compounds are reduced to amines with hydrochloric acid and an iron catalyst

Nitronates form by adding acids to nitro salts. Hydrolysis of the salts of nitro compounds yield aldehydes or ketones in the

Nef reaction

2.2. Aromatic nitro compounds

Reduction of aromatic nitro compounds with hydrogen gas over a platinum catalyst gives anilines.

The presence of nitro groups facilitates nucleophilic aromatic substitution.

3. NitrationNitration is a a general chemical process for the introduction of a nitro group into a chemical compound. Examples of nitrations are the conversion of glycerin to nitroglycerin and the conversion of toluene to trinitrotoluene. Both of these conversions use nitric acid and sulfuric acid.

3.1. Aromatic nitration

In "aromatic nitration," aromatic organic compounds are nitrated via an electrophilic aromatic substitution mechanism involving the attack of the electron-rich benzene ring by the nitronium ion. Alternative mechanisms have also been proposed, as the one involving single electron transfer (SET). Aromatic nitro compounds are important intermediates to anilines by action of a reducing agent. Benzene is nitrated by refluxing with concentrated sulfuric acid and concentrated nitric acid at 50 °C.

2 H2SO4 + HNO3 → 2 HSO4- + NO2

+ + H3O+ C6H6 + NO2

+ → C6H5NO2 + H+ H+ + H3O+ + 2 HSO4

- → H2O + 2H2SO4

The sulfuric acid is regenerated and hence acts as a catalyst. It also absorbs water.

Nitration of benzene

The formation of a nitronium ion (the electrophile) from nitric acid and sulfuric acid is shown below:

Reaction of nitric acid and sulfuric acid

- Scope

Selectivity is always a challenge in nitrations, The nitration of fluorenone is selective and yields a tri-nitro compound or tetra-nitro compound by modifying reaction conditions. Another example of trinitration can be found in the synthesis of phloroglucinol.

Other nitration reagents include nitronium tetrafluoroborate, a nitronium salt. This compound can be prepared from hydrogen fluoride, nitric acid, and boron trifluoride.

The substituents on aromatic rings affect the rate of this electrophilic aromatic substitution. Deactivating groups such as other nitro groups have an electron-withdrawing effect. Such groups deactivate (slow) the reaction and directs the electrophilic nitronium ion to attack the aromatic meta position. Deactivating meta-directoring substituents include sulfonyl, cyano groups, keto, esters, and carboxylates. Nitration can be accelerated by activating groups such as amino, hydroxy and methyl groups also amides and ethers resulting in para and ortho isomers.

The direct nitration of aniline with nitric acid and sulfuric acid, according to one source results in a 50/50 mixture of para and meta nitroaniline. In this reaction the fast-reacting and activating aniline (ArNH2) is in equilibrium with the more abundant but less reactive and deactivating anilinium ion (ArNH3

+), which may explain this reaction product distribution. According to another source a more controlled nitration of aniline starts with the formation of acetanilide by reaction with acetic anhydride followed by the actual nitration. Because the amide is a regular activating group the products formed are the para and ortho isomers. Heating the reaction mixture is sufficient to hydrolyze the nitroamide back to the nitroamine.

In the Wolfenstein-Boters reaction, benzene reacts with nitric acid and mercury nitrate to give picric acid.

4. Reduction of nitro compoundsThe chemical reactions described as reduction of nitro compounds can be facilitated by many different reagents and reaction conditions. Historically, the nitro group was one of the first functional groups to be reduced, due to the ease of nitro-group reduction.

Nitro-groups behave differently whether a neighboring hydrogen is present or not. Thus, reduction conditions can be initially classified by starting materials: aliphatic nitro compounds or aromatic nitro compounds. Secondary classifications are based upon reaction products.

4.1. Aliphatic nitro compounds

4.1.1. Reduction to hydrocarbons

Hydrodenitration (replacement of a nitro group with hydrogen) is difficult to achieve, but can be completed by catalytic hydrogenation over platinum on silica gel at high temperatures.

4.1.2. Reduction to amines

Aliphatic nitro compounds can be reduced to aliphatic amines using several different reagents:

Catalytic hydrogenation using platinum oxide (PtO2) or Raney nickel Iron metal in refluxing acetic acid Samarium diiodide

α,β-Unsaturated nitro compounds can be reduced to saturated amines using:

Catalytic hydrogenation over palladium-on-carbon Iron metal Lithium aluminium hydride [6] (Note: Hydroxylamine and oxime impurities are

typically found.)

4.1.3. Reduction to hydroxyl amines

Aliphatic nitro compounds can be reduced to aliphatic hydroxylamines using diborane.

4.1.4. Reduction to oximes

Nitro compounds are typically reduced to oximes using metal salts, such as stannous chloride or chromium(II) chloride. dditionally, catalytic hydrogenation using a controlled amount of hydrogen can generate oximes.

4.2. Aromatic nitro compounds

The reduction of aryl nitro compounds can be finely tuned to obtain a different products typically in high yields.

4.2.1. Reduction to anilines

Many methods for the production of anilines from aryl nitro compounds exist, such as:

Catalytic hydrogenation using palladium-on-carbon, platinum oxide, or Raney nickel

Iron in acidic media (Note: Iron is particularly well suited for this reduction as the reaction conditions are typically gentle and also because iron has a high functional group tolerance.)

Sodium hydrosulfite Sodium sulfide (or hydrogen sulfide and base) Tin(II) chloride Titanium(III) chloride Zinc

It is also possible to form a nitroaniline by reduction of a dinitroarene using sodium sulfide.

Metal hydrides are typically not used to reduce aryl nitro compounds to anilines because they tend to produce azo compounds.

4.2.2. Reduction to hydroxylamines

Several methods for the production of aryl hydroxylamines from aryl nitro compounds exist:

Raney nickel and hydrazine at 0-10 °C Electrolytic reduction Zinc metal in aqueous ammonium chloride

4.2.3. Reduction to hydrazo compounds

Treatment of nitroarenes with excess zinc metal results in the formation of N,N'-diarylhydrazine.

4.2.4. Reduction to azo compounds

Treatment of aromatic nitro compounds with metal hydrides gives good yields of azo compounds. For example, one could use:

Lithium aluminium hydride

Zinc metal with sodium hydroxide. (Excess zinc will reduce the azo group to a hydrazino compound.)