Reduction of nitro compounds
The 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.
- 1 Aliphatic nitro compounds
- 2 Aromatic nitro compounds
- 3 References
Aliphatic nitro compounds
Reduction to hydrocarbons
Aliphatic nitro compounds can be reduced to aliphatic amines using several different reagents:
- Catalytic hydrogenation using platinum(IV) 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 (Note: Hydroxylamine and oxime impurities are typically found.)
Reduction to hydroxylamines
- R-NO2 + 4 NH4Cl + 2 Zn → R-NH-OH + 2 ZnCl2 + 4 NH3 + H2O
Nitro compounds are typically reduced to oximes using metal salts, such as stannous chloride or chromium(II) chloride. Additionally, catalytic hydrogenation using a controlled amount of hydrogen can generate oximes.
Aromatic nitro compounds
The reduction of aryl nitro compounds can be finely tuned to obtain a different products typically in high yields.
Many methods for the production of anilines from aryl nitro compounds exist, such as:
- Catalytic hydrogenation using palladium-on-carbon, platinum(IV) 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.) (See Bechamp reduction)
- Sodium hydrosulfite
- Sodium sulfide (or hydrogen sulfide and base)
- Tin(II) chloride
- Titanium(III) chloride
Metal hydrides are typically not used to reduce aryl nitro compounds to anilines because they tend to produce azo compounds. (See below)
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
Reduction to hydrazino compounds
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.)
Reduction to azoxy compounds
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