Preparation and Reduction of Nitro Compounds

General preparation of nitroalkanes. A primary or secondary haloalkane, heated

with alcoholic silver nitrite ($\text{AgNO}_2$), undergoes nucleophilic substitution

to give predominantly the nitroalkane:

The nitrite ion is an ambident nucleophile -- it can attack through either its nitrogen

atom (giving the $\text{C}-\text{N}$ bonded nitro compound) or through one of its oxygen

atoms (giving the $\text{C}-\text{O}$ bonded alkyl nitrite). With the largely COVALENT

silver salt, the softer, more polarisable nitrogen end of the nitrite ion is the

preferred nucleophilic site, so the nitro compound dominates; the same substitution

carried out with the largely IONIC sodium nitrite instead favours attack through the

harder oxygen end, giving the alkyl nitrite as the major product -- exactly parallel to

the cyanide/isocyanide contrast met later in this chapter.

Reduction of nitro compounds to amines. The nitro group's greatest importance is as

a controlled route into a primary amine, and remarkably the SAME starting nitro compound

can be steered to different products purely by the choice of reducing agent and

conditions:

• $\text{Fe}/\text{HCl}$, $\text{Sn}/\text{HCl}$, or catalytic $\text{H}_2/\text{Pd-C}$ (or $\text{H}_2/\text{Ni}$, $\text{H}_2/\text{Pt}$) all carry the reduction FULLY through to the primary amine: $\text{C}_6\text{H}_5\text{NO}_2 \xrightarrow{6[\text{H}]} \text{C}_6\text{H}_5\text{NH}_2 + 2\text{H}_2\text{O}$. Catalytic hydrogenation is preferred industrially because it avoids the large volumes of metal-salt sludge that $\text{Fe}$/$\text{Sn}$ reductions leave behind.
• Zinc dust with neutral, aqueous ammonium chloride is a much milder reducing medium. It supplies only enough reducing power to stop the reaction at an intermediate oxidation stage, giving phenylhydroxylamine ($\text{C}_6\text{H}_5\text{NHOH}$) rather than carrying it through to aniline -- the mild, near-neutral conditions are …