Dinitration

Dinitration is a chemical process in which two nitro groups (-NO2) are introduced into a molecule. This is typically achieved through electrophilic aromatic substitution reactions, where a nitrating agent attacks an aromatic ring. The most common nitrating agent is a mixture of concentrated nitric acid and concentrated sulfuric acid, often referred to as "mixed acid." Sulfuric acid acts as a catalyst, protonating nitric acid to form the highly electrophilic nitronium ion (NO2+), which is the active species that attacks the aromatic substrate.

The first nitration of an aromatic compound usually proceeds readily. However, the introduction of a nitro group deactivates the aromatic ring towards further electrophilic substitution. This means that the second nitration step requires more vigorous conditions, such as higher temperatures or a more concentrated nitrating mixture, compared to the first nitration. The position of the second nitro group is influenced by the directing effects of the first nitro group. Nitro groups are meta-directors, meaning they tend to direct incoming electrophiles to the meta position relative to themselves. However, other substituents on the ring can also influence the regioselectivity of the dinitration.

Dinitration is a crucial step in the synthesis of many important compounds, including explosives like dinitrotoluene (DNT) and dinitrophenol (DNP). DNT is a precursor to toluene diisocyanate (TDI), a key component in the production of polyurethanes. Dinitrophenol has found use as a dye and a pesticide, though its use has declined due to toxicity concerns. The control of reaction conditions is vital in dinitration to achieve the desired product and minimize the formation of unwanted byproducts or over-nitrated species.