some naturally occurring nitro groups arose by the oxidation of amino groups. 2-Nitrophenol is an aggregation pheromone of ticks. Examples of nitro compounds are rare in nature. 3-Nitropropionic acid found in fungi and plants (Indigofera). Nitropentadecene is a defense compound found in termites. Nitrophenylethane is found in Aniba canelilla. Nitrophenylethane is also found in members of the Annonaceae, Lauraceae and Papaveraceae. In pharmaceuticals Despite the occasional use in pharmaceuticals, the nitro group is associated with mutagenicity and genotoxicity and therefore is often regarded as a liability in the drug discovery process. Reactions of aliphatic nitro compounds Reduction Nitro compounds participate in several organic reactions, the most important being their reduction to the corresponding amines: RNO2 + 3 H2 → RNH2 + 2 H2O Acid-base reactions The α-carbon of nitroalkanes is somewhat acidic. The pKa values of nitromethane and 2-nitropropane are respectively 17.2 and 16.9 in dimethyl sulfoxide (DMSO) solution. These values suggest an aqueous pKa of around 11. In other words, these carbon acids can be deprotonated in aqueous solution. The conjugate base is called a nitronate, they are formed as intermediates in the nitroaldol reaction and Nef reactions. Condensation reactions Nitromethane undergoes base-catalyzed additions to aldehydes in 1,2-addition in the nitroaldol reaction. Similarly, it adds to alpha-beta unsaturated carbonyl compounds as a 1,4-addition in the Michael reaction as a Michael donor. Nitroalkenes are Michael acceptors in the Michael reaction with enolate compounds. Biochemical reactions Many flavin-dependent enzymes are capable of oxidizing aliphatic nitro compounds to less-toxic aldehydes and ketones. Nitroalkane oxidase and 3-nitropropionate oxidase oxidize aliphatic nitro compounds exclusively, whereas other enzymes such as glucose oxidase have other physiological substrates. Reactions of aromatic nitro compounds Reduction of aromatic nitro compounds with hydrogen over metal catalysts gives anilines. Virtually all aromatic amines (anilines) are derived from nitroaromatics. A variation is formation of a dimethylaminoarene with palladium on carbon and formaldehyde: The Leimgruber–Batcho, Bartoli and Baeyer–Emmerling indole syntheses begin with aromatic nitro compounds. Indigo can be synthesized in a condensation reaction from ortho-nitrobenzaldehyde and acetone in strongly basic conditions in a reaction known as the Baeyer–Drewson indigo synthesis. Explosions Explosive decomposition of organo nitro compounds are redox reactions, wherein both the oxidant (nitro group) and the fuel (hydrocarbon substituent) are bound within the same molecule. The explosion process generates heat by forming highly stable products including molecular nitrogen (N2), carbon dioxide, and water. The explosive power of this redox reaction is