2-Nitrobenzaldehyde is an organic aromatic compound containing a nitro group ortho to formyl. 2-Nitrobenzaldehyde once was produced as an intermediate in the synthesis of the popular dye Indigo.
Synthesis[edit]
The main routes to nitrobenzaldehyde begin with the nitration of styrene and cinnamic acid followed by the conversions of the resulting 2-nitrostyrene and 2-nitrocinnamic acids. Cinnamaldehyde can also be nitrated, e.g. in a solution of acetic anhydride in acetic acid, in high-yield to 2-nitrocinnamaldehyde.[3] This compound is then oxidized to 2-nitrocinnamic acid, which is decarboxylated to the 2-nitrostyrene. The vinyl group can be oxidized in a number of different ways to yield 2-nitrobenzaldehyde.[4]
In one synthetic process, toluene is mono-nitrated at cold temperatures to 2-nitrotoluene, with about 58% being converted to the ortho- isomer, the remaining forming meta- and para- isomers.[5] The 2-nitrotoluene can then be oxidized to yield 2-nitrobenzaldehyde.[6][7]
Alternatively, 2-nitrotoluene as formed above can be halogenated to a 2-nitrobenzyl halide followed by oxidation with DMSO and sodium bicarbonate to yield 2-nitrobenzaldehyde, which is subsequently purified with the creation of a bisulfite adduct.[8]
The nitration of benzaldehyde produces mostly 3-nitrobenzaldehyde, with yields being about 19% for the ortho-, 72% for the meta- and 9% for the para isomer.[9] For this reason, the nitration of benzaldehyde to yield 2-nitrobenzaldehyde is not cost-effective.
Uses[edit]
2-Nitrobenzaldehyde is an intermediate in an early route to Indigo, a water-insoluble dye commonly used to dye jeans and other fabrics. In the Baeyer-Drewson indigo synthesis, 2-nitrobenzaldehyde condenses with acetone in basic aqueous solution to yield indigo in a one-pot synthesis.[10][11][12][13] The method was abandoned in the early part of the 20th century, being replaced by routes from aniline.[14]
Baeyer-Drewson Indigo Synthesis
Given its two relatively reactive groups, 2-nitrobenzaldehyde is a potential starting material for other compounds. Substituted 2-nitrobenzaldehydes can also be used to yield other important compounds based on indigo, such as Indigo carmine.
2-Nitrobenzaldehyde has been shown to be a useful photoremovable protecting group for various funcionalities.[15]