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My organic chem teacher recently mentioned the Mulliken-Barker test for Nitro compounds in a passing reference, mainly as some trivia as we went through Tollen's test for Carbonyls. I tried finding some info on the web and all I seemed to get was testing for nitrobenzene. I wanted to know if aliphatic nitro compounds also give the Mulliken Barker test. Some additional info on the mechanism of the test will also be appreciated!

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I wanted to know if aliphatic nitro compounds also give the Mulliken Barker test. Some additional info on the mechanism of the test will also be appreciated!

Since Mulliken-Barker test is based on reducing organic nitro compounds to their corresponding hydroxylamine first and then oxidizing it back to nitro group using Tollens reagent, theoretically the test should worked for both aliphatic and aromatic nitro compound likely. For instance following is the mechanism for the test for nitrobenzene (the gray-black precipitate indicates the positive test):

Mechanism of Mulliken-Barker test

However, the test is not clear cut for the presence of nitro group, because it can interfere with some other functional groups, which are present in the molecule (e.g., see Ref.1). Although the Ref.1 has given reliable solution(s) to address some interfering functional groups, these solution(s) would apply to only aromatic nitro-compounds.

The obstacle here is getting aliphatic nitro-compounds to reduce to their corresponding hydroxylamine cleanly, so that positive test is possible. Unlike nitroarenes, its aliphatic counterparts may not undergo clean transformation to corresponding hydroxylamines by $\ce{Zn/NH4Cl}$ or $\ce{Zn/CaCl2}$ reducing agent usually used in this test. The problem for both nitroarenes and nitroalkanes are they tend to go for full reduction to amine group, or mixture of other partially reduced byproducts, if the reaction conditions have not being maintained appropriately. For instance, nitroarenes and nitroalkanes can be easily converted to corresponding amines without going through hydroxylamine intermediate if the $\ce{Zn/NH4Cl}$ system is contaminated with common surfactants used in laboratory (e.g., glassware cleaning reagents)(Ref.2):

Chemoselective Reductions of Nitroaromatics

Also read the Ref.3 for some important aspects in converting nitro-organic reagents to their corresponding hydroxylamine derivatives.


Reference(s):

  1. Kaushik Basu, Suchandra Chakraborty, and Chandan Saha, "A closer look at the Mulliken-Barker test," Resonance 2013, 18, 845–848 (DOI: https://doi.org/10.1007/s12045-013-0108-1)(PDF).
  2. Sean M. Kelly and Bruce H. Lipshutz, "Chemoselective Reductions of Nitroaromatics in Water at Room Temperature," Org. Lett. 2014, 16(1), 98–101 (DOI: https://doi.org/10.1021/ol403079x).
  3. Min Yu, Dachen Ouyang, Liqiang Wang, and You-Nian Liu, "Catalytic Reduction of Aromatic Nitro Compounds to Phenylhydroxylamine and Its Derivatives," Molecules 2024, 29(18), Article # 4353 (30 pages) (DOI: https://doi.org/10.3390/molecules29184353).
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  • $\begingroup$ +1, but a question. You mention that nitroalkanes lack selectivity for reduction to the hydroxylamine, but then give an example where nitroarenes can also go wrong. Why exactly are nitroalkanes more vulnerable? $\endgroup$ Commented 1 hour ago

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