Nitration
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Nitration occurs with aromatic organic compounds via an electrophilic substitution mechanism involving the attack of the electron-rich benzene ring by the (nitryl) nitronium ion.
Benzene is nitrated by refluxing with concentrated sulfuric acid and concentrated nitric acid at 50°C.
(1) 2H2SO4 + HNO3 → 2HSO42- + NO2+ + H3O+
(2) C6H6 + NO2+ → C6H5NO2 + H+
(3) H+ + H3O+ + 2HSO42- → H2O + 2H2SO4
The sulfuric acid is regenerated and hence acts as a catalyst.
OChem-Reaction-NitrationBenzene.png
The formation of a nitronium ion (the electrophile) from nitric acid and sulfuric acid is shown below:
Chem-Mech-NitroniumFormationWH2SO4.png
Selectivity is always a challenge in nitrations, Fluorenone nitration is selective and yields a tri-nitro compound [1] or tetra-nitro compound [2] by tweaking reaction conditions just slightly.
Other nitration reagents include nitronium tetrafluoroborate which is a true nitronium salt. This compound can be prepared from hydrogen fluoride, nitric acid and boron trifluoride [3]. Aromatic nitro compounds are important intermediates to anilines by action of a reducing agent.
References
- [1] 2,4,7-Trinitrofluorenone E. O. Woolfolk and Milton Orchin Organic Syntheses, Coll. Vol. 3, p.837; Vol. 28, p.91 Article (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv3p0837)
- [2] 2,4,5,7-tetranitrofluorenone Melvin S. Newman and H. Boden Organic Syntheses, Coll. Vol. 5, p.1029; Vol. 42, p.95 Article (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv5p1029)
- [3] Benzonitrile, 2-methyl-3,5-dinitro- George A. Olah and Stephen J. Kuhn Organic Syntheses Annual Volume 47, page 56 , Article (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv5p0480)