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Nitration of 1,2-difluorobenzene using fuming nitric acid; 1,2-difluoro-4,5-dinitrobenzene

SyntheticPage 783
DOI: 10.1039/SP783
Submitted May 22, 2015, published May 27, 2015
Richard Knighton (r.c.knighton@warwick.ac.uk), Adrian Chaplin (a.b.chaplin@warwick.ac.uk)
A contribution from Chaplin group, University of Warwick


			Reaction Scheme: <img src="/images/empty.gif" alt="" />Nitration of <span id="csm1432735918751" class="csm-chemical-name" title="1,2-difluorobenzene">1,2-difluorobenzene</span> using fuming <span id="csm1432736029015" class="csm-chemical-name" title="nitric acid">nitric acid</span><img src="/images/empty.gif" alt="" />

Chemicals Used

1,2-difluorobenzene (Fluorochem)
Nitric acid, ACS Fuming 90% (Alfa Aesar)
Sulfuric acid, AR >95%  (Fisher)

 

Procedure

A two-necked round-bottomed flask was charged with 1,2-difluorobenzene (12.6 g, 10.9 mL) and a Teflon coated magnetic follower and then cooled to 0°C using an external ice bath. To the gently stirred and cooled solution, concentrated H2SO4 (40 mL) was added, followed by the slow addition (caution: highly exothermic reaction) of fuming (90%) HNO3 (100 mL) (Note A). A reflux condenser equipped with a scrubber (Note B) was fitted and the solution warmed slowly to ambient temperature and stirred for a further 2 hours. The reaction was heated slowly (caution: evolution of gaseous nitrogen oxides) over 2 hours to 100°C (Note C) and kept at this temperature for a further 12 hours. The reaction mixture was cooled to ambient temperature (Note D) and then poured slowly onto vigorously stirred crushed ice (ca 500 g, Note E). The solid was collected via vacuum filtration (water aspirator) and subsequently washed with copious amounts of H2O (ca 500 mL) until the washings were no longer acidic. The solid was dried in vacuo (< 10-2 mbar for 10 hours) to obtain the product as an off-white microcrystalline solid (12.8 g, 56%). 

Author's Comments

Caution: Fuming HNO3 is highly corrosive and a potent oxidising agent. Extreme care must be taken when working with this compound to ensure safety (Note A).

Note A: The reaction was performed in an otherwise empty fume cupboard; all non-removable steel fittings were covered with aluminium foil to prevent corrosion from the nitric acid vapours. Full personal protective equipment (PPE) was employed in addition to standard protective equipment; full face shield, blast shield, PVC apron and butyl rubber gloves (Caution: latex and nitrile gloves could ignite upon contact with fuming HNO3).  All glassware was oven dried prior to use to ensure the absence of combustible organic solvents, and all glass joints were mated with Teflon tape in place of potentially combustible grease. Fuming HNO3 must be used to obtain the di-nitrated product. The analogous reaction using concentrated (ca. 68%) HNO3 yields only 1,2-difluoro-4-nitrobenzene.
Note B: The scrubber consisted of an adapter connected to a Drechsel bottle (1 L) using PVC tubing which was vented by bubbling through H2O (1 L). The Drechsel bottle prevents suck back of water from the H2O scrubber into the reaction flask in the event of a power failure. 
Note C: The reaction was heated using an aluminium heating block to contain the spillage in the event of breakages and to prevent potentially dangerous contact of fuming HNO3 with organic based silicone oil.
Note D: In order to minimise the evolution of nitrogen oxides, heating was ceased and the scrubber was not removed until the internal reaction pressure had begun to decrease.
Note E: The ice was contained within a conical flask which was itself immersed in a large ice bath as an additional heatsink. The solution was diluted with distilled H2O if necessary to ensure all ice had melted.

 

Data

1H NMR (CDCl3, 400 MHz, 298 K): δ 7.86 (app. t, J = 8 Hz, 2H).

13C{1H} NMR (CDCl3, 126 MHz, 298 K): δ 152.0 (dd, 1JFC = 266 Hz, 2JFC = 15 Hz), 139.4 (br.), 115.2 – 115.9 (m).   

19F{1H} NMR (CDCl3, 282 MHz, 298 K):  δ 123.49 (s).


Lead Reference

X. de Hatten, D. Asil, R. H. Friend and J. R. Nitschke, J. Am. Chem. Soc. 2012, 134, 19170: DOI: 10.1021/ja308055s

Supplementary Information

Safe handling of HNO3 (01-Safe handling of HNO3.jpg)
PPE (02-PPE.jpg)
Reaction at ambient temperatur (03-Reaction at ambient temperature.jpg)
Reaction after heating for 2 h (04-Reaction after heating for 2 hours.jpg)
Final product (05-Final product.jpg)

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Keywords: aromatics/arenes, electrophilic, fluorinated aromatics, nitration, substitution

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