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Electrophilic bromination using NBS/ACN; 2,5-dibromo-1,4-dihexyloxybenzene

SyntheticPage 849
DOI: 10.1039/SP849
Submitted Sep 04, 2018, published Nov 26, 2018
Andres Rodriguez (andresrod56@gmail.com), Nicholas Marshall (nicholasm@usca.edu )
A contribution from Marshall Group @ USC Aiken
Reaction Scheme: <IMG src="/images/empty.gif">Electrophilic bromination using NBS/ACN<IMG src="/images/empty.gif">

Chemicals Used

Acetonitrile, HPLC-grade, Fisher Scientific

n-Bromosuccinimide, 99%, Acros

1,4-dihexyloxybenzene, prepared in our laboratory

Procedure

1,4-di-n-hexyloxybenzene (2.00 g, 7.18 mmol) was added to a 250 mL flat-bottomed flask with 2.68 g (15.1 mmol, 2.1 equiv) of NBS. 20 mL of acetonitrile was added. A stirbar and water-cooled condenser was added, and the mixture was brought to reflux by heating on an open stir plate. The reaction was continued under reflux for 2 hours. The mixture was allowed to cool to room temperature, causing a mass of white, needle-shaped crystals to form. The flask was cooled in a -20°C freezer and the white, crystalline product was collected by vacuum filtration on a Büchner funnel. The product was washed with cold acetonitrile until the yellow color of residual bromine was no longer visible, and dried in the air overnight. Yield; 2.71 g, 86%.

Author's Comments

  • The 1,4-dihexyloxybenzene was prepared by Williamson ether synthesis from hydroquinone and n-bromohexane in lab-grade denatured ethanol, using anhydrous potassium carbonate as the base.

  • Preparing the target material isn't difficult, but as our lab uses these substituted 1,4-dialkoxybenzenes regularly, we have found this technique to be the most convenient means of making them. The technique avoids the hazards and inconvenience of handling elemental bromine, and gives a very pure, crystalline crude product which does not require recrystallization for future use.

  • The acetonitrile used is an old bottle which is certainly contaminated with atmospheric moisture. The NBS is a powdery yellow grade which probably contains the typical traces of bromine and hydrobromic acid. We have no reason to believe that using purer grades of materials would improve the yield.

  • The yield is not diminished by allowing the reaction to proceed overnight, if it is convenient for the operator.

  • We have found that the dibromo product reacts in Suzuki coupling reactions about as well as the diiodo product (see Synthetic Page 762) and is easier to prepare. The Lead Reference contains an example of such a coupling.

Data

1H NMR (60 MHz, CDCl3 vs.TMS): ppm = 7.094 (s, 2H) 3.948 (t, 4H) 1.807 (m, 4H) 1.545 (m, 4H) 1.318 (m, 8H) 0.912 (t, 6H)
MP (open capillary, uncorr.): 56-57°C

Lead Reference

Synthesis of aryl bromides from arenes and N-bromosuccinimide (NBS) in acetonitrile — A convenient method for aromatic bromination. Canadian Journal of Chemistry, 2009, 87(2): 440-447, https://doi.org/10.1139/V08-176


Supplementary Information

1HNMR.tiff

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Get structure file (.cdx, .sk2, .mol)

Keywords: aromatics/arenes, aryl ether, bromination, carbocyclic compounds, NBS