Sodium nitrite, Fisher Scientific

Concentrated sulfuric acid

p-phenylenediamine, recrystallized from Fisher (see Notes)

14.0N glacial acetic acid, Fisher

Sodium tetrafluoroborate, Alfa Aesar

Methanol

Diethyl ether

DI water

Concentrated sulfuric acid (15 mL) was added to a 250-mL Erlenmeyer flask with a magnetic stirbar and placed on low heat. Sodium nitrite (1.40 g, 20.3 mmol) was added in small portions to the sulfuric acid with stirring, slowly enough to prevent the formation of brown nitrogen oxides. Once the sodium nitrite had fully dissolved the reaction was cooled in an ice bath, yielding a clear yellow-brown solution.

Recrystallized p-phenylenediamine (1.00g, 9.2mmol) and glacial acetic acid (10 mL) were combined in a 100-mL beaker and mixed with a glass-stirring rod until the p-phenylenediamine had fully dissolved, yielding a dark purple solution. The solution was then transferred into a 125-mL dropping funnel.

The p-phenylenediamine solution was then added dropwise over a course of 10 minutes to the sulfuric acid reaction with mixing, yielding a dark brown solution.The reaction was allowed to proceed in the ice bath for an additional 30 minutes.

A solution was made of sodium tetrafluoroborate (2.35g, 2.14 mmol) dissolved in a minimum amount of cold DI water. The cold tetrafluoroborate solution was then added to the reaction dropwise with a plastic transfer pipet, followed by 25 mL of ice-cold DI water. The reaction was allowed to proceed for an hour in the ice bath, yielding a suspension. The off-white bis(diazonium) salt was collected on a 60 mL glass Buchner filtering funnel with medium glass frit under water-aspirator suction, and washed with about 25 mL each of ice-cold DI water, methanol and diethyl ether. The resulting solid was then dried in a vacuum desiccator and the off-white product was collected. (1.62g, 5.3 mmol, 58% yield)

• p-Phenylenediamine from Fisher Scientific was recrystallized by crushing 5.00g (46.2 mmol) of the crude material in a mortar and pestle until it was a uniform powder. The recrystallization was done because the starting p-phenylenediamine was black oxidized crystals. (Attempts to use the crude material directly in the diazotization often gave a  sludgy solution which produced a poor yield of solid tetrafluoroborate salt)

• The powder was then transferred to a 500-mL Erlenmeyer flask with large stirbar. About 300 mL of DI water was added to the flask and brought to a boil on a hot plate. Decolorizing carbon was added in small amounts to the boiling solution; if too much decolorizing carbon is added at once, the solution tends to boil over. After an ample amount of decolorizing carbon was added the solution was black and opaque, and the solution was allowed to boil for an addition 30 minutes.

• In this time a hot vacuum filtration was set up with a 500-mL Erlenmeyer, large glass stemmed funnel, folded (but not fluted) filter paper and a small portion (about 10 mL) of DI water on medium heat. This setup allows steam from the small portion of water to rise and heat the funnel and paper to prevent premature crystallization. The solution was then filtered removing the decolorizing carbon and stirbar. The resulting solution was a clear magenta color.

• The solution was allowed to cool to room temperature before being placed in an ice bath, and the sides of the flask were scratched with a glass rod to help promote crystal formation. In many cases, crystals would not form at all until the crystallization was initiated by scratching, and formed instantly on this step. The crystals formed were collected via vacuum filtration and then dried in the vacuum desiccator, yielding about 2 grams of product. The p-phenylenediamine crystallizes as the hydrate, forming fluffy plate-like crystals that look surprisingly like vermiculite. In the dessicator, the crystals effloresce, becoming a loose powder.

• The crystals collected went from a light pink to a dark magenta color due to air oxidation over the course of several days.

• The p-phenylenediamine was crushed before the recrystallization to help it dissolve in water quickly.

• The addition of sodium nitrite is done over about a 10 minute time period, to avoid the formation of visible nitrogen oxides.

• The solution that results from the addition of the p-phenylenediamine solution at the 10 minute mark resembles thin brown turkey gravy, while at 30 minutes it resembles thick brown molasses. Comparing the “brown turkey gravy” solution to the “thick brown molasses” solution, the “brown turkey gravy” solution is relatively less viscous and opaque compared to the “thick brown molasses” solution.

• The 2.35g of sodium tetrafluoroborate dissolved in about 15 mL of ice-cold DI water. Pipetting the solution into the reaction made it easier to wash the sides of vessel down of any solution that might have splashed up during mixing.

• The suspension resulting from sodium tetrafluoroborate and water addition was  caramel colored.

• We prefilled 25-mL Erlenmeyer flasks with the water, methanol and diethyl ether; then capped them and placed them in the lab refrigerator at the beginning of the synthesis to allow ample time for them to get cold for the washings.

• Safety Note. As a precaution, no solids should be isolated or allowed to become dry from the reaction until after the sodium tetrafluoroborate is added. This is a standard precaution when working with any diazonium salt. We have prepared this salt and the hexafluorophosphate salt many times without incident. Both the tetrafluoroborate and hexafluorophosphate salts decompose smoothly in a melting point tube without deflagration, and burn with a smoky orange flame on a spatula tip in a Bunsen flame without violent reaction. However, care should be taken when working with any diazotization reaction, particularly in scale-up. The reaction should not be scaled up without calorimetry data on the product.

Melting point: 135-142°C (dec.)

1H NMR (vs D₂O = 4.8 ppm, 60 MHz): 9.26 (s,1H)

Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities. J. Org. Chem. 1997, 62 (21), 7512–7515.