9-Bromoanthracene: Synthesis, Impurity Analysis and Thermochemical Behavior Studies

9-Bromoanthracene is a kind of excellent fluorescent and phosphorescent luminescent material, which is widely used in optics, cosmetics, electronics, semiconductors and electroluminescent materials. The synthesis of 9-Bromoanthracene is a four step process which starts with dissolution of Anthracene in CHCl3, then N-bromosuccinimide is added, stirred and reacted continuiously. Then the CH2Cl2 solution was dried and the residue was recrystallized from anhydrous ethanol to give 9-Bromoanthracene.

Synthesis of 9-Bromoanthracene

Anthracene (50 g, 0.28 mol, 1 eq) and 1,3-dibromo-5,5-dimethylhydantoin (38.15 g, 0.133 mol, 0.475 eq) were combined in a 500 mL of ethyl acetate and brought to reflux. After about 5 min of reflux, the mixture became homogeneous, and yellow, and stayed light in color throughout the 2.5 h that it was refluxed. The reaction mixture allowed to cool and then washed with water to remove the 5,5-dimethylhydantoin. When water was added, the mixture turned greenish-blue and the color persisted through additional aqueous washes. The organic layer was dried over Na2SO4 and the greenish color gradually disappeared, and the mixture became orange-brown. The liquor was concentrated to dryness, to yield a brown solid residue. The solid was dissolved in hot THF (about 50 mL) and then slowly it was precipitated again with addition of acetonitrile (about 200 mL). The yellowish solid was isolated by filtration (36.4 g) and the process was repeated twice to yield two more crops of solid (total 23.8 g). The 3 crops were combined and analyzed by high performance liquid-chromatography (HPLC). The analysis indicated that only 89% of the reaction product was the desired 9-bromoanthracene, 9.3% was unreacted anthracene and 1% was the undesirable 9,10-dibromoanthracene. The 9,10-dibromoanthracene could not be removed by recrystallization.[1]

In the alternative process Step (b) includes preparation of 9-arylanthracene. This is illustrated for the preparation of 9-naphthylanthracene by the following procedure. 9-Bromoanthracene (5 g, 19.4 mmol, 1 eq, prepared by the procedure described above) was combined with 2-naphthylboronic acid (3 g, 17.5 mmol, 0.9 eq) in 30 mL toluene: The resulting suspension was degassed for 10 min under N2, then catalyst was added (0.5 mole %, 68 mg) followed by the addition of 10 mL of 2M Na2CO3 solution. The mixture was quickly heated to reflux, and stirred for 3 h (analysis by thin-layer chromatography (TLC) indicated the reaction was complete). The mixture was filtered hot through glass fiber filter paper, to remove Pd. The organic filtrate was separated, washed with water (twice with 25 mL) then dried over Na2SO4. The toluene extract was concentrated and left to crystallize slowly overnight. The solid formed was collected (3.37 g) and its purity solid was determined by HPLC analysis. Analysis indicated that 94% of the solid was the desired product, 9-naphthylanthracene. Three major impurities were present, including 1.2% of unreacted starting material, 9-bromoanthracene. There was 2.3% anthracene present. The presence of anthracene creates a problem because the next step in this alternative process, step 'c', involves a bromination and anthracene will brominate in both the 9- and 10-positions leading to further impurities. There was 1.3% of 9,10-di-(2-naphthyl)anthracene. This impurity is very undesirable because it is extremely difficult to remove. An attempt to remove 9,10-dinaphthylanthracene by recrystallization from heptane was unsuccessful and it continued to be present at a level of 1%.

Thermochemical and Vapor Pressure Behavior of Anthracene and Brominated Anthracene Mixtures

The present work concerns the thermochemical and vapor pressure behavior of the anthracene (1) + 2-bromoanthracene (2) and anthracene (1) + 9-bromoanthracene (3) systems. Solid-liquid equilibrium temperature and differential scanning calorimetry studies indicate the existence of a minimum melting solid state near an equilibrium temperature of 477.65 K at x1 = 0.74 for the (1) + (2) system. Additionally, solid-vapor equilibrium studies for the (1) + (2) system show that the vapor pressure of the mixtures depends on composition, but does not follow ideal Raoult’s law behaviour. The (1) + (3) system behaves differently from the (1) + (2) system. The (1) + (3) system has a solid solution like phase diagram. The system consists of two phases, an anthracene like phase and a 9-bromoanthracene like phase, while (1) + (2) mixtures only form a single phase. Moreover, experimental studies of the two systems suggest that the (1) + (2) system is in a thermodynamically lower energy state than the (1) + (3) system. A melt and quench-cool technique was used to prepare mixtures of anthracene + 2-bromoanthracene, and anthracene + 9-bromoanthracene with various compositions. The desired quantities (around 50 mg) of anthracene and 2-bromoanthracene or anthracene and 9-bromoanthracene were measured to ± 0.05 mg and sealed in a brass vessel. The vessel was heated to 498 ± 5 K in a fluidized bath and agitated for 5 min. Then the vessel was immediately placed into liquid nitrogen, which provided cooling at approximately 70–80 K·s−1 for the first 4 sec. The heating and cooling procedure was repeated 4 times.[2]

The melting behavior of the anthrance (1) + 9-bromoanthracene (3) system is different from that of the anthracene (1) + 2-bromoanthracene (2) system. This system exhibits solid-solution-like phase diagram, but it is not a real solid solution system. The thaw temperatures of mixtures with x1 < 0.5 are about 5 – 10 K lower than the melting point of the pure 9-bromoanthracene, while the thaw temperatures of mixtures with x1 > 0.5 tend to be significantly higher and increase almost linearly towards the melting point of anthracene with an increase of the mole fraction of anthracene in the mixtures. For the equimolar anthracene (1) and 9-bromoanthracene (3) mixture, two peaks appear during both the heating and cooling scan, which indicates the existence of two phases in that mixture. The high temperature crystallization peak at 421.8 K indicates the existence of one phase, while the low temperature crystallization peak, at 328.4 K indicates the existence of a second. It should be noted that the melting peak at about 370 K is associated with the low-melting phase. At x1 <0.50, this phase may described as a 9-bromoanthracene-like phase, but it is not true 9-bromoanthracene. There is no clear high temperature melting peak visible, because this other higher melting phase dissolves into the melt phase over a range of temperature. This higher melting phase is associated with the broad peak near 440K (corresponding with the liquidus temperatures. It is this phase that crystallizes at around 420 K upon cooling (421.7 K for x1 = 0.5). The shape suggests that the properties of this second phase vary continuously with composition, unlike the 9-bromoanthracene-like phase that defines the thaw point at x1 <0.50.

Two PAC mixture systems are investigated to study the influence of bromine substitution on anthracene on phase behavior and vapor pressure. The vapor pressure measurements on this system show that the phase behavior of the mixtures depends on the composition in a complicated way. For (1) + (2) mixtures with high mole fraction of anthracene (x1 > 0.7), the sublimation of the mixtures is dominated by loss of anthracene. For (1) + (2) mixtures with x1 ≤ 70, the vapor pressure falls slowly towards that of 2-bromoanthracene, achieving this value while there is still a significant amount of anthracene left in the mixture (x1 = 0.08 ± 0.02). The anthracene (1) + 9-bromoanthracene (3) system behaves quite differently from (1) + (2) system. The (1) + (3) system shows a solid solution like phase diagram, but forms two separate phases, i.e. an anthracene like and a 9-bromoanthracene like phase.

References

[1]EASTMAN KODAK - US2005/245752, 2005, A1

[2]Fu J, Suuberg EM. Thermochemical and Vapor Pressure Behavior of Anthracene and Brominated Anthracene Mixtures. Fluid Phase Equilib. 2013 Mar 25;342:10.1016/j.fluid.2012.12.036.