Synthesis of para-Benzoquinone
From hydroquinone Using CrO3 
Cool a solution of 33 g (0.33 mol) of hydroquinone in 150 ml of 60% acetic acid contained in a 600 ml beaker to below 5 °C in an ice bath. Dissolve 42 g (0.42 mol) of chromium trioxide (CAUTION !!!) in 70 ml of water, and add 30 ml of glacial acetic acid. By means of a separatory funnel with bent stem and supported over the beaker, add the chromium trioxide solution to the mechanically stirred hydroquinone solution at such a rate that the temperature does noot rise above 10 °C; the addition takes 2 hours. Filter the mixture at once and wash the quinone several times with 10 ml portions of ice cold water. Spread the material upon filter paper until dry but no longer or the quinone will be lost through sublimation. The yield of quinone (a bright yellow crystalline solid) is 21 g (66 %), M.P.: 115 °C. It darkens when exposed to light.
Impure quinone may be purified by placing it in a distilling flask attached to a condenser and passing a rapid steam into the flask: the quinone sublimes and collects in the receiver. It is separated from the water by filtration and dried; the M.P. is 116 °C. The vapour has a penetrating odour and attacks the eyes.
From Hydroquinone Using V2O5/NaClO3 
In a 1-litre round-bottomed flask, provided with a mechanical stirrer, place 0.5 g of vanadium pentoxide (catalyst), 500 ml of 2 % sulphuric acid, 55 g (0.5 mol) of hydroquinone and 30 g of sodium chlorate. Stir the mixture vigorously for about 4 hours. Greenish-black quinhydrone is first formed and this is converted into yellow quinone; the temperature of the mixture rises to about 40 °C (do not allow it to exceed this temperature). Cool the flask in running water, filter the mixture at the pump and wash it with 50 ml of cold water. Spread the material upon filter paper until dry (but no longer or the quinone will be lost through sublimation) or dry in a dessicator over anhydrous calcium chloride. The yield is 45 g (83 %); M.P.: 111-112 °C. The crude quinone may be purified by steam distillation as in the method above, or by recrystallisation from boiling light petroleum, B.P.: 100-120°C (12 ml per gram): the resulting pure, bright yellow quinone has a M.P.: 115°C and recovery is about 95%.
From Hydroquinone Using V2O5/NaClO3 (another variation) 
110g (1 mole) of hydroquinone, 60g (0.56 mole) of sodium chlorate and 0.5g of vanadium pentoxide were placed in a 2-liter flask with 1 liter of 2% sulfuric acid. The mixture was stirred by means of an electric motor for four hours, at the end of which time the hydroquinone had been completely converted to light yellow quinone. Because the temperature of the mixture rose to about 40°C during the course of the reaction the flask was cooled in an ice mixture. The yellow solid was brought upon a filter, washed with 100 cc. of cold water and dried in a desiccator over calcium chloride yielding 90g of light yellow quinone melting at 110-112°C. Extraction of the combined filtrate and washings with four 100 ml portions of benzene gave an additional 14g of quinone. On recrystallization from ligroin (bp 90-120°C) a 90% yield (97g) of pure quinone melting at 111-113°C was obtained.
From Hydroquinone using Sodium Dichromate 
In a 100 mL beaker dissolve 2 g Hydroquinone in 40 mL water at 50°C. Cool this solution to 20° C and slowly add 2 g of conc. H2SO4 (the beaker must be immersed in a cool water bath to prevent overheating). Then drop by drop add a Na2Cr2O7 solution (prepared from 2.8g sodium dichromate and 1.5 ml water) to the beaker and stir constantly with glass rod. The temp must not exceed 30° C. first you will see green/black precipitate which then turns yellow. The mixture is then cooled to 10° C, precipitate is filtered out with Buchner funnel and recrystalized from little benzene (approx. 10 mL). The melting point of pure p-Benzoquinone is 115°C.
From hydroquinone using Potasium Dichromate [by Spiceboy]
In a 500 ml Erlenmeyer flask, place 23 grams of hydroquinone. Add 17.5ml 93% H2SO4 *in* 250ml dH2O, with gentle warming until the hydroquinone dissolves completely. When it does, plunge the flask into an icebath with vigorous swirling, to obtain a very fine suspension of crystals. Upon removal of the flask from the ice bath, add 35g of K2Cr207 (dissolved in 60 ml dH2O) in about five portions with swirling, slowly. It'll take about 5-10 minutes. You may need to monitor the reaction temp to keep it at 25°C or so.
In the early stage of oxidation, a precipitate separates that is a green-blackish color, this is the so-called quinhydrone complex, and is a good sign. In the later stages, as the reaction completes, the color of the precip changes to yellow-green, almost day-glo-ish. Elapsed time; 65 min. Cool mixture to 5 degrees. Filter crystals on a buchner. They consist of benzoquinone. Wash the crude crystals with a SMALL portion of ice-cold dH2O. This is good enough to use as is, but sublimation is a straight-forward alternative for you purity freaks. The bright yellow crystals decolorize upon standing, so its probably a good idea to make this on an as-needed basis. Yields: about 14g or 60%+
From Hydroquinone using hydrogen peroxide 
55 g of hydroquinone, 115 g of isopropyl alcohol as a reaction medium, and 1 g of iodine as a catalyst were charged into a 300 ml four-necked glass reaction vessel with a stirrer, a dropping funnel, and a thermometer. Then stirring was started, and after 58 g of a 35% hydrogen peroxide solution were added over about 3 hours while keeping the temperature at 30-35°C, the temperature was raised to about 45°C, which was kept for about 3 hours to complete the reaction. After the completion of the reaction, the reaction mixture was cooled gradually to about 15°C, and the reaction product was filtered by suction. The reaction product was washed with a small amount of isopropyl alcohol, and dried under reduced pressure to yield 49.5 g of p-benzoquinone. The yield was 91.6% (based on the charged hydroquinone, hereinafter the same being applied), the melting point was 111-112°C, and the purity as measured by HPLC was 99.0%.
The filtered mother liquor containing isopropyl alcohol that had been obtained by filtering the reaction product by suction was distilled under normal pressures or reduced pressure in a usual manner to recover aqueous isopropyl alcohol containing about 12% of water. The recovered isopropyl alcohol can be used again as a reaction medium in the next reaction.
From Aniline using Potassium Dichromate 
A solution of 20g of aniline in a mixture of water (600 ml) and concentrated sulfuric acid (160g, 80ml) is placed in a stout beaker immersed in ice-cold water and continous stirring is begun. During the course of an hour 20g of finely powdered potassium dichromate are added in portions of about 1g at a time, care being taken that the temperature does not rise above 10°C. An even better method is to add a solution of 20g of sodium dichromate in 100 ml of water from a dropping funnel. In either case the mixture is left in a cool spot overnight, and then a further quantity of potassium dichromate (33g), or a solution of sodium dichromate (40g in 200 ml of water), added under conditions similar to the above. After four or five hours the mass is extracted three times with ether, the ethereal solution dried with calcium chloride, and the ether removed by evaporation. The crude quinone is purified by distillation in steam, or by sublimation, and forms orange-yellow needles with a characteristic pungent odour. Melting-point, 116°. Yield, 19g (82% of theory).
A: H2O2 (60%) (10 ml, 0.22 mol) was added dropwise with stirring to a solution of dihydroxybenzene (0.20 mol), I2 (10 mmol), and concentrated H2SO4 (10 ml) in MeOH (600 ml) at RT. After 4h part of the formed quinone crystallized and was collected by filtration. The methanolic solution was poured in 100 ml of water and extracted with ether, and the quinones were purified by flash chromatography on silica gel (hexane-ethyl acetate, 3:1). Yield: 97%
B: A procedure similar to A has been utilized with dihydroxybenzenes, which have sufficient solubility in water, such as hydroquinone, 2-methylhydroquinone, and 2,6- dimethylhydroquinone, with the only difference that water was used instead of MeOH. In this way the quinone directly crystallized from the aqueous solution and was easily separated by simple filtration. Yield: 93%
E: The same procedure as in A was used, with the difference that 10 mmol of Br2 was utilized instead of I2. Yield: 52%
From Hydroquinone using Nitrogen oxide [6,7]
A suspension of hydroquinone in DCM under an oxygen atmosphere remained essentially intact, even after prolonged stirring at RT. However, upon the addition of catalytic amounts of nitrogen dioxide the slurry turned spontaneously into a clear (homogeneous) solution over the course of 1 day, and pure 1,4 benzoquinone was isolated in 94% yield.
Hydroquinone itself reacted somewhat slowly due to the initial precipitation of dark-blue quinhydrone crystals, which in turn disappeared upon continued stirring at RT to afford a clear yellow solution of benzoquinone in quantitative yield.
General procedure: Hydroquinone (100 mmol) is suspended in DCM (95 ml) and the flask purged with O2. The side arm of the flask is equipped with an O2-filled balloon to maintain the oxygen atmosphere. The mixture is cooled in a ice-acetone bath to approx. -10°C, and a solution of NO2 in DCM (5 ml, 0.2M) is added with the aid of a hypodermic syringe. The reaction mixture is stirred until the slurry yields a clear yellow solution. Evaporation of the solvent in vacuo leads to crystalline quinone which upon GC and NMR analysis is found to be free of impurities (>1%)
By Catalytic oxidation of hydroquinones using NaNO2/HCl 
This one can't bee beated for price/quantity ratio:
General procedure: Hydroquinone 9 (24.4g, 100 mmol) and sodium nitrite (345mg, 5 mmol) were suspended in DCM (100 ml) in 250 ml flask equipped with a side arm. The flask was purged with O2 and stoppered with a rubber septum and an O2-filled balloon attached to the side arm. The flask was cooled in an ice-acetone bath (approximately -10°C), and 1 drop of concentrated HCl added. The reaction mixture was stirred for 3h (hydroquinone 24h), at which time a clear yellow solution was obtained. It was dried over MgSO4 and filtered; the removal of solvent gave quinone 9q in quantitative yield (24g) (benzoquinone, 94% yield).
By Oxidation of Hydroquinone using (NH4)2Ce(NO3)6 (CAN) and (NH4)2Cr2O7 on SiO2 
Oxidation of Hydroquinone by the Cerium(IV)/SiO2 Reagent; Typical Procedure
Hydroquinone (0.11g, 1 mmol) in dichloromethane (15mL) is added with stirring to cerium(IV)/SiO2 reagent [6.94g, 2.1 mmol of cerium(IV)]. The mixture was stirred for 5 minutes and filtered. The residue is washed with dichloromethane (2 × 15mL). Evaporation of solvent from the combined filtrates gives product (0.106g) as a bright yellow crystalline solid which is found to be pure p-benzoquinone by 1H-N.M.R. (authentic sample from Aldrich). It is furthes purified by recrystallization from ether; mp 116°C.
Repetition of the oxidation on a 5 mmol scale yields 0.525g of p-benzoquinone.
Oxidation of Hydroquinone by the Chromium(VI)/SiO2 Reagent
Hydroquinone (0.11g, 1 mmol) in dichloromethane (10mL) is added with stirring to the chromium(VI)/SiO2 reagent [2.07g, 2.4 mmol of chromium(VI)]. The mixture is stirred for 5 minutes and filtered. The residue is washed with dichloromethane (2 × 15mL). Evaporation of solvent from the combined filtrates gave pure p-benzoquinone (0.104g). A 5 minute reaction with 5 mmol of hydroquinone yields 0.52g of pure p-benzoquinone.
Preparation of Cerium(IV)/SiO2 and Chromium(VI)/SiO2 Reagents
To an efficiently stirred solution of ceric ammonium nitrate (20g) in MeOH (200mL), dichloromethane (200mL) followed by silica (200g, Baker Analyzed, 40-140 mesh) is added. The mixture is stirred for additional 15 minutes and the solvents are evaporated on a rotavapor. The resulting solid is dried for 3 h at 55° C on rotavapor, when free-flowing yellow powder is obtained. At this stage the yellow powder of cerium(IV)/SiO2 reagent weights 221.2g, suggesting that 1.2g of the solvent is retained. It is used as such for the oxidations. For this reagent 6.06g contain 1 mmol of cerium(IV).
A similar procedure using ceric ammonium nitrate (40g), silica (200g), MeOH (300mL) and dichloromethane (200mL) gives the 20% cerium(IV)/SiO2; yield: 242g [3.32g of the reagent contain 1 mmol of cerium(IV)].
Ammonium dichromate (40g) is dissolved in warm (50° C) 50:1 MeOH/H2O (600mL MeOH and 12mL H2O) and to this silica (200g) is added with stirring. After 10 minutes, the solvent is evaporated and the residue is dried for 10 h at 55° C on rotavapor. The 20% chromium(VI)/SiO2 reagent is obtained as a free-flowing orange powder; yield: 250.4g [0.79g of the reagent contain 1 mmol of chromium(VI)].
Other MethodsOrganic Syntheses: Vanadium Pentoxide/Sodium Chlorate oxidation of Hydroquinone
Organic Syntheses: Sodium Dichromate oxidation of p-Hydroxyaniline
Organic Syntheses: Sodium Dichromate oxidation of Hydroquinone
Alkyl Quinones from Alkylphenols - J. Org. Chem. 48, 2932-2933 (1983)
Catalytic Oxidation of para-Substituted Phenols with NO2/O2 - Tet. lett. 39, 2261-2264 (1998)
 A. I. Vogel, Practical Organic Chemistry, 5th Ed, p1025