the present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included herein . in this specification and in the claims which follow , reference will be made to a number of terms which shall be defined to have the following meanings . the singular forms “ a ”, “ an ” and “ the ” include plural referents unless the context clearly dictates otherwise . “ optional ” or “ optionally ” means that the subsequently described event or circumstance may or may not occur , and that the description includes instances where the event occurs and instances where it does not . as used herein the term “ polycarbonate ” refers to polycarbonates incorporating structural units derived from one or more dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates . as used herein , the term “ melt polycarbonate ” refers to a polycarbonate made by the transesterification of at least one diaryl carbonate with at least one dihydroxy aromatic compound . “ bpa ” is herein defined as bisphenol a and is also known as 2 , 2 - bis ( 4 - hydroxyphenyl ) propane , 4 , 4 ′- isopropylidenediphenol and p , p - bpa . as used herein the terms “ aryl ” and “ aromatic radical ” refer to a radical having a valence of at least one and comprising at least one aromatic ring . examples of aromatic radicals include phenyl , pyridyl , furanyl , thienyl , naphthyl , phenylene , and biphenyl . the term includes groups containing both aromatic and aliphatic components , for example a benzyl group , a phenethyl group or a naphthylmethyl group . the term also includes groups comprising both aromatic and cycloaliphatic groups for example 4 - cyclopropylphenyl and 1 , 2 , 3 , 4 - tetrahydronaphthalen - 1 - yl . as used herein the terms “ alkyl ” and “ aliphatic radical ” refer to a radical having a valence of at least one and consisting of a linear or branched array of atoms which is not cyclic . the array may include heteroatoms such as nitrogen , sulfur and oxygen or may be composed exclusively of carbon and hydrogen . examples of aliphatic radicals include methyl , methylene , ethyl , ethylene , hexyl , hexamethylene and the like . as used herein the terms “ cycloalkyl ” and “ cycloaliphatic radical ” refer to a radical having a valance of at least one and comprising an array of atoms which is cyclic but which is not aromatic , and which does not further comprise an aromatic ring . the array may include heteroatoms such as nitrogen , sulfur and oxygen or may be composed exclusively of carbon and hydrogen . examples of cycloaliphatic radicals include cyclopropyl , cyclopentyl cyclohexyl , 2 - cyclohexylethy - 1 - yl , tetrahydrofuranyl and the like . the present invention relates to a method of transforming a phenol 1 into a mixture of a p - bromophenol ii and a p - benzoquinone iii wherein , in each of structures i , ii and iii , r 1 is independently at each occurrence a c 1 - c 20 alkyl group , c 4 - c 20 cycloalkyl group , or a c 4 - c 20 aryl group , and n is an integer from 0 to 4 . the product mixture may also comprise a corresponding o - bromophenol by - product , as well as more highly brominated by - products . as used herein , “ ortho -” refers to the 2 - or 6 - position on the aryl ring relative to the carbon in the 1 - position attached to the hydroxy group . likewise , “ meta -” refers to the 3 - or 5 - position , and “ para -” refers to the 4 - position . furthermore , “ o -” refers to ortho -; “ m -” refers to meta -; and “ p -” refers to para -. the present invention also relates to a method for preparing 4 , 4 ′- dihydroxybiphenyl compounds and 1 , 4 - hydroquinone derivatives from the mixture comprising a p - bromophenol ii and a p - benzoquinone derivative ti . an unexpected advantage of the present invention is that conversion of the p - benzoquinone derivative to the desired 1 , 4 - hydroquinone can be conducted in the presence of the p - bromophenol ii , as well as any other brominated by - products , in the same piece of equipment , without degradation of the bromophenols . furthermore , the 1 , 4 - hydroquinone product , as well as the p - bromophenol , may then each be separated from the resulting mixture . the isolated p - bromophenol may be reductively coupled to form the desired dihydroxybiphenyl compound . in addition , many of the reagents used in the process , such as the copper catalyst , unreacted hydroxyaromatic compound , and unreacted hbr may also be recycled into a subsequent process . thus , in one embodiment of the present invention a product mixture comprising p - bromophenol ii and benzoquinone iii is subjected to a separation step to provide p - bromophenol ii and p - benzoquinone iii in purified form . a significant advantage of the method of the present invention over known methods is that it provides the intermediate p - benzoquinone iii which may be transformed under very mildly reducing conditions to hydroquinone derivative iv wherein r 1 and n are defined as in structures i - iii . purified p - bromophenol ii may be transformed via reductive coupling into 4 , 4 ′- dihydroxybiphenyl derivative v wherein r 1 and n are defined as in structures i - ii . physical methods which may be used to separate a mixture comprising p - bromophenol ii and p - benzoquinone iii into purified forms of h and iii include fractionation by distillation , vacuum distillation , steam distillation , sublimation , crystallization , zone refining and like techniques , or a combination of one or more of such techniques . typically , fractional distillation and crystallization are preferred . in one embodiment , the mixture of p - bromophenol ii and p - benzoquinone iii prepared by the method of the present invention is separated by fractional distillation and recrystallization into purified forms of p - bromophenol ii and p - benzoquinone iii having purities in excess of 90 percent , preferably in excess of 95 percent , and still more preferably in excess of 98 percent . in one embodiment a mixture comprising 4 - bromophenol ( cas no . 106 - 41 - 2 ) and 1 , 4 - benzoquinone ( cas no . 105 - 51 - 4 ) prepared from phenol using the method of the present invention , is separated by fractional distillation and recrystallization of the resultant distillation fractions to provide purified 4 - bromophenol and 1 , 4 - benzoquinone having purities in excess of 90 percent . in another embodiment , the mixture comprising p - bromophenol ii and p - benzoquinone iii is subjected to mild reduction of the p - benzoquinone component to hydroquinone iv prior to separation from p - bromophenol ii and other by - products . reduction methods for transforming benzoquinones into the corresponding hydroquinones are well known in the art and include catalytic reduction using hydrogen and a noble metal catalyst , for example , reduction with hydrogen gas using palladium on carbon as the catalyst at ambient temperature and pressure . other reduction catalysts include , but are not limited to , raney nickel , palladium on alumina , palladium on barium carbonate , palladium on barium sulfate , palladium on strontium carbonate , palladium on calcium carbonate , and the like . the method of the present invention is carried out by first contacting a hydroxyaromatic compound with hydrogen bromide ( hbr ), a source of copper , optionally an organic solvent , and oxygen gas at a temperature in a range between about 20 ° c . and about 250 ° c ., preferably between about 30 ° c . and about 150 ° c ., and even more preferably between about 40 ° c . and about 100 ° c . the amount of hydrogen bromide plays a key role in producing high levels of benzoquinone product . the amount of hydrogen bromide should range from about 0 . 01 to about 0 . 2 moles , preferably range from about 0 . 02 to about 0 . 15 moles , and still more preferably range from about 0 . 04 to about 0 . 1 moles of hydrogen bromide per mole of the hydroxyaromatic compound . hydrogen bromide may be employed in any form ; examples include gaseous hbr , aqueous hbr ( hydrobromic acid ), and hbr in solution in a polar solvent , such as acetic acid , for example . hydrobromic acid may be employed at any concentration , including the commercially available 48 % aqueous solution ( i . e . a mixture of hydrogen bromide and water comprising about 48 percent by weight hbr and about 52 percent by weight water ). typically , the hydroxyaromatic compound is a phenol bearing no substituent in the position para to the phenolic oh group , for example structure i wherein r 1 is independently at each occurrence a c 1 - c 20 alkyl group , c 4 - c 20 cycloalkyl group , or a c 4 - c 20 aryl group , and n is an integer from 0 to 4 . examples of phenols having structure 1 include phenol , o - cresol , m - cresol ; 2 , 6 - xylenol ; 2 , 5 - xylenol ; 2 , 3 , 5 - xylenol ; 2 , 3 , 6 - xylenol ; 2 - ethylphenol , 2 - propylphenol , 2 - tert - butylphenol , 2 - phenylphenol , and the like . typically , phenol and o - cresol are preferred . the method of the present invention requires only catalytic amounts of copper but typically involves the use of a substantial stoichiometric excess of oxygen . typically , the hydroxyaromatic compound is reacted in the presence of aqueous hbr , a copper species , an organic solvent ( optional ), and oxygen in a reaction vessel equipped for intimate mixing of the reactants and operation at pressures higher than atmospheric pressure . in one embodiment , the reactants other than oxygen are first charged to a batch reaction vessel and stirring is begun . subsequently a stoichiometric excess of oxygen gas is introduced into the reaction vessel to provide a partial pressure of oxygen over the reaction mixture ranging from about 1 atm to about 140 atm , preferably from about 3 atm to about 100 atm , and still more preferably from about 7 atm to about 34 atm . in an alternate embodiment , the reactants are introduced into a flow reactor , for example a continuous stirred tank reactor , agitated and exposed to oxygen under conditions corresponding to partial pressure of oxygen over the reaction mixture ranging from about 1 atm to about 140 atm , preferably from about 3 atm to about 100 atm , and still more preferably from about 7 atm to about 34 atm . in almost any embodiment of the invention , the oxygen gas may be used advantageously in the form of pure oxygen , air , an oxygen - enriched air mixture , a synthetic mixture of oxygen and one or more other gases ( for example , a synthetic mixture of oxygen and nitrogen ), or any other convenient source of oxygen which may be used as the stoichiometric oxidant . reaction vessels for use according to the method of the invention include stirred tank reactors , continuous stirred tank reactors and the like . as mentioned , the source of copper employed is present in a catalytic amount and can be any copper source capable of producing copper ions under the reaction conditions . thus , even elemental copper or a mixture of copper compounds may be used according to the method of the invention . as used herein , the term “ copper catalyst ” and “ source of copper ” have the same meaning and are used interchangeably . it has been found most convenient , however , to employ a catalytic amount of copper in the form of a single soluble copper compound , such as a copper halide . the initial oxidation state of the copper compound employed does not appear to be critical , so that cuprous halides and cupric halides may be employed with reasonable interchangeability . typically , the source of copper will be selected from the group consisting of cuprous chloride , cuprous bromide , cuprous iodide , cupric chloride , cupric bromide , and cupric iodide . owing to their greater overall stability cupric halides are typically preferred . typically , the source of copper employed is used in an amount sufficient to provide a concentration of copper ion in the reaction mixture in an amount ranging from about 0 . 001 to about 0 . 200 moles , preferably from about 0 . 01 to about 0 . 1 moles , and still more preferably from about 0 . 03 to about 0 . 07 moles of copper ion per mole of hydroxyaromatic compound . the method of the present invention may be advantageously carried out in the presence of an organic solvent which may be a pure solvent , or a mixture of solvents . typically it is preferred that the organic solvent comprise an organic nitrile solvent , for example acetonitrile . organic nitrile solvents are illustrated by acetonitrile , propionitrile , butyronitrile , isopropyinitrile , benzonitrile , and mixtures thereof . in some embodiments an organic nitrile solvent is used as a mixture with one or more solvents selected from the group consisting of aliphatic ethers , aromatic ethers , aliphatic alcohols , aromatic alcohols , ketones , halogenated alkanes , halogenated aromatics , amides , aliphatic hydrocarbons , and aromatic hydrocarbons . typically , the organic solvent is present in an amount ranging from about 0 . 01 to about 1 . 0 liters , preferably from about 0 . 01 to about 0 . 5 liters , and sill more preferably from about 0 . 1 to about 0 . 4 liters of organic solvent per mole of hydroxyaromatic compound employed . in addition , it is often beneficial to add water to the reaction mixture . in this embodiment , water is added to the reaction mixture in an amount corresponding to about 0 . 4 moles to about 5 moles , preferably from about 1 mole to about 5 moles , and still more preferably from about 1 mole to about 3 moles of water per mole of hydroxyaromatic compound . in one embodiment , phenol is reacted under the conditions of the present invention to provide a mixture of 4 - bromophenol with 1 , 4 - benzoquinone . this embodiment of the present invention is practiced by contacting in a reaction mixture at a temperature in a range from about 20 ° c . to about 150 ° c ., phenol with hydrogen bromide , at least one source of copper selected from the group consisting of cupric bromide or cuprous bromide , acetonitrile ( optional ); and oxygen gas . the hydrogen bromide is present in an amount corresponding to less than 0 . 2 moles of hydrogen bromide per mole of phenol . the source of copper is present in an amount ranging from about 0 . 01 to about 0 . 20 moles of copper per mole of phenol . the acetonitrile , when present , is present in an amount ranging from about 0 . 01 to about 1 . 0 liters of acetonitrile per mole of phenol , and the “ contacting ” is carried out under a partial pressure of oxygen ranging from about 1 to about 140 atm . the product obtained is a mixture comprising 4 - bromophenol and 1 , 4 - benzoquinone . in an alternate embodiment , o - cresol is reacted under the conditions of the present invention to provide a mixture of 4 - bromo - 2 - methylphenol with 2 - methyl - 1 , 4 - benzoquinone . this embodiment of the present invention is practiced by contacting in a reaction mixture at a temperature ranging from about 20 ° c . to about 150 ° c ., o - cresol with : hydrogen bromide , at least one source of copper selected from the group consisting of cupric bromide or cuprous bromide , acetonitrile ( optional ), and oxygen gas . the hydrogen bromide is present in an amount corresponding to less than 0 . 2 moles of hydrogen bromide per mole of o - cresol . the source of copper is present in an amount ranging from about 0 . 01 to about 0 . 20 moles of copper per mole of o - cresol . the acetonitrile , when present , is present in an amount ranging from about 0 . 01 to about 1 . 0 liters of acetonitrile per mole of phenol , and the “ contacting ” is carried out under a partial pressure of oxygen ranging from about 1 to about 140 atm . the product obtained is a mixture comprising 4 - bromo - 2 - methylphenol and 2 - methyl - 1 , 4 - benzoquinone . typically , the percentage of a phenolic compound having structure 1 , converted to p - bromophenol ii and p - benzoquinone iii is at least 20 percent , preferably at least 25 percent , and still more preferably at least 30 percent . typically , the selectivity for the production of p - bromophenol ii is in a range from about 20 to about 80 percent . this means that for every mole of phenolic compound 1 converted to products , between about 0 . 2 and about 0 . 8 moles of a p - bromophenol ii is produced . correspondingly , the selectivity for the production of p - benzoquinone iii is in a range from about 80 to about 20 percent . other products , for example 2 - bromophenols , or more highly brominated products , may be formed during the reaction such that the sum of the selectivities for the p - bromophenol ii and the p - benzoquinone iii need not be 100 %. as previously mentioned , the method of the present invention described above produces a product mixture comprising a p - bromophenol ii and a 1 , 4 - benzoquinone iii derivative iii . the source of copper is also included in the product mixture . however , in another aspect of the invention , the 1 , 4 - benzoquinone derivative iii is subsequently reduced to the corresponding 1 , 4 - hydroquinone derivative iv in the presence of the p - bromophenol product 11 . in this embodiment , prior to reduction of the 1 , 4 - benzoquinone derivative iii , the source of copper may easily be removed from the product mixture , thereby eliminating the need to isolate the benzoquinone from the mixture . beneficially , the copper catalyst may then be reused in a subsequent oxybromination reaction of hydroxyaromatic compound 1 , such as that previously described . removal of the copper catalyst from the product mixture may be facilitated by adding additional hbr and water to create two phases . typically , aqueous hbr ( 48 wt . %) is added , as well as additional water . in this case , the total amount of hbr ( 48 wt . %) and water added to the product mixture generally does not exceed 20 % ( up to about 20 %) of the weight of the initial copper - containing reaction mixture . in addition , the ratio of 48 % hbr to water that is added typically ranges from about 1 : 1 ( 24 % hbr ) to about 1 : 9 ( 4 . 8 % hbr ). however , as will be obvious to one of ordinary skill in the chemical arts , the amounts of hbr and water added to the product mixture may be adjusted accordingly when other forms of hbr are employed . the copper catalyst , unused hbr and water reside predominantly in the aqueous phase , and the oxybromination products , i . e . p - bromophenol ii , as well as any corresponding o - bromophenol by - products , and the 1 , 4 - benzoquinone derivative , reside predominantly in the organic phase . the two phases may then be separated using conventional techniques , such as by decantation , or by using a separatory funnel , a separation tank , or a continuous separation colurnn , for example , and the organic phase is removed . the aqueous phase comprising the source of copper and unreacted hydrogen bromide may then be recycled , all or in part , into a subsequent oxybromination reaction , such as previously described . often , to avoid the accumulation of water , a portion of the water may be removed from the aqueous phase prior to recycling . this can be done by commonly known techniques , such as by vacuum distillation . often an organic solvent , such as one or more of the organic nitriles previously listed , may be present with the reactants in the initial oxybromination reaction , as well as in the product mixture . in this embodiment , prior to the addition of hbr and water , at least a portion of the organic solvent may optionally be removed from the product mixture , typically by vacuum distillation , in order to obtain phase separation . however , to facilitate evaporation and removal from the product mixture , the organic solvent should have a boiling point lower than that of water , such as acetonitrile , which has a boiling point of 81 - 82 ° c . advantageously , the 1 , 4 - benzoquinone iii included in the organic phase can then be reduced to the corresponding 1 , 4 - hydroquinone iv without separating the 1 , 4 - benzoquinone derivative iii from the p - bromophenol ii or any by - products . the reduction may be conducted using conventional techniques such as by using hydrogen gas as the reducing agent , and a metal catalyst at ambient temperature and pressure . as used herein , the term “ metal catalyst ” means a reduction catalyst , such as palladium or nickel . examples include , but are not limited to , raney nickel , palladium on carbon , palladium on alumina , palladium on barium carbonate , palladium on barium sulfate , palladium on strontium carbonate , palladium on calcium carbonate , and other supported palladium catalysts . typically , the metal catalyst will be heterogeneous , but need not be . excess hydrogen gas is introduced into the reaction vessel to provide a partial pressure of hydrogen over the reaction mixture ranging from about 1 atm to about 140 atm , preferably from 2 atm to about 100 atm , and still more preferably from about 3 atm to about 34 atm . after this step , a mixture of the p - bromophenol ii and the 1 , 4 - hydroquinone derivative iv remains in the reaction vessel . the reduction catalyst may then be removed , typically by filtration , and may be reused in a subsequent reduction of benzoquinone , if desired . in one embodiment , when the hydroxayaromatic compound i is phenol , the desired hydroquinone derivative iv is 1 , 4 - hydroquinone . in another embodiment , the hydroxayaromatic compound 1 is o - cresol , and the hydroquinone derivative iv is 2 - methyl - 1 , 4 - hydroquinone . optionally , prior to the reduction of the benzoquinone derivative iii , the organic phase may be washed with water at least one time , but preferably up to three times , to remove any traces of the copper catalyst , as well as any traces of unreacted hbr . the wash water is then removed and may be combined with the aqueous phase for recycling to another oxybromination reaction . again , much of the water is typically removed prior to recycling . after the reduction of benzoquinone iii is complete , the 1 , 4 - hydroquinone derivative iv and the p - bromophenol ii may be separated from the resulting mixture . separation of these compounds into their purified forms may be effected by art - recognized methods , such as those previously listed , including fractionation by distillation , vacuum distillation , steam distillation , sublimation , crystallization , zone refining and like techniques , or a combination of one or more of such techniques . again , fractional distillation is preferred , because , for example , in one embodiment , wherein a mixture of 4 bromophenol and 1 , 4 - hydroquinone is produced from phenol , there is a difference of more than 55 ° c . between the boiling points of 4 bromophenot ( 238 ° c .) and 1 , 4 - hydroquinone ( 295 ° c .). purities in excess of 90 percent are obtained , but are preferably in excess of 95 percent , and still more preferably in excess of 98 percent . one to three distillations are generally sufficient . distillation may be conducted under reduced pressure to minimize thermal decomposition . after separation , the purified 1 , 4 - hydroquinone derivative iv obtained from the conversion of the hydroxyaromatic compound , may then be used in the synthesis of various copolymers , as previously described . the p - bromophenol ii fraction may undergo reductive coupling to form a 4 , 4 ′- dihydroxybiphenyl compound having structure v . reductive coupling reactions are fully described , for example , in u . s . pat . no . 5 , 177 , 258 . in one embodiment , wherein phenol is the hydroxyaromatic compound 1 and 4 - bromophenol is the p - bromophenol ii , the 4 , 4 ′- dihydroxybiphenyl derivative v formed is 4 , 4 ′- dihydroxybiphenyl , i . e ., biphenol . in another embodiment , when o - cresol is the starting compound , the p - bromophenol ii is 4 - bromo - 2 - methylphenol , and the 4 , 4 ′- dihydroxybiphenyl derivative v is 2 , 2 ′- dimethyl - 4 , 4 ′- dihydroxybiphenyl . the 4 , 4 ′- dihydroxybiphenyl derivative v may then be used in the synthesis of copolymers , as previously described . the following examples are set forth to provide those of ordinary skill in the art with a detailed description of how the methods claimed herein are carried out and evaluated , and are not intended to limit the scope of what the inventors regard as their invention . unless indicated otherwise , parts are by weight , temperature is in ° c . the reagents , reactants , and catalysts used in the reactions described herein are readily available materials . an amber 3 dram vial was charged with phenol ( 1 . 33 ml , 15 . 00 mmol ), 48 percent hydrobromic acid ( 0 . 118 ml , 1 . 0 mmol of hydrogen bromide ), cupric bromide ( 340 milligrams , 1 . 5 mmol ) and acetonitrile ( 3 . 18 ml ). the vial was loaded into an aluminum block , placed inside an autoclave and pressurized with air ( 1000 psi ). upon heating at 65 ° c . for 2 hours the autoclave was allowed to cool and the product mixture was analyzed by gas chromatography . the product mixture was found to contain 4 - bromophenol ( 6 . 50 % by weight ), 2 - bomophenol ( 2 . 00 % by weight ) and p - benzoquinone ( 6 . 75 % by weight ). these product concentrations corresponded to 54 percent overall conversion of phenol to products with a selectivity for 4 - bromophenol of 23 percent and a selectivity for p - benzoquinone of 37 percent . an amber 3 dram vial was charged with phenol ( 1 . 33 ml , 15 . 00 mmol ), 48 percent hydrobromic acid ( 0 . 118 ml , 1 . 0 mmol of hydrogen bromide ), cupric bromide ( 340 milligrams , 1 . 5 mmol ) and acetonitrile ( 3 . 18 ml ). the vial was loaded into an aluminum block , placed inside an autoclave and pressurized with air ( 500 psi ). upon heating at 65 ° c . for 2 hours the autoclave was allowed to cool and the product mixture was analyzed by gas chromatography . the product mixture was found to contain 4 - bromophenol ( 6 . 0 % by weight ), 2 - bromophenol ( 2 . 30 % by weight ) and p - benzoquinone ( 3 . 25 % by weight ). these product concentrations corresponded to 35 percent overall conversion of phenol to products with a selectivity for 4 - bromophenol of 30 percent and a selectivity for p - benzoquinone of 26 percent . an amber 3 dram vial was charged with phenol ( 1 . 25 ml , 14 . 00 mmol ), 48 percent hydrobromic acid ( 0 . 118 ml , 1 . 00 mmol of hydrogen bromide ), cupric bromide ( 156 milligrams , 0 . 70 mmol ), water ( 0 . 45 ml , 25 . 00 mmol ) and acetonitrile ( 3 . 18 ml ). the vial was loaded into an aluminum block , placed inside an autoclave and pressurized with air ( 68 atm ). upon heating at 65 ° c . for 2 hours the autoclave was allowed to cool and the product mixture was analyzed by gas chromatography . the product mixture was found to contain 4 - bromophenol ( 3 . 96 % by weight ), 2 - bomophenol ( 1 . 25 % by weight ) and p - benzoquinone ( 5 . 17 % by weight ). these product concentrations corresponded to 39 percent overall conversion of phenol to products with a selectivity for 4 - bromophenol of 20 percent and a selectivity for p - benzoquinone of 41 percent . an amber 3 dram vial was charged with o - cresol ( 1 . 47 ml , 14 . 00 mmol ), 48 percent hydrobromic acid ( 0 . 118 ml , 1 . 0 mmol of hydrogen bromide ), cupric bromide ( 156 milligrams , 0 . 7 mmol ), water ( 0 . 45 ml , 25 mmol ) and acetonitrile ( 2 . 91 ml ). the vial was loaded into an aluminum block , placed inside an autoclave and pressurized with air ( 68 atm ). upon heating at 65 ° c . for 2 hours the autoclave was allowed to cool and the product mixture was analyzed by gas chromatography . the product mixture was found to contain 4 - bromo - 2 - methylphenol ( 5 . 5 % by weight ), methyl - 1 , 4 - benzoquinone ( 6 . 40 % by weight ), and 6 - bromo - 2 - methylphenol ( 0 . 6 % by weight ). these product concentrations corresponded to 45 percent overall conversion of o - cresol to products with a selectivity for 4 - bromo - 2 - methylphenol of 22 percent and a selectivity for methyl - p - benzoquinone of 40 percent . an amber 3 dram vial was charged with m - cresol ( 1 . 46 ml , 14 . 00 mmol ), 48 percent hydrobromic acid ( 0 . 118 ml , 1 . 0 mmol of hydrogen bromide ), cupric bromide ( 156 milligrams , 0 . 7 nimol ), water ( 0 . 45 ml , 25 mmol ) and acetonitrile ( 2 . 92 ml ). the vial was loaded into an aluminum block , placed inside an autoclave and pressurized with air ( 68 atm ). upon heating at 65 ° c . for 2 hours the autoclave was allowed to cool and the product mixture was analyzed by gas chromatography . the product mixture was found to contain 4 - bromo - 3 - methylphenol ( 6 . 2 % by weight ), methyl - p - benzoquinone ( 15 . 84 % by weight ). these product concentrations corresponded to 67 percent overall conversion of m - cresol to products with a selectivity for 4 - bromo - 3 - methylphenol of 17 percent and a selectivity for methyl - p - benzoquinone of 65 percent . to a 3 - dram vial were charged 1 . 37 ml ( 15 . 56 mmol ) of phenol , 0 . 112 g ( 0 . 5 mmol ) of cupric bromide , 1 . 39 ml ( 12 . 50 mmol ) of 48 % hydrobromic acid and 2 . 20 ml of acetonitrile . the vial was sealed with a cap containing a hole to allow for air flow during the reaction and placed in an aluminum block . the block was placed in a 450 - ml autoclave reactor , pressurized to 34 . 0 atm with air and heated at 65 ° c . for 1 hour . it was then cooled to room temperature and depressurized . the resulting mixture was analyzed by vapor phase chromatographic analysis and shown to contain 7 . 39 % phenol , 28 . 63 % 4 - bromophenol , 5 . 59 % 2 - bromophenol and 0 . 68 % 2 , 4 - dibromophenol , corresponding to 72 % phenol conversion , with 82 % 4 - bromophenol selectivity and a total monobromophenol selectivity of 98 %. to a 3 - dram vial were charged 1 . 59 ml ( 15 . 39 mmol ) of o - cresol , 0 . 112 g ( 0 . 5 nmmol ) of cupric bromide , 1 . 48 ml ( 12 . 47 mmol ) of 48 % hydrobromic acid and 1 . 92 ml of acetonitrile . the vial was sealed and located as in example 1 , pressurized to 34 . 0 atm with air and heated at 65 ° c . for 1 hour . it was then cooled to room temperature and depressurized . the resulting mixture was analyzed by vapor phase chromatographic analysis and shown to contain 13 . 23 % o - cresol , 27 . 74 % 4 - bromo - 2 - methylphenol and 0 . 93 % 6 - bromo - 2 - methylphenol , corresponding to 56 % o - cresol conversion , with 96 % 4 - bromo - 2 - methylphenol selectivity and a total monobromophenol selectivity of 99 %. to a 3 - dram vial were charged 1 . 37 ml ( 15 . 56 umnol ) of phenol , 0 . 112 g ( 0 . 5 mmol ) of cupric bromide , 1 . 30 ml ( 6 . 85 mmol ) of a 30 % solution of hydrogen bromide in acetic acid and 2 . 31 ml of acetonitrile . the vial was sealed and located as in example 1 , pressurized to 34 . 0 atm with air and heated at 65 ° c . for 1 hour . it was then cooled to room temperature and depressurized . the resulting mixture was analyzed by vapor phase chromatographic analysis and shown to contain 14 . 12 % phenol , 19 . 15 % 4 - bromophenol and 1 . 15 % 2 - bromophenol , corresponding to 44 % phenol conversion , with 96 % 4 - bromophenol selectivity and a total monobromophenol selectivity of 100 %. results from example 1 and comparative examples 1 - 3 are gathered in table 1 and illustrate the surprising effect of acid concentration on the level of benzoquinone produced . the procedure of example 1 was followed . a fraction of the acetonitrile is then evaporated from the product mixture at 60 ° c . under vacuum , such that it represents a mass fraction of less than 20 % of the resulting remaining mixture . to the resulting mixture is added 2 g of hbr ( 48 wt . % in water ) and 3 g water , whereupon , a two - phase mixture is formed at 50 ° c . the phases are separated and analyzed . the aqueous phase contains & gt ; 98 % of the cu fed into the reactor originally . the organic phase contains most of the 2 - bromophenol , 4 - bromophenol , and 1 , 4 - benzoquinone (& gt ; 75 % of each component ). the organic phase is then washed with water , and the water wash is combined with the aqueous phase already collected . the combined aqueous phase is then partially evaporated to remove a fraction of the water . the remaining mixture , comprising water , hbr , and a cu compound , can then be used as the source of cu catalyst and hbr in a subsequent oxidationibromination reaction . after the water wash has been removed , the organic phase is then sent to a reduction reaction : pd / c catalyst is added to the organic phase to form a slurry , and the mixture is then subjected to h 2 pressure ( 6 . 8 atrn ) for 3 hours . the resulting mixture is filtered to separate the pd / c catalyst , and the liquid stream is analyzed . 1 , 4 - benzoquinone is converted to 1 , 4 - hydroquinone (& gt ; 95 %), while the bromophenols are relatively unaffected (& gt ; 95 % remaining from the mixture fed to the reduction reaction ). the invention has been described in detail with particular reference to preferred embodiments thereof , but it will be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of the invention .