Production of hydroquinones
This application is a continuation-in-part of Ser. No. 372,874, filed Apr. 29, 1982, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     It is known to oxidize phenol to p-benzoquinone in the presence of copper catalysts; see for example U.S. Pat. No. 3,987,068. Improvements to these copper catalyzed oxidations of phenol to p-benzoquinone have been disclosed and claimed in U.S. Ser. No. 284,893, filed July 20, 1981, and U.S. Ser. No. 339,965, filed Jan. 18, 1982, which disclosures are hereby incorporated by reference. 
     Heretofore, p-benzoquinone has been converted to hydroquinone by use of various catalysts. Thus, for example, Sabatier passed quinone vapors mixed with hydrogen over a reduced copper catalyst (e.g., a heterogeneous vapor phase system) to obtain hydroquinone (Comptes rendus Ac. Sc. vol. 146, p. 457, 1908 and vol. 172 p. 733, 1921), but in this process the catalyst has a very limited life. R. Cornubert and J. Phelisse, Compt. Rend. 229, 460 (1949) disclose use of Raney nickel to convert quinone to hydroquinone. U.S. Pat. No. 2,495,521 discloses nickel, cobalt, or copper catalyzed heterogeneous vapor phase hydrogenation of benzoquinone in the presence of steam. Popova et al., Chem. Abs. 53,275 (1959) disclose hydrogenation of benzoquinone with nickel, platinum and palladium--calcium carbonate. M. Calvin, J. Am. Chem. Soc., 61, 2230 (1939) discloses use of a copper-quinoline complex derived from cuprous acetate to promote hydrogenation of purified quinone, but this process is limited in that, at most, only one-half mole of hydrogen is taken up per mole of the Copper I--quinoline complex and thus the hydrogenation is very inefficient and perhaps not catalytic. Also of interest is the disclosure of Yananaka et al., Bull. Inst. Phys. Chem. Research (Tokyo) 14, 31 1935 which reduces quinones to oxy compounds with copper and Al 2  O 3  -promoted nickel. 
     BRIEF STATEMENT OF THE INVENTION 
     It has now been found that a good yield of hydroquinone and substituted hydroquinones is readily obtained when the p-benzoquinone compound formed in a reaction mass obtained by oxidation of a phenol with a copper catalyst is hydrogenated directly, i.e., without isolation, and without any additional catalyst. It is quite obvious that such a &#34;one-pot&#34; process provides a highly efficient means to obtain hydroquinone and substituted hydroquinones which are commercial chemicals much used in the photographic industry and as inhibitors used in the stabilization of rubber compounds. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Phenols and substituted phenols used in this invention are converted into substituted hydroquinones in accord with the process of the invention as illustrated by the following equation: ##STR1## where the ring substitutents: R 1 , R 2 , R 3  and R 4  may be the same or different radicals selected from the group consisting of hydrogen, halo, cyano, alkyl and alkoxy having from 1 to 12 carbon atoms, phenyl, naphthyl, phenylalkyl and alkylphenyl radicals having from 7 to 16 carbon atoms and R 1  and R 2  or R 3  and R 4  may be joined to form an aliphatic, aryl or hetero ring. When the R groups as defined above are other than hydrogen, R 1 , R 2 , R 3  and R 4  may be in any position except the para position. Specific R groups are exemplified by chloro, cyano, methyl, ethyl, propyl, butyl, isobutyl, tert-butyl, pentyl, phenyl, p-tolyl, p-anisyl, methoxy, t-butoxy, phenoxy, p-methylphenoxy, and the like. Preferred phenols useful in the process are phenol, o-chlorophenol, o-cresol, m-cresol, 2,5- and 2,6-di-t-butylphenol, 2-t-butylphenol, 2,6-dimethylphenol, and 1-naphthol. When R 1  and R 2  or R 3  and R 4  are joined the resulting 1,4 -hydroquinone will be those derived from naphthalene, quinoline, isoquinoline, chroman (dihydrobenzopyran), indole and the like. 
     The oxidation of phenol to p-benzoquinone with a copper catalyst is, as indicated, well known in the art and any of such processes may be used in the first step of this invention. Preferably a process such as that disclosed in U.S. Pat. No. 3,987,068 will be used, which disclosure is hereby incorporated by reference. Most preferably, the hydrogenation step of this invention will be used with the processes disclosed in (a) Ser. No. 284,893, filed July 20, 1981, where a divalent copper catalyst promoted with an alkali metal hydroxide is used (molar ratio of base to copper being no greater than about 2.0) and may also be further enhanced with water in an amount less than about 10% by volume of the reaction solution, and (b) Ser. No. 339,965, filed Jan. 18, 1982, where a monovalent copper catalyst promoted with water is used (preferably 1 to 4 moles of water per mole of phenol). 
     The hydrogenation step in the process will be carried out in a homogeneous, liquid phase system simply by first removing oxygen from the system and then pressuring in hydrogen to a pressure of about 500 to about 5000 psig (preferably about 1000 to about 3000) and effecting the hydrogenation at a temperature between about 100° and about 200° C., preferably from about 125° to about 175° C. 
    
    
     In order to further exemplify the process the following examples are given: 
     EXAMPLE 1 
     500 mmole of phenol is oxidized in 350 ml of acetonitrile containing 35 mmole of CuCl 2 . An oxygen containing gas (39% O 2 , 61% N 2 ) is continuously sparged through the mixture at 65° C. and 750 psi at 500 ml/hr. After four hours the reaction mixture contains four mmole of phenol, 263 mmole of benzoquinone and six mmole of p-chlorophenol (99% conv., 53% selectivity). The oxygen is flushed from the system and hydrogen gas is admitted to a pressure of 3,000 psi. The mixture is heated for fifteen hours at 175° C. to give 135 mmoles of hydroquinone, 2 mmoles unreacted p-benzoquinone, 17 mmoles catechol, 6 mmoles of p-chlorophenol, 17 mmoles of phenol and a large amount of quinhydrone (a 1:1 complex of hydroquinone and p-benzoquinone). Yield calculations are difficult because of the low accuracy of the quinhydrone analysis, but a yield of up to about 71% is believed to be achieved. 
     EXAMPLE 2 
     In an experiment similar to Example 1 except that the CuCl 2  catalyst is promoted with LiOH (one mole per mole of CuCl 2 ) selectivity to p-benzoquinone is about 70% at 99% conversion. The reaction mixture is hydrogenated by pressurizing 2000 psi of hydrogen on it at 175° C. to give a yield of hydroquinone about the same as in Example 1. 
     EXAMPLE 3 
     Similar results are obtained when the process of Example 1 is carried out with a CuCl catalyst promoted with 500 mmole of water. 
     EXAMPLE 4 
     In an experiment similar to Example 1 except that o-cresol is used in place of phenol, a good yield of methyl-p-benzoquinone is obtained which is hydrogenated to 1,4-dihydroxy-2-methylbenzene. 
     EXAMPLE 5 
     Following the detail of Example 2 except that m-cresol is used in place of phenol, 1,4-dihydroxy-2-methylbenzene is obtained in satisfactory yield. 
     EXAMPLE 6 
     When Example 1 is repeated except that 2,6-dimethylphenol is used in place of phenol, a good yield of 1,4-dihydroxy-2,6-dimethyl-benzene is obtained. 
     EXAMPLE 7 
     In an experiment similar to Example 2 except that 2,6-di-t-butylphenol is used in place of phenol, 2,6-di-t-butyl-1,4-dihydroxybenzene is obtained in good yield. 
     EXAMPLE 8 
     When Example 1 is repeated except that o-t-butylphenol is used in place of phenol tert-butylhydroquinone is obtained in a quite satisfactory yield. 
     EXAMPLE 9 
     Following the details of Example 1 except that 2,5-di-tert-butylphenol is used in place of phenol, 2,5-di-tert-butylhydroquinone is obtained in good yield.