1. Field of the Invention
The present invention relates to a process for producing alkyl-substituted hydroquinone compounds.
2. Description of Related Art
Hydroquinone compounds having mono-, di- or trialkyl-substituted aromatic ring have been industrially used as raw materials or intermediates for pharmaceutical or agricultural chemicals, resins, various additives, polymerization inhibitors, industrial chemicals and the like. Particularly, trimethylhydroquinone has been widely used as an intermediate for vitamin E, a pharmaceutical, a polymerization inhibitor and a resin additive. In particular, it is much in demand as an intermediate for vitamin E.
Conventionally, it is known that hydroquinone compounds having alkyl-substituted aromatic ring are produced by gaseous phase reaction in which a hydroquinone compound and an alcohol are vaporized and passed through a catalyst phase or by liquid phase reaction utilizing the Friedel-Crafts reaction. For example, JP-A-7-265710 discloses a process for producing hydroquinone compound having methylated aromatic ring by reacting, in gaseous phase, hydroquinone and methanol using basic magnesium carbonate, a fine powder of phenol resin, manganese oxalate as a catalyst. This process, however, has a problem that it needs a complicated reaction apparatus. In addition, JP-A-58-72530 discloses a process for producing trimethylhydroquinone by contacting, in gaseous phase, hydroquinone and methanol heated to 400.degree. C. with a catalyst such as iron oxide, manganese oxide, chromium oxide and the like. This process, however, has a problem that about 7% by mole of tetramethylhydroquinone is produced as a byproduct. Moreover, U.S. Pat. No. 4,060,561 discloses a process for producing trimethylhydroquinone from p-methoxyphenol and methanol.
In the known processes described above, the reaction is usually carried out in gaseous phase or in liquid phase. When the reaction is carried out in liquid phase, a strong catalyst such as a Lewis acid, phosphoric acid or the like is required causing a corrosion problem of apparatus. On the other hand, when the reaction is carried out in gaseous phase, there is a problem that the apparatus becomes complicated by the presence of preheating part, evaporation part, reaction part and condensing part and also needs to be large-sized.
It is desired for a process for producing a mixture of mono-, di- and trimethylhydroquinones from hydroquinone or mono- or dimethylhydroquinone and methanol with less production of tetramethylhydroquinone as a byproduct and allowing easier isolation and purification of trimethylhydroquinone which is a useful intermediate.
An object of the invention is to provide a process for producing hydroquinone compounds having alkyl-substituted aromatic ring from a hydroquinone compound and an alcohol without using a highly corrosive catalyst and allowing the reaction in a relatively small reactor.
An object of the invention is also to provide a process for producing a mixture of mono-, di- and trimethylhydroquinones from hydroquinone or mono- or dimethylhydroquinone and methanol with less production of tetramethylhydroquinone as a byproduct.
Under these circumstances, the present inventors have conducted an extensive study on a process for producing hydroquinone compounds having alkyl-substituted aromatic ring from hydroquinone compound sand alcohols. As the result, the present inventors have found that the above problems can be resolved by reacting hydroquinone compounds with alcohols which are in a supercritical state and thus completed the present invention.