A process for the production of dialkyl-polysulfides is disclosed by U.S. Pat. No. 3,022,351, according to which dialkyl-polysulfides are obtained as a mixture of dissolved sulfur, dialkyl-disulfides, dialkyl-trisulfides, dialkyl-tetrasulfides, dialkyl-pentasulfides and dialkyl-hexasulfides by reacting alkyl-mercaptans and sulfur in the presence of oxides, hydroxides, alcoholates or sulfides of alkali metals or alkaline earth metals or amines as a catalyst and alcohols as an auxiliary catalyst.
Dialkyl-polysulfides obtained by said process contain highly copper-strip corrosive components such as dissolved sulfur, dialkyl-tetrasulfides, dialkyl-pentasulfides, dialkyl-hexasulfides and the like at considerably high contents, and it is excessively difficult to separate purely dialkyl-trisulfides, which have a minimum grade of said copper-strip corrosivity, from said dialkylpolysulfides.
As a classical process for the production of dialkyl-trisulfides, a process is known in which alkyl-mercaptans are reacted with sulfur dichloride in accordance with the following reaction equation (1). EQU 2.multidot.RSH+SCl.sub.2 .fwdarw.RS.sub.3 R+2HCl (1)
However, sulfur dichloride (SCl.sub.2) is existing under equilibrium with sulfur monochloride as shown in the following reaction equation (2). Accordingly, sulfur dichloride always contains approximately 10% of sulfur monochloride (S.sub.2 Cl.sub.2). EQU 2SCl.sub.2 .revreaction.S.sub.2 Cl.sub.2 +Cl.sub.2 ( 2)
For this reason, the sulfur monochloride also reacts with alkyl-mercaptans to produce dialkyl-tetrasulfides as shown in the following reaction equation (3). EQU 2RSH+S.sub.2 Cl.sub.2 .revreaction.RS.sub.4 R+2HCl (3)
Thus, dialkyl-tetrasulfides, which are highly copper-strip corrosive, are contained in the reaction mixture obtained through the classical process. In order to isolate dialkyl-trisulfides from the reaction mixture, it is necessary to distill out dialkyl-trisulfides by using high-vacuum distillation equipment. Moreover, during the distillation, sulfur by-produced by decomposition of the bottom liquid at the distillation column is sublimated and easily mixes and dissolves into the products of dialkyl-trisulfides. Therefore, the above process has a defect to increase the copper-strip corrosivity of the products dialkyl-trisulfides.
As a process for selectively preparing dialkyl-trisulfides, the process is reported on J. Org. Chem. Vol. 32, page 3833 (1967) and J. Org. Chem. Vol. 31, page 601 (1966) by B. D. Vineyard, wherein the products dialkyl-trisulfides are obtained as a distillate after n-butylamine and alcohol are distilled off at a reduced pressure distillation of a reaction mixture obtained from the reaction of alkyl-mercaptans and sulfur in alcoholic solvent in the presence of n-butylamine as catalyst. However, this process also has the same defect as shown in the case of the classical process using sulfur dichloride, because the produced dialkyl-trisulfides are obtained as a distillate. Therefore, it cannot be employed as an industrial manufacturing process of dialkyl-trisulfides.
It is an object of the present invention to provide a process for the selective production of dialkyl-trisulfides employable as an industrial manufacturing process.
Inventors had eagerly researched to achieve the above object. As a result, they found that the reaction to produce dialkyl-polysulfides proceeds with stoichiometrically and dialkyl-polysulfides are synthesized at high yield by reacting alkyl-mercaptans and sulfur in the presence of magnesium oxide catalyst even without adding alcohols (as auxiliary catalysts), and further found that the selectivity to produce dialkyl-trisulfides in the dialkyl-polysulfides changes over a wide range by changing the quantity of sulfur at gram atom ratio against alkyl-mercaptans (mol), hence they had completed this invention.