09-17-02 23:49
No 357415
From Organikum 15th edition:
WARNING: Thiophenols have a very strong, lasting, terrible, smell, and they can cause severe skin problems. All manipulation of these compounds should be carried out inside a good fume hood, or in a specialy designed laboratory room. All glassware should be handled with rubber gloves and, after the reaction, washed with a KMnO4 solution.
General technique for the preparation of thiophenols:
In a 3 neck 250ml round bottom flask, equiped with a reflux condenser, stirrer and addition funnel, are heated to reflux, under strong agitation, 50ml of glacial acetic acid, 12,5g of red P and 0,6g I2. 0,15 mol of the sulphonyl chloride are slowly added, so as to make the reaction start, wich is evident by the appearance of I2 vapours(solid sulphonyl chlorides are introduced through the condenser). The heat source in then removed, and the addition of the sulphonyl chloride is regulated so as to mantain the boiling of the reaction mixture, taking care that the reaction doesn´t get so violent as to let I2 escape from the top of the condenser. After all as benn added, the mixture is refluxed for two hours, 9ml of water are added and the reflux is mantained for another hour. The products are isolated with a steam distillation.
In the case of liquid Thiophenols the distilate is placed in a sep funnel, the organic layer separated, and the aqueous fase extracted with chloroform. The pooled organic phases are dried with Na2SO4 and the thiophenol is vacuum distilled, after solvent distilation. Solid thiophenols are vaccum filtered and recrystalized from MeOh/water.
Yields range from 50 to 85%, on a variety of substituted benzeneshulphonyl chlorides(aliphatic side chains, phenol ethers and halogenides).
Refs:
Chem Ber. 99, 375(1966)
Z. Chem 10, 449(1970)
(Chief Bee)
09-18-02 01:06
No 357439
Zinc/Sulfuric Acid can also be used for the reduction of sulfonyl chlorides to thiophenols:
The following reaction is also a very vigorous one and must be performed in a well ventilated place. To a solution of 400 mL 25% H2SO4 (V/V) in a beaker at least 2 L in size, there was added 54 g of 2,5-dimethoxybenzenesulfonyl chloride, and the mixture was heated on a steam bath. The yellow crystals of the acid chloride floated on the surface of the aqueous layer. There should be 80 g of zinc dust at hand. A small amount of Zn dust was placed at one spot on the surface of this chapeau. With occasional stirring with a glass rod, the temperature was allowed to rise. At about 60 or 70 °C an exothermic reaction took place at the spot where the zinc was placed. Additional dollups of zinc were added, and each small exothermic reaction site was spread about with the glass stirring rod. Finally, the reaction spread to the entire solid surface layer, with a melting of the acid chloride and an apparent boiling at the H2O surface. The remainder of the 80 g of zinc dust was added as fast as the size of the reaction container would allow. After things subsided again, the heating was continued for 1 h on the steam bath. After the reaction mixture had cooled to room temperature, it was filtered through paper in a Buchner funnel, and the residual metal washed with 100 mL CH2Cl2. The two-phase filtrate was separated, and the lower, aqueous phase was extracted with 2x75 mL CH2Cl2. The addition of 2 L H2O to the aqueous phase now made it the upper phase in extraction, and this was again extracted with 2x75 mL CH2Cl2. The organic extracts were pooled (H2O washing is more trouble than it is worth) and the solvent removed under vacuum. The light amber residue (30.0 g) was distilled at 70-80 °C at 0.3 mm/Hg to yield 25.3 g 2,5-dimethoxythiophenol as a white oil.
Ref: Pihkal #40, http://www.erowid.org/library/books_onli
(Hive Addict)
09-18-02 13:47
No 357622
You could also use LAH for this reduction, although I agree it's a bit wasteful...