SYNTHESIS OF O-ETHYL S-ETHYL DITHIOCARBONATE

CAUTION: These preparations must be conducted in an efficient fume cupboard.

Potassium O-ethyI dithiocarbonate:

• Into a 500-ml round-bottomed flask, fitted with a reflux condenser, place 42g (0.75 mol) of potassium hydroxide pellets and 120g (152 ml) of absolute ethanol. 

• Heat under reflux for 1 hour. 

• Allow to cool and decant the liquid from the residual solid into another dry 500-ml flask; add 57g (45 ml, 0.75 mol) of carbon disulphide (1) slowly and with constant shaking. 

• Filter the resulting almost solid mass, after cooling in ice, on a sintered glass funnel at the pump, and wash it with three 25 ml portions of ether. 

• Dry the potassium O-ethyl dithiocarbonate in a vacuum desiccator over silica gel. The yield is 74g. If desired, it may be recrystallised from absolute ethanol but this is usually unnecessary.

O-EthyI S-ethyl dithiocarbonate:

• Place 32g (0.2 mol) of potassium O-ethyl dithiocarbonate and 50 ml of absolute ethanol in a 500-ml round-bottomed flask provided with a double surface condenser. 

• Add 32g (1 6.5 ml, 0.205 mol) of ethyl iodide. 

• No reaction appears to take place in the cold. 

• Heat on a water bath for 3 hours: a reaction sets in within 15 minutes and the yellow reaction mixture becomes white owing to the separation of potassium iodide. 

• Add about 150 ml of water, separate the lower layer and wash it with water. 

• Dry it with anhydrous calcium chloride or anhydrous calcium sulphate and distil collecting O-ethyl S-ethyl dithiocarbonate at 196-198 °C. The yield is 23g (77%).

Notes to keep in mind:

1. Carbon disulphide is toxic and has a dangerously low flash point.

Cognate preparation: O-Ethyl S-butyl dithiocarbonate

• Use 32g (0.2 mol) of potassium O-ethyl dithiocarbonate, 37g (23 ml, 0.2 mol) of butyl iodide and 50 ml of absolute ethanol. 

• Reflux on a water bath for 3 hours. 

• Pour into 150 ml of water, saturate with salt (in order to facilitate the separation of the upper layer), remove the upper xanthate layer, wash it once with 25 ml of saturated salt solution and dry with anhydrous calcium chloride or anhydrous calcium sulphate. 

• Distil under reduced pressure and collect the pale yellow O-ethyl S-butyl dithiocarbonate at 90-91 °C/4mmHg. The yield is 34g (95%).

O-[1,2,2-Trimethylpropyl] S-methyl dithiocarbonate:

• In a 2-litre three-necked flask equipped with a stirrer, reflux condenser and dropping funnel, prepare potassium t-pentoxide by dissolving 48.5g (60 ml, 0.55 mol) of 2-methyl-butan-2-ol (t-pentyl alcohol) in 750 ml of dry toluene and adding in portions 21.5g (0.55 mol) of potassium metal (1) and refluxing gently until reaction is complete. 

• Then add 51g (63 ml, 0.5 mol) of 3,3-dimethylbutan-2-ol to the hot solution slowly and with stirring. 

• Cool and add slowly 57g (0.75 mol) of carbon disulphide. 

• When the reaction has subsided, cool the resulting orange xanthate suspension to room temperature, add 78g (34 ml, 0.55 mol) of methyl iodide and heat on a water bath for 4-5 hours. 

• Filter, remove toulene and the residual alcohols on a rotary evaporator (fume cupboard) and distil the product under reduced pressure collecting the xanthate ester at 85-87 °C/6mmHg (2). The yield is 63g (70%).

Notes to keep in mind:

1. Great care must be taken in the handling of potassium and of methyl iodide.

2. Use a 500-ml flask fitted with a short fractionating column; the distillation is accompanied by considerable frothing.