Quinoline - sulphur poison

The stock solution of quinoline-sulphur poison is prepared by refluxing I g. of sulphur with 6 g. of quinoline for 5 hours and diluting the resulting brown liquid to 70 nJ. with xylene which has been purified by distilling over anhydrous aluminium chloride. The addition of the quinoline - sulphur poison ensures that the reduction does not proceed beyond the aldehyde stage it merely slows up the reaction and has no harmful effects. 

For use in the Rosenmund reduction (Expt 6.120) the catalyst is moderated by the addition of the appropriate quantity of a quinoline-sulphur poison prepared in the following manner. Heat under reflux 1 g of sulphur with 6g of quinoline for 5 hours and dilute the resulting brown liquid to 70 ml with xylene which has been purified by distillation over anhydrous aluminium chloride. Thiourea (about 20% by weight of the palladium-barium sulphate catalyst) may also be used as a catalyst poison. 

RCHjOH + H, - > RCHg + H,0 and to products produced by their interaction with acid chlorides. It is generally considered that the reduction of the aldehyde can be prevented by the use of an appropriate catalyst poison or regulator , which inactivates the catalyst towards the reduction of the aldehyde but not to the acid chloride. The poison usually contains sulphur, t.g., quinoline-sulphur or thiourea. Such a regulator is not always necessary and it has been stat that the decisive factors are to keep the temperature near the lowest point at which hydrogen chloride is liberated and to arrest the reaction as soon as one mol of hydrogen chloride is evolved. The reduction is illustrated by ... 

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