JYRIMIDINES 283 



(a) thymine and Cytosine. 



The precipitate is suspended in hot water and decomposed with hydro- 

 chloric acid. Traces of silver chloride which remain in solution are removed 

 by hydrogen sulphide and the filtrate from silver sulphide is evaporated in 

 vacuo at 60 to a small volume. Thymine separates out during evaporation 

 and on cooling. It is filtered off and recrystallised from hot water containing 

 animal charcoal. 



The filtrate containing cytosine is carefully evaporated to dryness to re- 

 move hydrochloric acid and dissolved in a small amount of water. Some 

 thymine may remain undissolved. The solution contains cytosine hydro- 

 chloride from which the picrate or platinichloride may be made. Cytosine is 

 obtained by treating the concentrated solution with ammonia ; it crystallises 

 out and is purified by recrystallisation from water. 



(b) Uracil and Cytosine. 



The precipitate of pyrimidine silver compounds is suspended in hot water 

 and decomposed with hydrogen sulphide. If barium be present, it is quanti- 

 tatively removed with sulphuric acid and the solution is concentrated. The 

 cytosine is precipitated by slowly adding a hot saturated solution of picric 

 acid. The cytosine picrate is recrystallised from water, dissolved in 5 per 

 cent, hydrochloric acid and separated from picric acid by extracting the solu- 

 tion with ether. The solution of cytosine hydrochloride yields cytosine as above. 



The uracil is obtained from the filtrate by acidifying with sulphuric acid, 

 extracting the picric acid with ether, removing the sulphuric acid with baryta, 

 evaporating to a small volume and allowing it to crystallise out. The im- 

 pure crusts of crystals are recrystallised firstly from hot water containing charcoal 

 and then from 5 per cent, sulphuric acid. 



Properties. 



Thymine generally crystallises from water in rosettes of small platelets, 

 sometimes in the form of needles. 



It is soluble with difficulty in cold water -4 parts in 100 parts of water at 

 25 but easily in hot water. It is slightly soluble in alcohol. It sublimes if 

 carefully heated and melts when heated in a capillary at 32 1 with decomposition. 



It does not form salts with acids, but a potassium salt has been prepared. 



It combines with silver nitrate forming a compound which is precipitated 

 by ammonia or baryta ; the compound is so'uble in excess of ammonia, but 

 not of baryta. 



It is precipitated by mercuric nitrate, but not by phosphotungstic acid. 



It is identified by its melting-point, sublimation and analysis. 



Cytosine crystallises in colourless glistening plates containing iH 2 O which 

 is given off at 100. It decomposes at 320-325. It is soluble with difficulty in 

 water i part in 1 29 parts of water at 25. Cytosine forms salts with acids ; the 

 chief salt is the picrate which turns brown on heating at 255 and melts at 270. 



It forms also double salts with platinum chloride, etc. 



It behaves like thymine towards silver nitrate and ammonia or baryta. 



It is slowly precipitated by mercuric sulphate and also by phosphotung- 

 stic acid. 



It is converted into uracil by the action of nitrous acid. 



Uracil forms a white crystalline powder consisting of needles arranged in 

 clusters. It partially sublimes on heating and gives off red vapours. 



It is soluble with difficulty in cold water, more easily in hot and is almost 

 insoluble in alcohol and ether. 



It behaves like thymine towards silver nitrate and ammonia or baryta. It 

 is precipitated by mercuric nitrate, but not by phosphotungistic acid. 



