184 GUANIDIN. 



8. Guanidin. CN 8 H 6 . NH 2 



NH = C . 



NH 2 . 



Although this substance does not occur in the free state in any 

 tissue or fluid of the animal body, it is of considerable interest, 

 for it has been obtained by the direct oxidation of proteids (p.!61j 

 and may be made to yield urea by treatment with boiling dilute 

 sulphuric acid or baryta water. NH : C (NH 2 ) 2 + H 2 = (NH 2 ) 2 CO 

 -+- NH 3 . Further, it affords a connecting link between the xanthin 

 series and kreatin (p. 143), the latter substance being, as already 

 stated, methylguanidin-acetic acid, while guanidin is itself the chief 

 product of the oxidation of guanin. 



It may be readily synthetised in several ways; of these its 

 formation by the action of alcoholic ammonia on chlorpicrin (tri- 

 chlornitromethan) CC1 3 (NO 2 ) or on cyanogen iodide shows clearly 

 its constitution. In the first case 



CC1 8 (N0 2 ) + 3NH 3 = NH : C (NH 2 ) 2 + 3HC1 + HN0 2 . 



In the second CNI + 3NH 3 = NH : C (NH 2 ) 2 + NHJ, or in 

 other words guanidin may be regarded as a compound of cyana- 

 mide and ammonia CN . NH 2 + NH 3 = NH : C (NH 2 ) 2 . The 

 relationship to kreatin may now be at once made evident by 

 comparing the reaction just given with that for the synthesis of 

 kreatin from cyanamide and sarkosin : 



NH, 



CN . NH a -4- CH 2 . NH(CH 3 ). COOH = NH : C / 



X N(CH 3 ).CH 2 .COOH. 



Xanthin derivatives. 



The monomethyl (?) derivative of xanthin (heteroxanthin) has 

 already been described, as also one of the possible dimethyl derivatives, 

 viz. paraxanthin. 



When the (silver or) lead salt of xanthin (PbC 5 H 2 N 4 2 ) is dried 

 and heated in sealed tubes at 100 with methyl iodide, iodide of lead 

 is formed together with dimethyl-xanthin. 1 The substance thus 

 obtained is identical with theobromin, long known as the character- 

 istic alkaloidal constituent of cocoa-beans, the fruit of Theobroma 

 cacao. A third presumably dimethyl derivative of xanthin has re- 

 cently been described as occurring in tea, viz. theophyllin. 2 When 

 the silver salt of theohroinin is further treated as above with methyl 

 iodide it is converted into methyl-theobromin or trimethylxanthin, 

 which is identical with the vegetable alkaloid, long known under the 

 synonymous names of theine or caffeine, as occurring in the leaves or 

 seeds of many plants such as tea and coffee, also in the Brazilian 

 * guarana ' prepared from the fruit of Paulinia sorbiiis, in ' mate ' of 



1 E. Fischer, loc. cit. (sub xanthin). 



2 Kossel, Zt.f. physiol. Chem. Bd. xin. (1889), S. 298. 



