— 193 — 



m last-named acid must be regarded as being the most labile and the richest 

 ^ in energy, and the allo-acid as the most stable and the most deficient in 

 energy. Between the two stands Liebermann's ^so-acid. 



A second paper by Erlenmeyer 1 ) deals with the isomeric acids of the 

 cinnamic acid series. It has been demonstrated by various methods tha 

 synthetic cinnamic acid is a uniform crystallising mixture of storacic and 

 hetero-cinnamic acids. But, as we have already mentioned in previous 

 references to the matter, the objection has been raised that the difference 

 between hetero- and storacic-cinnamic acid may have been caused by 

 some impurity in the hetero-cinnamic acid. It is scarcely possible, in 

 certain circumstances, to invalidate this objection, especially if it be as- 

 sumed that the impurity is so small as to be no longer detectable by the 

 available methods. In order to disprove this hypothesis of contamination, 

 Erlenmeyer has made a symposium of the differences which have been 

 found to exist in the past and recently between storacic and synthetic 

 cinnamic acids on the one hand and storacic and hetero-cinnamic acids 

 on the other. He has next described two methods by which it is possible 

 to give convincing proof of the structural identity of storacic and hetero- 

 cinnamic acids. 



In common with cinnamic acid, benzaldehyde also appears to occur 

 in several modifications. From the detailed communications by Erlenmeyer, 

 G. Hilgendorff and T. Marx 2 ) on separation and conversion in the benz- 

 aldehydes we abstract the following particulars: In Perkin's reaction the 

 natural benzaldehyde, before being deprived of its hydrocyanic acid, gives 

 rise to storacic-cinnamic acid, and after the elimination of hydrocyanic 

 acid, to synthetic, and in some cases to hetero-cinnamic acid. The be- 

 haviour of the natural benzaldehyde before and after being deprived of its 

 hydrocyanic acid is very different, not only under condensation but also 

 under distillation. It would appear that natural benzaldehyde before the 

 abstraction of its hydrocyanic acid consists principally of storacic benz- 

 aldehyde, whereas benzaldehyde deprived of its hydrocyanic acid is com- 

 posed of a mixture of storacic and hetero-benzaldehydes, from which under 

 condensation synthetic cinnamic acid is generated. But it is not possible 

 to base upon the distillation a method of separating the various benz- 

 aldehydes. The conditions are more favourable when the separation, which 

 is based upon the varying rates of velocity of the reaction, is taken as 

 the basis, but this method has the disadvantage that only one of the two 

 |cmponents can be recovered, as the other undergoes a change in the 

 course of the reaction. Erlenmeyer looks for the causes of the differences 

 in the cinnamic acids and benzaldehydes in the benzene-nucleus itself, and 

 it is to be assumed with v. Baeyer that it is possible for the benzene 



!) Biochem. Zeitschr. 34 (1911), 355. 

 2 ) Biochem. Zeitschr. 34 (1911), 386. 



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