753 



In dilute (75 7o) alcoliolic solution the transformation takes place 

 slowly and spontaneously' and only at the end of 14 days has the 

 greater part of the |i-acid disappeared '). 



On repeated recrystallisation from ether or dissolving in absolute 

 alcohol, the /i-acid is generally converted into the «-acid. In some 

 cases, however, it will remain unchanged for weeks. "Wie es 

 scheint spielt auch bei diesen Umwandlungen die Belichtung eine 

 wichtige Rolle, die aber noch der Aufkltirung bedarf")". This last 

 remark looks to me as if he has occasionally mistaken the crystals. 

 Here, where the temperature is generally 10— 20° higher, the trans- 

 formation of /?-cinnamic acid in water, benzene or alcohol proceeded 

 very rapidly in a few days. 



In a dry state and at the ordinary tem[)erature the «-cinnamic 

 acid keeps unchanged for a very long time. Tlie fine powder 

 obtained by pouring an alcoholic solution of cinnamic acid into water 

 i.s but very slowly converted even after an addition of (J-cinnamic 

 acid. A specimen that had been mixed with 1 Vo of cinnamic acid 

 after being kept in the dark for over a month gave, on illuminating 

 0.5 gram during one morning, 0.09 gram of «-acid and 0.10 gram 

 of ^-truxillic acid. 



On heating, however, (he /:?-cinnamic acid, even without previous 

 fusion, is changed very rapidly into «-cinnamic acid. The higher 

 the temperature, the more rapidly the transformation. 



From all these data it thus ai)pears that at the ordinary and also 

 at a higher temperature the «-cinnamic acid is the stable modification 

 and that the /i-acid, for this temperature range, is always metastable. 



From the ready change of the /?-cinnamic acid into the «-acid it 

 may be explained that concentrated solutions, which commence to 

 crystallise at a higher temperature than the dilute ones, give «-cin- 

 namic acid, whereas from dilute solutions which crystallise at a 

 lower temperature, /i-crystals are deposited. 



We have already stated previously with a few words that — 

 considering it has been generally found in tiie case of the organic 

 acids that in some solvents (benzene, chloroform etc.) they occur at 

 great concentration, almost e.xclusively, and at low concentrations 

 still partially, as double molecules — we must also assume that in 

 the solid condition at least double molecules occur. 



According to Beckmann') the fornnila of the double molecule in 

 a general form would be as follows: 



"^Ber. 39, 1583. (1906). 



2) Ber. 42, 509. (1909). 



3) Z. f. ph. Gh. 6, 469. (1890). 



