THE CHEMICAL ACTION OF LIGHT 265 



takes place in the solid thymoquinone, as when this is spread 



out on the walls of a large flask (by dissolving in a minimum 



of ether and then evaporating the solvent) it becomes colourless 



on exposure to light. The polymerisation does not take place 



in solutions of the quinone. From the point of view of 



structure, it is interesting to note that although thymoquinone, 



O : C a H 2 MePr^ : O, rapidly polymerises, />-xylylquinone, which 



differs from it only by containing methyl in place of isopropyl, 



is not susceptible to light. The same is true of durylquinone, 



dibromothymoquinone and nitrosothymol,OH.N:C G H 2 MePr^:0. 



In 1895 Bertram and Kursten ' observed that when dry 



cinnamic acid is exposed to sunlight, it is transformed into a 



dipolymeride, a-truxillic acid : 



C 6 H 5 . CH . CH . C0 2 H 

 2C 6 H 5 : CH : CH . C0 2 H -> | | 



C 6 Hs . CH . CH . CO,H 



This change, like that of thymoquinone, does not take place 

 in solution, if either alcohol or ether or acetone be used as 

 solvent ; but when cinnamic acid is suspended in paraldehyde it 

 is partly converted into its polymer. Stilbene, on the other 

 hand, dissolved in benzene (in absence of air, so as to prevent 

 autoxidation) is converted into distilbene : 2 



2Ci 4 H 12 = (C 14 H 12 ) 2 



If coumarin dissolved in absolute alcohol be exposed to light 2 

 hydrodicoumarin is formed : 



/CH : CH /CH 2 . C = C . CH 2 \ 



2C 6 H 4 < I -> C 6 H 4 <( I I >C B H 4 



XO .CO no .CO CO. 0/ 



CO.CH-CH.O 



i I I I 



(or possibly O CH-CH CO 



C B H 4 C 6 H 4 



Dibenzylideneacetone, CHPh : CH . CO . CH : CHPh, resini- 

 fies under the influence of light giving a dimeric form. 3 The 

 different behaviour of the isomers safrole and isosafrole, 



1 Ber. 1895, 28, 387. 



2 Ciamician and Silber, Ber. 1902, 35, 4128 ; Atti R. Accad. Lincei, 1903, 12, 

 ii. 528 ; Ber. 1903, 36, 4266. 



3 Atti R. Accad. Lincei, 1909, 18, i. 216 ; Ber. 1909, 42, 1386. 



