THE CHEMICAL ACTION OF LIGHT 255 



the influence of light is dealt with later (p. 266) ; the remarkable 

 self-reduction which takes place in presence of traces of iodine 

 is considered on p. 261. 



Anisaldehyde behaves in alcoholic solution in the same 

 manner as benzaldehyde, giving rise to hydranisoin but the 

 action is much slower. The behaviour of vanillin, 1 on the other 

 hand, is quite special in its character, oxidation occurring when a 

 solution of the aldehyde in alcohol, ether or acetone is exposed 

 to light ; the product is dehydrovanillin, which can also be 

 obtained by the oxidation of vanillin with weak oxidising agents, 

 thus : 

 2CHO.C 6 HKOMe)OH + = H„0 + CHO.C 6 H,(OH)(OMe).C s H,(OH)(OMe).CHO 



Puxeddu 2 has recently shown that the photochemical 



oxidation of vanillin does not depend on the presence of 



atmospheric oxygen but takes place equally well in sealed tubes 



when it is dissolved in benzene or toluene ; it is probably a 



case of self-reduction in which vanillyl alcohol is also formed. 



The action is always particularly rapid, dehydrovanillin beginning 



to separate from the solution after ten minutes' insolation. It is 



interesting that the ethylic and methylic ethers of vanillin 



behave in an entirely different way. Thus when a solution of 



methylvanillin or of ethylvanillin in alcohol, benzene, toluene or 



acetic acid is exposed in sunlight, the corresponding vanillic 



acid is formed : 



OMe OMe 



CHO COOH 



Puxeddu gives a rather complex explanation of these changes 

 which involves the assumption that a dibenzylidene derivative 

 [in the case of the methylic ether, this has the structure 

 (OMe) 2 . C 6 H 3 . CH : CH . C 6 H 3 (OMe) 2 ] is formed as intermediate 

 product ; but no trace of such a compound could be isolated. 

 It seems more probable that what really occurs is self-oxidation 

 involving the formation of the vanillic acid and the corresponding 

 vanillyl alcohol such as actually has been shown by Mascarelli to 

 take place in the case of benzaldehyde in presence of traces 

 of iodine (see p. 261). 



When benzaldehyde dissolved in benzylic alcohol is exposed 



1 Atti R. Accad. Lincei, 1901, 10, i. 92. J Ibid. 191 1, 20, ii- 718. 



17 



