THE CHEMICAL ACTION OF LIGHT 



263 



tion of water-vapour by sunlight into hydrogen and hydrogen 

 peroxide is suggestive from this standpoint. 



It is well known that aldehydes undergo oxidation in the 

 air, especially rapidly in light, forming peroxides and finally 

 the corresponding acids ; benzaldehyde especially behaves in 

 this way. Ketones also oxidise spontaneously in the air under 

 the action of light ; acetone, for example, gives a mixture of 

 acetic and formic acids : l 



CHs.CO.CH, + 3O 



CH a .CO.OH + H.CO.OH 



Unsaturated compounds, such as stilbene, 2 readily undergo 

 autoxidation under the influence of light ; in the case of stil- 

 bene, the action occurs even when the solid is left in a desiccator 

 exposed to sunlight, benzaldehyde being first formed and finally 

 benzoic acid, a peroxide perhaps being produced as an inter- 

 mediate product : 



C 6 H 5 . CH O 



II + II 



C 6 H 5 .CH O 



Stilbene 



C 6 H 5 . CH . O 



C 6 H 5 .CH.O 



Peroxide 



2C 6 H 5 .CHO 



Benzaldehyde 



A striking example of a somewhat similar character is that 

 observed by the writer 3 in the case of solid 3 : 6-dibromo-/3- 

 naphthaquinone, which is transformed almost quantitatively 

 when exposed to bright light into 3 : 6-dibromo-2-hydroxy-i 14- 

 naphthaquinone (compare p. 266) : 



+ 



Br 



The well-known transformation of chloroform into carbonyl 

 chloride under the action of light is a somewhat similar case : 



CHCU + O -=► C0C1 2 +HC1 



Some striking instances have recently been observed by 

 Ciamician and Silber 4 of the oxidation of such stable hydro- 



1 Atti R. Accad. Lincei, 1903, 12, i. 235 ; Ber. 1903, 36, 1575. 

 * Atti R. Accad. Lincei, 1903, 12, ii. 528 ; Ber. 1903, 36, 4266. 

 s Brit. Ass. Report, " Isomeric Naphthalene Derivatives," 1902. 

 4 Atti R. Accad. Lincei, 191 1, 20, ii. 673. 



