272 PHOTOSYNTHESIS 



dehyde hypothesis and the supposition that carbon dioxide is reduced by 

 hydrogen. Other modifications of Baeyer's theory and the attempts which 

 have been made to verify this will be discussed later. 



c. The Condensation of Formaldehyde by Means of Alkali. 



Turning to the fourth assumption in Baeyer's theory, that the formal- 

 dehyde formed by reduction of carbon dioxide is condensed to sugars 

 under conditions existing in the plant, it must be realized that this is an 

 important feature of the theory and was based upon the discovery of 

 Butlerow of the condensation of his dioxymethylene (trioxymethylene) 

 to a sugar-like substance by means of alkalies. Shortly after Baeyer had 

 suggested that the condensation of formaldehyde to a sugar was to be 

 ascribed to the splitting out of water from CH2(OH)2 Wiirtz,^^ on the 

 basis of his discovery of aldol, concluded that the condensation of formalde- 

 hyde was also an aldol condensation and thus modified Baeyer's theory. 

 "Oncongoit d'ailleurs que le plus simple des aldehydes, I'aldehyde formique 

 puisse prendre naissance dans les procedes de la vegetation, par la reduc- 

 tion partielle d'une molecule d'acide carbonique 



CO^ -f- H^O — O^ = CH^O 



et que la condensation de plusieurs molecules d'aldehyde formique puisse 

 donner naissance a des hydrates de carbon, a la fois alcools et aldehydes 

 au meme titre et par le meme procede que la condensation de deux molecules 

 d'aldehyde ordinaire produit Taldol." Wiirtz therefore provided the first 

 rational conception of the reactions involved in the condensation of 

 formaldehyde to hexose sugars. 



Many modifications of Butlerow's method of condensing formaldehyde 

 have been devised, but in principle these do not differ greatly from the 

 original method ; the modifications consist in the main in the use of weaker 

 alkalies and lower temperatures than were used by Butlerow. Thus 

 Loew ^* introduced the use of calcium hydroxide at 15 to 20° and called 

 the product obtained formose. Lobry de Bruyn and van Ekenstein ^^ 

 found that an excellent condensing agent is lead hydroxide. There are 

 many other substances which bring about the condensation : the 

 hydroxides of potassium, sodium, magnesium and barium, finely divided 

 zinc and lead, basic lead salts and sodium acetate. With weak alkaline 

 solutions higher temperatures increase the condensation, although above 

 about 70° the alkalies also decompose the condensation product. 



Formose obtained by any of these methods is a mixture of a number 

 of substances, the nature of which has for the most not been definitely 

 determined. Of considerable significance in relation to the origin of sugars 



"Wiirtz, Compt. rend., 74, 1366 (1872). Kekule, Ann. Chem., 162, 77 (1872). 

 "Loew, Jour, prakt. Chem., 33, 321, 34. 51 (1886) ; 92, 133 (1915). Ber. chcm. 

 Ges., 22, 470, 478 (1889). 



"Lobry de Bruyn and van Ekenstein, Rec. trav. chim., 18, 309 (1899). 



