ISOMERIC CHANGE 241 



The equilibrium between the sugars, glucose, mannose, and 

 fructose which is set up by contact with alkali * may be attri- 

 buted, like the interconversion of gluconic and mannonic acids, 

 to the migration of a hydrogen atom between carbon and 

 oxygen, since each sugar may be regarded as a ketonic form 

 of the " enolic " compound 



CH 2 OH . CHOH . CHOH . CHOH . COH : CHOH. 



The inversion of an asymmetric carbon atom which accompanies 

 the interconversion of a- and /3-glucose has already been explained 

 (p. 235) as due to a hydrolysis of the lactone ring which converts 



the asymmetric group )C n into — CH(OH) 2 ; the change 



/ OH 

 appears therefore at first sight to be a widely different process 

 from that which converts glucose into mannose. It should be 

 noted, however, that each may be explained by the addition and 

 removal of water, the enormous contrast between the velocities 

 of the two changes being attributed to the great readiness of 

 the lactonic sugar to hydrolyse, coupled with a great reluctance 

 on the part of the resulting hydrate to yield up a molecule of 

 water in such a way as to produce the " enolic " form of the 

 sugar which forms the bridge between glucose and mannose. 



(b) Optical Inversion of Asymmetric Tin and Nitrogen. — Pope 

 and Peachey have recorded an extremely facile racemisation 

 in the case of the methylethylpropyl stannic iodides. 2 This 

 probably depends on the dissociation of the stannic compound 

 with formation of an inactive derivative of divalent tin. 

 .CH 2 



C2Hi Nsn^ $ CH g I + C 2 H 5 .Sn.C,H 7 £ C,H *Sbb/ 1 



C3H/ I C 3 H 7 / \CH 8 



1 Lobry de Bruyn, Rec. Trav. Chim. 1895, 14, 201. 



16 



2 Proc. C/iem. Soc. 1900, 16, 116 



