36 



CARBOHYDRATES 



The optical rotation of D-fructose, however, is very different from that 

 of D-ghicose, being —92.4° as compared to +52.7° (see Table 3-2). 



Fructose and other ketoses can be distinguished from aldoses by the 

 resorcinol test, which consists in the production of a blood-red color 

 when a ketose is boiled with a solution of resorcinol in hydrochloric acid. 

 Since other ketoses are not common, a positive resorcinol test is a' good 

 indication of the presence of D-fructose or of other carbohydrates which 

 produce D-fructose on hydrolysis (sucrose, raffinose, melezitose, inulin, 

 etc.). 



Table 3-2 



Melting points and optical rotations of common sugars 



Sugar 

 D-Xylose 



Melting 

 point [°C]* 

 145 



Optical Melting Optical 



rotation [a] v'f Sugar point [°C] rotation [a]n 



+ 18.8 L-Sorbose 161 —43.4 



— 104.5 D-Glucosamine .. 110 +70 tt 



+ 104.5 D-Glucuronif acid 156 y +36.3 



—23.7 Lactose 202 +52.6 



+52.7 Sucrose 188 +66.5 



+80.2 Maltose 103 +130.4 



+ 14.2 Cellobiose 225 +34.6 



—92.4 Trehalose 97 +178.3 



lJ—^\.^ IWOC Xll/ 



D-Arabinose 160 



L-Arabinose 160 



D-Ribose 87 



D-Glucose 146 



D-Galactose 167 



D-Mannose 163 



D-Fructose 104 



* Of form obtained most commonly. 



t Specific rotation of the equilibrium mixture of a and (3 forms (if any) measured 

 in water at or near 20°C. 

 tt Hydrochloride. 



Ketoses are also differentiated from aldoses by greater resistance to 

 mild oxidative treatments. For example, under proper experimental con- 

 ditions aldoses can be converted almost quantitatively into the corre- 

 sponding aldonic acids by oxidation with iodine in alkaline solution 

 according to the equation: 



CHO 

 I 

 (CHOH)^ + I2 + 3NaOH 



CH.OH 



Aldose 



COONa 



I 

 (CHOH)^ + 2NaI + 2H2O 



CH2OH 



Aldonic acid 

 (sodium salt) 



Under identical conditions ketoses remain practically unaffected. 



h'Sorbose 



This ketohexose is found in nature only in the fermented juice of 

 mountain ash berries, where it undoubtedly arises as a result of bacterial 

 oxidation of D-sorbitol. As an industrial product, however, it has acquired 



