Hemiacetal groups structure

Note. The term potential aldehydic carbonyl group refers to the hemiacetal group arising from ring closure. Likewise, the term potential ketonic caibonyl group refers to the hemiketal structure (see 2-Carb-5). 

T-glycosidic linkage. Trehalose is not a reducing sugar because there is no hemiacetal group in this structure. Instead it has two acetal type linkages, much like sucrose. 

Ans. Refer to Figs. 18-5, 18-6, and 18-7. If we write out the complete structure for a molecule of any of these polysaccharides, the very last glucose unit at the far right has a hemiacetal group. However, this hemiacetal unit is only one out of thousands or tens of thousands of glucose units. The concentration of hemiacetal is too low to be experimentally detected by the Benedict s test or optical rotation. 

In analogy with pyranoses, the furanoses are also in equilibrium with the open-chain forms. The new functionality which appears in the cyclic forms is hemiacetal, the structure already mentioned in our discussions on alcohol addition to the carbonyl group. The hemiacetals can easily undergo hydrolysis yielding the open chain monosaccharide molecule. 

Hemiacetal group n. Functional groups derived from carbonyl groups by addition of one molecule of an alcohol, of the general structure shown -C-OH-OR-. Morrison RT, Boyd RN (1992) Organic chemistry, 6th edn. Prentice-Hall, Englewood Cliffs, NJ. 

Structure. As shown by G. Tanret, the sugar is hydrolyzed to D-fructose and D-glucose by yeast a-D-glucosidase and, hence, must be an a-D-glucoside. This conclusion receives confirmation from the lack of hydrolysis of the disaccharide by almond emulsin 110). The above evidence and the resistance of the sugar to oxidation by alkaline hypoiodite solutions, coupled with its reduction of alkaline copper salt solutions, show that the unsubstituted hemiacetal group belongs to the fructose component. The mutarotation of turanose resembles that of fructose and probably is caused... 

The branched-chain sugars streptose, apiose and cordycepose (p. 18) which are found only in glycosidic union must, in their natural states, have a hemiacetal ring structure formed in the latter two instances by union of C-i to either of the CHjOH groups attached to C-3.This introduces an additional... 

Compounds that contain both carbonyl and alcohol functional groups are often more stable as cyclic hemiacetals or cyclic acetals than as open chain compounds Examples of several of these are shown Deduce the structure of the open chain form of each... 

Carbohydrates are marvelous molecules In most of them every carbon bears a functional group and the nature of the functional groups changes as the molecule mterconverts between open chain and cyclic hemiacetal forms Any approach to understanding carbohydrates must begin with structure... 

---