Ch_322b_22.08

Ch_322b_22.08 - Mutarotation Ordinary D-(+)-glucose has a...

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Mutarotation Ordinary D-(+)-glucose has a mp = 146 o C. But when D-(+)-glucose is crystallized from an aqueous solution kept above 98 o C, a second form of D-(+)-glucose is obtained with a mp = 150 o C. Some Observations These two forms of D-(+)-glucose have different specific rotations: Form A mp 146 o C [ ! ] D 25 = +112 o Form B mp 150 o C [ ! ] D 25 = +18 o When either form of glucose is placed in water, the specific rotation changes to a final value of [ ! ] D 25 = +52.7 o . This change in rotatory power is called mutarotation . Both acids and bases speed up the rate of mutarotation.
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An Explanation of Mutarotation Mutarotation is due to the interconversion of the !" and #" anomers of D-(+)-glucose through the aldohexose form. The two isolated forms of D-glucose, with their different physical properties, are the two anomers. The final rotatory power of an aqueous solution of D-glucose (+52.7 o at 25 o C) results from an equilibrium mixture of the two anomers. in solution
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Ch_322b_22.08 - Mutarotation Ordinary D-(+)-glucose has a...

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