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mutarotation is not possible without the presence of an amphiprotic solvent ??

mutarotation is not possible without the presence of an amphiprotic solvent ??

Grade:12th pass

1 Answers

Aarti Gupta
askIITians Faculty 300 Points
9 years ago

When a monosaccharide is dissolved in water, the optical rotatory power of the solution gradually changes until it reaches a constant value.A freshly-prepared aqueous solution of α-D-glucose has a specific rotation,of +112.2°. And when this solution is allowed to stand, the rotation falls to +52.7° and remains constant at this value. The final stage can be attained more quickly either by heating the solution or by adding some catalyst which may be an acid or an alkali. This gradual change in specific rotation is known as mutarotation or changing rotation.
According to Lowry, mutarotation can take place only in the presence of an amphiprotic solvent.Amphiprotic solvent is a solvent which can act both as an acid and a base.For ex- Water is one such solvent.Lowry and Faulkner showed that mutarotation is checked in cresol solution which is an acidic solvent and in pyridine solution which is an basic solvent.But it takes place in mixture of pyridine and cresol. It is assumed that when mutarotation takes place, the ring opens and then recloses either in the inverted position or in the original position. The existence of the open chain form in mutarotation is proved by certain evidences also. For example, the absorption spectra of fructose and sorbose in aqueous solution indicate the presence of open chain forms but aldoses give negative results.

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