Definition of mutarotation and how it depends on temperature

Definition of mutarotation

When a monosaccharide  is dissolved in amphoteric solvent like water, the optical rotatory  power of the solution gradually changes until it reaches a constant value . 
This change in value of the specific rotation with time is known as mutarotation . 

  

For example, a  freshly  prepared solution of ordinary D(+)glucose has an initial specific rotation of +112,when this solution is allowed to stand , ultimately the specific rotation of the solution falls to +52.7. 

A solution of the second form of D(+)glucose has an initial specific rotation of +18.7 , but slowly the specific rotation of this solution rises and reach to +52.7 and an equilibrium is established during this change.

The explanation for this mutarotation lies in the existence of an equilibrium  between the open-chain form of D(+)glucose and the α and β forms of the cyclic hemiacetals.

X-ray analysis has confirmed that ordinary D(+)glucose has the α-configuration at the anomeric carbon atom and that the higher melting form has the β-configuration.

The concentration of the open-chain D(+)glucose in solution at equilibrium is very small. 

Assuming that the concentration of the open-chain  form is negligible, by use of the specific rotations in the preceding figures, we can calculate the percentages of the α and β anomers present in equilibrium.  

These percentages, 36% α-anomer and 64% β-anomer, are accord with a greater stability for β-D-(+) glucopyranose.

How does mutarotation depends on temperature ?

The rate of mutarotation  is directly proportional  to the temperature that is with increasing temperature the rate of mutarotation increases  and the rate decreases with decreasing temperature .

How does mutarotation depends on acid-base ?

The entire process of mutarotation occurs in amphoprotic solvent like water or in a mixture of acid and base. Mutarotation does not occur only in acid or base . 

But in addition of slightly acid or a small amount base to the solution , the rate of mutarotation  increases.

Why sucrose does not exhibit  mutarotation ?

Ordinary table sugar is a disaccharide called sucrose. Sucrose is a non reducing sugar, it gives negative tests with Benedict’s and Tollen’s solutions. Sucrose does not  undergo mutarotation . 

These means that neither the glucose nor the fructose portion of sucrose has a hemiacetal group. 

Thus, the two hexoses must have a glycosidic linkage that involves C1 of glucose and C2 of fructose, for only in this way will both carbonyl groups be present as full acetals.

Why maltose undergoes mutarotation ?

Unlike sucrose, maltose is a reducing sugar. It gives positive tests with Fehlinh’s Benedict’s and Tollen’s solutions. 

Maltose exists in two anomeric forms , α-anomer with specific rotation +168 , and β-anomer with specific rotation +112. Besides this maltose has a hemiacetal group.

For the above reason , maltose anomers undergoes mutarotation to yield an equilibrium mixture  with constant specific rotation +136.

Summary:




What is mutarotation ?
How does mutarotation depends on temperature ?
Why sucrose does not undergo mutarotation ? 
Why maltose undergoes mutaratation ?

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