Why does mutarotation occur only in specific media?

I was reading a book on *biomolecules*, and read that *muta-rotation* in hexoses, can happen only in media like water, which can act as an acid and as a base. Why is this so?


Essentially, it is just the formation of the open chain form, and then back to the other cyclic anomer, right? ***What role does the medium have in this process***?





> 
> Essentially, it is just the formation of the open chain form, and then back to the other cyclic anomer, right?
> 
> 
> 


Precisely, mutarotation involves interconversion of the α-and β-forms (cyclic anomers) of the monosaccharide with intermediate formation of the linear aldehyde (or ketone).




---


The mutarotation of glucose is a general acid- base-catalysed reaction which involves two proton transfers. Studies have proposed that these proton transfers are either carried out in a stepwise or concerted manner, with the latter alternative possibly incorporating several water molecules in a cyclic transition state.


[![enter image description here](https://i.stack.imgur.com/6eX1g.png)](https://i.stack.imgur.com/6eX1g.png)


Two possible mechanisms of mutatoration have been investigated:


* The first involves an intramolecular proton transfer *i.e acid-catalyzed reaction.*
* The second uses one solvent water molecule to assist proton transfer *i.e water-catalyzed process.*


There is some experimental evidence1 for the **solvent participation** in the mutarotation reaction that appears from the studies of primary solvent effects on the rates of glucose and tetramethylglucose observed in $\ce{H2O + D2O}$ mixtures. In addition the absence of intramolecular proton transfer can be deduced from the small values found for the mutarotation rate of glucose in organic solvents.



> 
> Eigen1 proposed that the two proton transfers could proceed
>  in aqueous solution via a pathway involving two or more water
>  molecules in a cyclic hydrogen-bonded transition state, where the
>  intramolecular proton transfer does not take place in aqueous
>  solution.
> 
> 
> 


Previous studies2 have shown that the reaction order with respect to water is zero for an acid-catalyzed reaction involving a stepwise mechanism, however in aqueous solution without added catalyst, water molecules **must act as an acid or base** and it is possible that the mutarotation reaction does proceed via a cyclic-concerted mechanism involving about three water molecules.



> 
> The results obtained in experimental work in the newer
>  studies1 have shown that mutarotation is a
>  **solvent-assisted process** that obeys pseudo first-order kinetics. Besides that, the first water molecule from the solvent acts as a
>  catalyst, and plays a decisive role, since it completely changes the
>  mutarotation mechanism.
> 
> 
> 


*So a solvent is necessary, the evidence been seen in proton transfers in reactions that do occur via cyclic concerted mechanism as proposed in these studies.*




---


References:


1. A theoretical study of glucose mutarotation in aqueous solution. A. M. Silva et al. / Carbohydrate Research 341 (2006) 1029-1040. Carbohydrate Research 341 (2006) 1029-1040.
2. The Mutarotation of Glucose in Dimethylsulfoxide and Water Mixtures. Ballash and Robertson CAN. J. CHEM VOL. 51. 1973.
3. CM Garrett, Grisham Biochemistry


 9