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I have recently read about mutarotation of glucose specifically

I am familiar with how cyclic structures like substituted cyclo-hexanes exist in an equilibrium of both their chair conformers.

The thing is, for mutarotation, the interconversion takes place via an open-chain form, which is I think supported by enough experimental evidence like the specific rotation of equilibrium mixture.

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But, can ring flip take place the old-school way, that is without opening the ring in a solvent like CCl4 or Benzene

Or is the ring too strained to cross the energy barrier?

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    $\begingroup$ Cyclohexane does not require change of chiral centers to change conformation. R stays R and S stays S. For mutatotation, you need a change in a chiral center, which requires a bond to break. $\endgroup$
    – Karsten
    Commented Aug 23, 2021 at 11:07

1 Answer 1

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The short answer is no.

  • Experiment with a model kit (example) and recognize changing conformers only is about wiggling/rotating bonds which may be gradual in many tiny steps.

  • The transition from one isomer to an other (e.g., $\alpha{} \rightarrow \beta{}$-glucose) is binary like using a light switch; instead of on/off, it is either (R), or (S) – or, in case of double bonds, either (E), or (Z). This requires breaking existing/creating new bonds. Solvent, temperature, radiation, pressure, reagents, catalysts may affect the energetic barrier to achieve this, but normally this change requires surpassing a much higher energetic barrier than the typical rotation around a $\ce{C-C}$ single bond.

While simplified,* this description often is sufficient for everyday like chemistry, including your example about the inversion/mutarotation of glucose dissolved in water. In benzene or $\ce{CCl4}$, this reaction however probably will proceed at a much lower rate of conversion than in water because the assumed mechanism includes steps of protonation/deprotonation and these two solvents are not as good "proton shuttles" (as a Bronsted acid/base) as is water.

From a practical perspective, either form of glucose will not dissolve well in benzene, or $\ce{CCl4}$. Mutarotation however is observed for many sugars e.g., permethylated glucose in benzene and surfactants (e.g., described by Fendler et al.), or pyridone (e.g., Rony) and still of interest today (e.g., Dujardin et al.).

*) My answer here points to some exceptions from this simplification.

References

Fendler, E. J.; Fendler, J. H.; Medary, R. T.; Woods, V. A. Catalysis by reversed micelles in non-polar solvents : mutarotation of 2,3,4,6-tetramethyl-α-D-glucose in benzene. J. Chem. Soc. D 1971, 1497–1497; doi 10.1039/C29710001497.

Rony, P. R. Polyfunctional catalysis. I. Activation parameters for the mutarotation of tetramethyl-D-glucose in benzene. J. Am. Chem. Soc. 1968, 90, 2824–2831; doi 10.1021/ja01013a018.

Dujardin, N.; Dudognon, E.; Willart, J.-F.; Hédoux, A. H; Guinet, Y.; Paccou, L.; Descamps, M. Solid State Mutarotation of Glucose. J. Phys. Chem. B 2011, 115, 1698–1705; doi 10.1021/jp109382j.

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