CCCXXXIX.—Studies of dynamic isomerism. Part XXIV. Neutral-salt action in mutarotation

An Unexpected Pathway to Enantiomerization of Hemithioketals in Toluene Involving a Dimeric Transition State: A Combined Experimental and Computational Study

“…To understand the origin of the exceedingly large value of the activation barrier, it is important to remember that a key‐role in the ring opening process of hemiacetals and hemiketals is normally played by solvent molecules (in particular water) that assist the proton transfer 8. If we take into account that we are emulating a process occurring in toluene, the large computed barrier is not surprising and agrees with the evidence that in dry benzene the anomeric inter‐conversion of glucopyranose is nearly not observed 9…”

Transition States for Glucopyranose Interconversion

“…Schmid 36 treated the reaction as a single step and found values similar to the present data for heat and entropy of activation in glucose anomerization (17.2 kcal/mol and -24 cal/mol K, respectively). It has also been noted that the slow isotope exchange of [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] O]D-glucose in water as compared to mutarotation rates definitively rules out a significant role of an aldehydrol form 37,38 , although this form has been observed in solution 39 with the other tautomers.…”

Influence of the Carboxylic Function on the Degradation of d-Galacturonic Acid and Its Polymers

“…Figure 1 shows the different linearized temporal progressions of the solutions examined, whereby the y-axis represents the time-dependent rotation angle alteration (ln(α t − α ∞ )) as described by Lowry and Smith. 17 To examine whether the high reactivity of D-galacturonic acid is due to its carboxylic functionality, an organic acid with a comparable pK a value to that of uronic acid (pK a = 3.51 18 ) was mixed in equal concentrations with D-galactose, and the mutarotation rate constant of the mixture was measured. Formic acid with a pK a of 3.75 19 was used, and the results of the mixture were compared to those of the reducing sugar D-galactose alone.…”

“…According to the literature, the presence of the open-chain structure of pentoses and hexoses has an effect on the degradation speed. − To examine whether this is also the case for completely dissolved d -galacturonic acid, uronic acid was polarimetrically measured and the mutarotation speed was compared to those of d -galactose and d -glucose in combination with formic acid. We used formic acid, as it has a similar p K a value to that of d -galacturonic acid.…”