Electrophilic Addition of Water to Alkenes and Alkynes Hydration

Cyclic Forms of Saccharides and Concerted Proton Transfers 

Hemiacetals in five- and six-membered rings are considerably more stable than acyclic hemiacetals. They constitute the form in which many saccharides exist under biological conditions. Glucose exists primarily in its pyranose (six-membered ring) form, although furanose (five-membered ring) forms are present also. Both the pyranose and furanose forms have anomeric isomers, known as a and p. These are shown below for the pyranose form. The open chain form is present in neutral water at ambient temperature only to an extent of approximately 0.003% of the total equilibrium. In the context of saccharides, interconversion between the hemiacetals and the open chain forms occurs in a reaction called mutarotation (see below). This reac- 

The catalysis of mutarotation in organic solvents can be accomplished using simple compounds that are capable of tautomerization reactions. As shown below, simultaneous deprotonation and protonation of a hemiacetal with 2-pyridone leads to ring-opiening. Bond rotation and closing of the ring interconverts the anomers. The 2-pyridone catalyzes this interconversion 7000 times faster than a mixture of phenol and pyridine. 

The Mechanism of Acid-Base Catalysis in Non-Aqueous Solvents.  

3 Electrophilic Addition of Water to Alkenes and Alkynes Hydration 

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ELECTROPHILIC ADDITION OF WATER TO ALKENES AND ALKYNES HYDRATION... 

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