11. The numbered carbons in the open-chain forms correspond to the same numbered carbons in the hemiacetal forms. 12. Similarly, in excess alcohol, the aldehyde, its hemiacetal , and its acetal all exist in solution. 13. In crystal form, three conformers occur as hemiacetal dimers with a 1, 4-dioxane ring skeleton: 14. Hemiacetal results from addition of the alcohol's hydroxyl group to the carbon in the C = O bond.15. The cyclic hemiacetal forms of aldoses can open to reveal an aldehyde and certain ketoses can undergo tautomerization to become aldoses. 16. D-ribose in itself is a hemiacetal and in equilibrium with the pyranose "'3 " '. 17. The resulting molecule has an hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose. 18. Though the cyclic forms of sugars are usually heavily favoured, hemiacetals in aqueous solution are in equilibrium with their open-chain forms. 19. In this mechanism, the terminal olefin undergoes hydroxymercuration to produce the first intermediate, compound "'2 "', a hemiacetal . 20. The formation of the hemiacetal causes carbon number 1, which is symmetric in the open-chain form, to become asymmetric in the cyclic version.
