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Cyclic hemiacetals often form readily, especially when they are 5- and 6-membered rings. In this case, a hydroxy group reacts with a carbonyl group within the same molecule to undergo an intramolecular cyclization reaction. [6] Formation of a general cyclic hemiacetal Structures of some readily isolable hemiacetals and hemiketals.
This forms an intramolecular hemiacetal. If reaction is between the C-4 hydroxyl and the aldehyde, a furanose is formed instead. [1] The pyranose form is thermodynamically more stable than the furanose form, which can be seen by the distribution of these two cyclic forms in solution. [2]
In contrast, the formation of cyclic hemiacetals involves a single molecule reacting with itself, making the reaction more favorable. Another way to understand the stability of cyclic hemiacetals is to look at the equilibrium constant as the ratio of the forward and backward reaction rate.
The chemical structure of ribose in its furanose form. The wavy bond indicates a mixture of β-ribofuranose and α-ribofuranose. The furanose ring is a cyclic hemiacetal of an aldopentose or a cyclic hemiketal of a ketohexose.
In contrast to variations of R, both R' groups are organic fragments. If one R' is a hydrogen, the functional group is instead a hemiacetal, while if both are H, the functional group is a ketone hydrate or aldehyde hydrate. Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water.
If the sugar is an aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating a hemiacetal with five- or six-membered ring, respectively. If the sugar is a 2-ketohexose, it can only involve the hydroxyl in carbon 5, and will create a hemiketal with a five-membered ring.
In organic chemistry, a lactol is a functional group which is the cyclic equivalent of a hemiacetal (−CH(OH)O−) or a hemiketal (>C(OH)O−). The compound is formed by the intramolecular , nucleophilic addition of a hydroxyl group ( −OH ) to the carbonyl group ( C=O ) of an aldehyde ( −CH=O ) or a ketone ( >C=O ).
The original version of the Kiliani–Fischer synthesis proceeds through cyanohydrin and aldonic acid lactone intermediates. The first step is to react the starting sugar with aqueous cyanide (typically NaCN); the cyanide undergoes nucleophilic addition to the carbonyl group of the sugar (while sugars tend to exist mainly as cyclic hemiacetal, they are always in chemical equilibrium with their ...