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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.
The glucose molecule can exist in an open-chain (acyclic) as well as ring (cyclic) form—due to the presence of alcohol and aldehyde or ketone functional groups, the form having the straight chain can easily convert into a chair-like hemiacetal ring structure commonly found in carbohydrates.
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]
The resulting molecule has a hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form. In these cyclic forms, the ring usually has five or six atoms.
However, in order for anomers to exist, the sugar must be in its cyclic form, since in open-chain form, the anomeric carbon atom is planar and thus achiral. More formally stated, then, an anomer is an epimer at the hemiacetal/hemiketal carbon atom in a cyclic saccharide. [1] Anomerization is the process of conversion of one anomer to the other.
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 ...
For a cyclic hemiacetal the reaction is intramolecular so the nucleophile is always held close to the carbonyl group ready to attack, so the forward rate of reaction is much higher than the backward rate. Many biologically relevant sugars, such as glucose, are cyclic hemiacetals. Cyclic hemiacetals
Reducing form of glucose (the aldehyde group is on the far right). A reducing sugar is any sugar that is capable of acting as a reducing agent. [1] In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent.