We know that aldoses (and ketoses) react with alcohols to give first hemiacetals (and hemiketals) and then acetals (and ketals), i.e.
Since monosaccharides contain a number of hydroxyl groups and an aldehyde or a keto group, therefore, any one of the –OH groups (usually C4 or C5 in aldohexoses and C5 or C6 in ketohexoses) may combine with the aldehyde or the keto group to form intramolecular hemiacetal or hemiketal.
As a result, the open chain formulae do not represent the actual structures of the monosaccharides. Their actual structures are cyclic involving five or six membered rings containing an oxygen atom. The five membered ring containing one oxygen atom because of its similarity with furan is called the furanose form and the six membered ring containing one oxygen atom because of its resemblance with pyran is called the pyranose form. In nut shell, all the monosaccharides (pentoses and hexoses) in the free state always exist in the pyranose form. However, in the combined state some monosaccharides such as ribose, 2-deoxyribose, fructose etc., usually exist in the furanose form.
Cyclic Structure of Glucose – Anomers
We have discussed above that monosaccharides have cyclic hemiacetal or hemiketal structures. To illustrate, let us first consider the example of D-glucose. During hemiacetal formation C5 – OH
of glucose combines with the C1 – aldehydic group. As a result, C1 becomes chiral or asymmetric and thus has two possible arrangements of H and OH groups around it. In other words, D-glucose exists in two stereoisomeric forms, i.e., a-D-glucose and b-D-glucose as shown below:
In a-D-glucose, the OH group at C1 is towards right while in b-D-glucose, the OH group at C1 is towards left. Such a pair of stereoisomers which differ in configuration only around C1 are called anomers and the C1 carbon is called Anomeric carbon (or glycosidic carbon. The cyclic structures of monosaccharides can be better represented by Haworth Projection formulae. To get such a formula for any monosaccharide (say a-and b-D-glucose), draw a hexagon with its oxygen atom at the upper right hand corner. Place all the groups (on C1, C2, C3 and C4) which are present on left hand side in structures I and II, above the plane of the ring and all those groups on the right hand side below the plane of the ring.
The terminal – CH2OH group is always placed above the plane of the hexagon ring (in D-series). Following the above procedure, Haworth Projection Formulae for a-D-glucose (I) and b-D-glucose (II) are obtained as shown below:
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