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Carbohydrates are defined as the polyhydroxy carbonyl compounds or compounds which give polyhydroxy carbonyl compounds on hydrolysis.

Reaction of aldoses

  1. Mild oxidation: Mild oxidation of aldoses gives aldonic acid. Br2 water or alkaline solution of iodine oxidizes only the aldehydic group to give aldonic acids.
  2. Strong oxidation: Strong oxidation of aldoses oxidizes both –CHO group and terminal –CH2OH group into –COOH to give aldaric acid.
  3. Reduction of sugar: Sugar can be reduced into corresponding alcohols by variety of reducing agents such as high pressure catalytic hydrogenation (Ni/H2), NaBH4, and Na/Hg electrolytic reduction in acidic medium.
  4. Reaction of aldose as ketose with phenylhydrazine
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  5. Reaction with acid chlorides and acid anhydrides
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  6. Reaction with PCl5
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  7. Formation of glycosides: Glucose reacts with methyl alcohol in the presence of dry HCl to form α- and β-methyl glycoside of glucose. The reaction takes place only on OH of hemi-acetylic carbon. Other hydroxyl groups are unreactive.
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  8. Reaction of carbonyl group (aldehydic group):
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  9. Reaction with HCN
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  10. Reaction with HIO4: Glucose consumes 5 mol of HIO4 to give 5 mol of formic acid and 1 mol of formaldehyde. Fructose consumes 5 mol of HIO4 to give 3 mol of formic acid, 2 mol of formaldehyde, and 1 mol of CO2.
  11. Epimerization: Glucose can also be epimerized by treating with hot concentrated alkali. Some glucose is also converted into fructose.
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Cyclic structures of monosaccharides

Many five-membered and six-membered monosaccharides occur in cyclic form. Cyclic structures of monosaccharides are established by many experiments. The cyclic structure is due to intramolecular hemiacetal formation between aldo/keto group and OH of any one carbon.
The ring formed is generally six-membered (pyranose) or five-membered (furanose). Each cyclization results in creation of a new asymmetric center apart from the existing ones.
The isomers resulting from cyclizations are called anomers. For example, when d-glucose (open structure) cyclize, it gives α-d-glucose and β-d-glucose.
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When ordinary α-d-glucose is dissolved in water, it has specific rotation of +111° which on standing gradually changes until it reaches a constant value of +52.7°. This phenomenon is known as mutarotation. Mutarotation is observed because of equilibrium between open-chain structure and cyclic structures of monosaccharides in solutions. The specific rotation of β-d-glucose is +19°, and equilibrium mixture contains 36.2% and 63.8% α- d-glucose and β-d-glucose, respectively.


All the disaccharides are crystalline solids, soluble in water, and fall into two classes, the reducing sugars and the non-reducing sugars.
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  1. Sucrose (cane-sugar) C12H22O11: On hydrolysis with dilute acids, sucrose yields an equimolecular mixture of d(+)-glucose and d(–)-fructose.
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    Sucrose is not a reducing sugar, i.e., it will not reduce Fehling’s solution. It does not form an oxime or an osazone and does not undergo mutarotation.
  2. Maltose (malt sugar) C12H22O11: Maltose is another disaccharide which is made up of two α-d-glucose units joined together by glycosidic linkage in such a way that one anomeric OH (C-1) is joined with OH (C-4) in other unit.
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    Another example of disaccharide is cellobiose which is β-d-glucopyranose, i.e., it is made up of two γ-d-glucose units. Lactose is another disaccharide which is made up of d-glucose and d-galactose.

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