13.2 Chemical properties of monosaccharides

Chemical properties of monosaccharides.

• Oxidation Reactions:
• Sugars can undergo oxidation reactions, where they lose electrons or gain oxygen atoms.
• Classification of Sugars:
• Sugars can be classified as reducing or non-reducing based on their reactivity to certain reagents like Tollen's and Fehling's solutions.
• If a sugar is oxidized by these reagents, it is termed a reducing sugar.
• The reaction typically results in the formation of a silver mirror or cuprous oxide precipitate.

• Reduction Reactions:
• Reduction involves the gain of electrons or loss of oxygen atoms.
• Sodium Borohydride Reduction:
• Aldoses can be reduced using sodium borohydride, resulting in the reduction of the carbonyl group at the end of the aldose chain.
• This reduction process results in the formation of an alditol, where the carbonyl group is converted to a hydroxyl group.
• Configurational Changes:
• Reduction to an alditol accomplishes configurational changes produced by oxidation to an aldonic acid.

Glycoside bond formation.

• Introduction:
• Glycosides are important derivatives formed from the reaction of monosaccharides with alcohols or amines in the presence of specific catalysts. These reactions lead to the formation of stable acetal or N-glycoside linkages, which are crucial in various biological processes and synthetic applications.
• *check video section for visuals
• Reaction Mechanism:
• Hemiacetal Reactivity:
• Hemiacetals react with alcohols in the presence of an acid catalyst, resulting in the loss of water and formation of acetals.
• Glycoside Formation:
• Acetal derivatives formed from the reaction of a monosaccharide with an alcohol are called glycosides.
• In glycosides, the "-ose" suffix of the sugar name is replaced by "-oside", and the alcohol group name is placed first.
• Water Loss:
• Glycoside formation typically involves the loss of an equivalent of water.
• Stability:
• Glycosides are stable to base and alkaline oxidants like Tollen's reagent.
• Acetals are stable under neutral or basic conditions.
• Reversibility:
• Since acid-catalyzed aldolization is reversible, glycosides can be hydrolyzed back to their alcohol and sugar components by aqueous acid.
• Reactivity with Tollens Reagent and Fehling’s Solution:
• Open-Chain vs. Cyclic Form:
• Tollens reagent and Fehling’s solution react with the open-chain form of the sugar, which has a free aldehyde or ketone group.
• If the sugar is in a cyclic acetal form, it cannot open to the free carbonyl compound, resulting in a negative Tollens test.
• Reduction Test:
• Hemiacetals are easily opened and react with Tollens reagent and Fehling’s solution, indicating a reducing sugar.
• However, acetal derivatives formed from glycoside formation are stable and do not react with these reagents, making them non-reducing sugars.