Chemical reactions of Carbohydrates

 

Chemical reactions of Carbohydrates

Tautomerization / Enolization

The process of shifting a hydrogen atom from one carbon atom to another to produce enediols is known as tautomerization. Sugars possessing an anomeric carbon atom undergo tautomerization in alkaline solutions. When glucose is kept in an alkaline solution for several hours, it undergoes isomerization to form D-fructose and D-mannose. This reaction is known as the Lobry de Bruyn-von Ekenstein transformation, which forms a common intermediate, namely enediol, for all three sugars. The enediols are highly reactive, hence, sugars in alkaline solution are powerful reducing agents.

Important example: Glucose, Fructose, and Mannose can convert into each other through a common enediol form.

Reducing properties

Sugars are classified as reducing or non-reducing. The reducing property is attributed to the free aldehyde or keto group of the anomeric carbon. Many laboratory tests are employed to identify sugars' reducing action. These include Benedict’s test, Fehling’s test, Barfoed’s test, etc. Reduction is much more efficient in the alkaline medium than in the acid medium. The enediol forms (explained above) or sugars reduce cupric ions (Cu2+) of copper sulphate to cuprous ions (Cu+), which form a yellow precipitate of cuprous hydroxide or a red precipitate of cuprous oxide, as shown in the figure

Mechanism

·       Alkaline Environment (Role of Sodium Carbonate):

o   Sodium carbonate makes the solution alkaline. In alkaline conditions, aldoses and ketoses undergo tautomerization to form enediols (compounds with strong reducing ability). For example, glucose changes into its enediol form.

·       Enediol as Reducing Agent:

o   The enediol form of the sugar is a strong reducing agent. It reduces blue Cu²⁺ ions (from copper sulfate) to red Cu⁺ ions.

·       Formation of Precipitate:

o   Cu⁺ ions form cuprous oxide (Cu₂O), which is a brick-red precipitate. The amount and intensity of the red precipitate correlate with the amount of reducing sugar present.

·      ·       Role of Chelating Agent (Sodium Citrate):

o   Copper (II) ions (Cu²⁺) in alkaline solution would normally react with carbonate to form insoluble copper carbonate, which is useless in the test.

o   Sodium citrate binds Cu²⁺ ions (chelates them), keeping them soluble and available for the redox reaction.

 

                Reducing Sugar (enediol form) + Cu²⁺ (blue) → Cu⁺ → Cu₂O (brick-red precipitate)

Oxidation

Depending on the oxidizing agent used, the terminal aldehyde (or keto) or the terminal alcohol or both, the groups may be oxidized. For instance, consider glucose :

1. Oxidation of the aldehyde group (CHO -> COOH) results in the formation of gluconic acid.

2. Oxidation of the terminal alcohol group (CH2OH->COOH) leads to the production of glucuronic acid.

 

Site of Oxidation	Product Formed	Example
Aldehyde group (C1)	Aldonic acid	Gluconic acid
Primary alcohol (C6)	Uronic acid	Glucuronic acid
Both C1 and C6	Aldaric acid	Glucaric acid

Reduction

When treated with reducing agents such as sodium amalgam, the aldehyde or keto group of a monosaccharide is reduced to the corresponding alcohol, as indicated by the general formula

 

Sugars can be reduced to form sugar alcohols.

• Aldoses → reduced to alditols.
• Example:
• Glucose → Sorbitol
• Mannose → Mannitol

Dehydration

When treated with concentrated sulfuric acid, monosaccharides undergo dehydration with the elimination of 3 water molecules. Thus, hexoses give hydroxymethyl furfural while pentoses give furfural on dehydration. These furfurals can condense with phenolic compounds alpha-naphthol, to form coloured products. This is the chemical basis of the popular Molisch test. In case of oligo and polysaccharides, they are first hydrolysed to monosaccharides by acid, and this is followed by dehydration.

Osazone formation

Phenylhydrazine in acetic acid, when boiled with reducing sugars, forms osazones in a reaction summarized in Fig. As is evident from the reaction, the first two carbons (C1 and C2) are involved in osazone formation. The sugars that differ in their configuration on these two carbons give the same type of osazones, since the difference is masked by binding with phenylhydrazine. Thus, glucose, fructose, and mannose give the same type (needle-shaped) osazones. Reducing disaccharides also gives osazones— maltose sunflower-shaped, and lactose powderpuff-shaped.

Glycosidic bond formation

It is a key chemical reaction in carbohydrates where the anomeric hydroxyl group (–OH) of one monosaccharide reacts with the hydroxyl group of another monosaccharide or alcohol. This reaction is a type of condensation reaction, meaning that a molecule of water (H₂O) is removed during the process. The result is the formation of a glycosidic bond, which links the two sugar molecules. This bond can occur in different orientations, such as α (alpha) or β (beta), depending on the position of the –OH group on the anomeric carbon. For example, when two glucose molecules join through an α-1,4-glycosidic bond, they form maltose, a disaccharide. Similarly, lactose is formed by a β-1,4-glycosidic bond between galactose and glucose. When many such monosaccharide units are joined by glycosidic bonds, they form polysaccharides like starch, glycogen, and cellulose. In starch, glucose units are primarily linked by α-1,4-glycosidic bonds, with some α-1,6-branches, while cellulose consists of β-1,4-linked glucose units, which makes it structurally rigid and indigestible to humans.