Comprehensive Guide to Carbohydrate Chemistry: Structure, Classification, and Reactivity

Definition and Fundamental Properties of Carbohydrates

• Definition: Carbohydrates are organic compounds composed of Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$), generally following the ratio $(CH_2O)_n$. Chemically, they are polyhydroxy aldehydes or ketones and their derivatives, or substances that yield these through hydrolysis.

• General Formula: $(CH_2O)_n$, where $n$ is the number of carbon atoms, typically 3 or more.

• Occurrence in Nature:

  • They are the most abundant organic compounds on Earth.

  • Produced by plants via photosynthesis.

• Photosynthesis Reaction:

  • $6CO_2 + 6H_2O + \text{light energy} \xrightarrow{\text{chlorophyll}} C_6H_{12}O_6 + 6O_2$

• Dietary Sources: They provide approximately $4\,kcal/g$. Sources include cereals, fruits, milk, vegetables, legumes, and honey.

• Biological Roles:

  • Primary energy source (immediate and storage).

  • Structural components (e.g., cellulose in plant cell walls).

  • Protective and buffering agents (e.g., mucoproteins).

  • Cell recognition and communication (membrane-bound glycans).

Classification of Carbohydrates

• Monosaccharides: The simplest units that cannot be hydrolyzed further. Usually contain 3-7 carbon atoms.

  • Triose ($C_3$): Aldose (Glyceraldehyde); Ketose (Dihydroxyacetone).

  • Tetrose ($C_4$): Aldose (Erythrose); Ketose (Erythrulose).

  • Pentose ($C_5$): Aldose (Ribose, Deoxyribose); Ketose (Ribulose, Xylulose).

  • Hexose ($C_6$): Aldose (Glucose, Galactose, Mannose); Ketose (Fructose, Sorbose).

  • Heptose ($C_7$): Aldose (Sedoheptulose); Ketose (Sedoheptulose).

• Oligosaccharides: Yield 2-10 monosaccharide units on hydrolysis. Linked by glycosidic bonds.

  • Disaccharides (2 units): Sucrose ($Glc + Fru$), Lactose ($Glc + Gal$), Maltose ($Glc + Glc$).

  • Trisaccharides (3 units): Raffinose ($Gal + Glc + Fru$).

  • Tetrasaccharides (4 units): Stachyose ($Gal + Gal + Glc + Fru$).

• Polysaccharides: Yield more than 10 monosaccharide units. They can be linear or branched.

  • Homopolysaccharides: Composed of one type of monosaccharide (e.g., Starch, Glycogen, Cellulose, Chitin).

  • Heteropolysaccharides: Composed of two or more different types of monosaccharides (e.g., Hyaluronic acid, Heparin, Pectin).

Detailed Chemical Structure and Stereochemistry

• Fischer Projections:

  • Horizontal bonds represent atoms coming toward the viewer.

  • Vertical bonds represent atoms moving away from the viewer.

• Functional Groups:

  • Hydroxyl (-OH): Present in all carbohydrates; increases polarity and enables H-bonding.

  • Carbonyl (Aldehyde, -CHO): Present in Aldoses at $C_1$; reactive and reduces $Cu^{2+}$ or $Ag^+$.

  • Carbonyl (Ketone, >C=O): Present in Ketoses at $C_2$.

• Ring Structures (Cyclization): In aqueous solution, monosaccharides exist mainly as cycles.

  • Hemiacetal Formation: Reaction between the $C_1$ aldehyde and a hydroxyl group (usually at $C_5$ or $C_4$).

  • Hemiketal Formation: Reaction between the $C_2$ ketone and a hydroxyl group.

  • Pyranose: A 6-membered ring.

  • Furanose: A 5-membered ring.

• Anomers: Diastereomers that differ only in configuration at the anomeric carbon ($C_1$ for aldoses, $C_2$ for ketoses).

  • $\alpha$-Anomer: Hydroxyl group on the anomeric carbon is "down" (opposite to the $CH_2OH$ group at $C_6$ in D-sugars).

  • $\beta$-Anomer: Hydroxyl group is "up" (same side as the $CH_2OH$ group).

• Mutarotation: The change in specific rotation of a solution due to the interconversion between $\alpha$ and $\beta$ anomers through the open-chain form until equilibrium is reached.

  • $\alpha$-D-Glucose rotation: $+112^{\circ}$.

  • $\beta$-D-Glucose rotation: $+18.7^{\circ}$.

• Chair Conformation: The most stable 3D arrangement for Pyranoses. Substituents prefer equatorial positions to minimize steric hindrance.

Nomenclature Rules

• D and L Configuration: Based on the chiral center farthest from the carbonyl group.

  • D-Series: -OH is on the right.

  • L-Series: -OH is on the left.

• Systematic Naming of Disaccharides:

  • Maltose: $\alpha$-D-Glucopyranosyl-(1-4)-D-Glucopyranose.

  • Lactose: $\beta$-D-Galactopyranosyl-(1-4)-D-Glucopyranose.

  • Sucrose: $\alpha$-D-Glucopyranosyl-(1-2)-$\beta$-D-Fructofuranoside.

• Generic Suffixes:

  • -ose: Sugar.

  • -ulose: Ketose form.

  • -onic acid: Oxidized at $C_1$.

  • -uronic acid: Oxidized at $C_6$ (-COOH group).

  • -oside: Sugar unit in a glycosidic linkage.

Reactions of Sugars due to Carbonyl and Carbinol Groups

• Carbonyl Group Reactions:

  1. Cyanohydrin Formation: Reversible reaction with $HCN$.

  2. Oxime Formation: Reaction with hydroxylamine ($NH_2OH$).

  3. Osazone Formation: Reaction with excess Phenylhydrazine ($C_6H_5NHNH_2$). It involves the first two carbons ($C_1$ and $C_2$). Sugars differing only at $C_1$ or $C_2$ (e.g., Glucose, Fructose, Mannose) yield the same osazone. This is useful for converting an aldose to a ketose.

• Carbinol (-OH) Group Reactions:

  1. Protonation: In acidic media, forms an oxonium ion ($OH_2^+$), facilitating mutarotation.

  2. Ester Formation: Reacts with acid chlorides (e.g., Acetyl chloride) or anhydrides to form esters like Glucose triacetate.

  3. Ether Formation: Reacts with alkyl halides (e.g., Methyl iodide with base) to form methyl ethers like Glucose trimethyl ether.

  4. Glycoside Formation: Anomeric -OH reacts with alcohols ($ROH$) in acid catalyst to form an acetal (glycoside).

  5. Anhydro Sugar Formation: Intramolecular dehydration between two -OH groups (e.g., 1,6-anhydro-$\beta$-D-glucose).

Oxidation of Sugars to Sugar Acids

• Aldonic Acids: Oxidation of $C_1$ aldehyde to -COOH. Reagent: Mild oxidizing agents like Bromine water ($Br_2/H_2O$). Example: D-Glucose $\rightarrow$ D-Gluconic acid.

• Uronic Acids: Oxidation of $C_6$ primary alcohol to -COOH. Example: D-Glucose $\rightarrow$ D-Glucuronic acid (vital for liver detoxification).

• Saccharic (Aldaric) Acids: Oxidation of both $C_1$ and $C_6$ to -COOH. Reagent: Strong oxidizing agents like dilute Nitric acid ($HNO_3$). Example: D-Glucose $\rightarrow$ D-Glucaric acid.

• Ketose Oxidation: Ketoses must first isomerize to aldoses via an enediol form before oxidation can occur.

Reduction of Sugars to Sugar Alcohols (Polyols)

• Mechanism: Carbonyl reduction at $C_1$ (aldoses) or $C_2$ (ketoses) using $NaBH_4$, $LiAlH_4$, or catalytic hydrogenation ($H_2/Ni$).

• Stability: Sugar alcohols are non-reducing, stable to mild alkali, and do not show mutarotation.

• Common Sugar Alcohols:

  • D-Glucose: Sorbitol (D-Glucitol); used as a sweetener and humectant.

  • D-Mannose: Mannitol; used as an osmotic diuretic.

  • D-Galactose: Dulcitol (Galactitol).

  • D-Ribose: Ribitol.

  • D-Xylose: Xylitol (non-cariogenic, dental health).

• Ketose Reduction: Fructose yields two epimeric alcohols: Sorbitol and Mannitol.

Reactions in Alkaline Media

• Strong Alkali (Conc. $NaOH$ or $KOH$ + Heat):

  • Causes Retro-aldol cleavage of the carbon-carbon bond.

  • Pathway: Aldose $\rightarrow$ Enediolate (unstable) $\rightarrow$ Cleavage between $C_2$ and $C_3$ $\rightarrow$ smaller fragments.

  • Final products on prolonged treatment: Formic acid (formate) and Acetic acid (acetate).

• Weak Alkali (Dilute $NaOH$):

  • Lobry de Bruyn-van Eckenstein Transformation: Reversible rearrangement of aldose/ketose via an enediol intermediate.

  • Sequential process: Aldose $\xrightarrow{OH^-}$ Enediolate $\rightarrow$ Aldonic acid salt $\xrightarrow{H_3O^+}$ Aldonic acid $\xrightarrow{-H_2O}$ $\delta$-Lactone.

  • The $\delta$-Lactone is a six-membered cyclic ester formed between the $C_1$ carboxyl and $C_5$ hydroxyl.

Structural and Biological Significance of Polysaccharides

• Starch (Plant Storage):

  • Amylose: Linear $\alpha(1-4)$ linkages.

  • Amylopectin: $\alpha(1-4)$ chain with $\alpha(1-6)$ branches.

• Glycogen (Animal Storage): Highly branched; stored in liver and muscle. Features $\alpha(1-4)$ chains and $\alpha(1-6)$ branches.

• Cellulose (Plant Structural): Linear unbranched chains with $\beta(1-4)$ linkages. Insoluble in water.

• Heteropolysaccharides (Glycosaminoglycans - GAGs):

  • Highly hydrophilic, negatively charged (due to sulfate/uronic acid groups).

  • Hyaluronic Acid: $GlcA + GlcNAc$. Found in synovial fluid and vitreous humor; lubricates joints.

  • Chondroitin Sulfate: $GlcA + GalNAc$ (sulfated). Found in cartilage; provides compression resistance.

  • Heparin: Heavily sulfated; found in mast cells. Functions as a powerful natural anticoagulant.

  • Keratan Sulfate: $Gal + GlcNAc$ (sulfated). Found in cornea and cartilage.

  1. Clinical Relevance: Defects in degradation leads to Mucopolysaccharidoses (MPS) (e.g., Hurler or Hunter syndromes).

Qualitative and Color Tests for Identification

• General Test:

  • Molisch's Test: Carbohydrate + conc. $H_2SO_4$ $\rightarrow$ Furfural + $\alpha$-naphthol $\rightarrow$ Violet ring.

• Reducing Sugar Tests (Detects free aldehyde/ketone):

  • Tollens Test: Reduces $[Ag(NH_3)_2]^+$ to a Silver Mirror.

  • Fehling's Test: Reduces $Cu^{2+}$ to Brick-red $Cu_2O$ precipitate.

  • Benedict's Test: Semi-quantitative color scale (Blue $\rightarrow$ Green $\rightarrow$ Yellow $\rightarrow$ Orange $\rightarrow$ Brick-red).

  • Nylander Test: Reduces Bismuth(III) to Bluish-black Bismuth metal.

  • Picric Acid Test: Reduces to Picramic acid (Yellow to Orange).

• Specific Classification Tests:

  • Barfoed's Test: Differentiates monosaccharides (reduces in 2-3 mins) from disaccharides (slower).

  • Bial's (Orcinol) Test: Detects Pentoses (Blue-green color).

  • Seliwanoff's Test: Differentiates Ketoses (Cherry-red color develops rapidly) from Aldoses.

  • Iodine Test: Detects Starch (Blue-black complex).

  • Tauber's Benzidine Test: Specific for Pentoses (Blue-green compound).