Carbohydrates: Introduction and Classification

Carbohydrates: Introduction

• Most abundant naturally occurring organic compounds.
• Produced via photosynthesis.
• Historically named "carbohydrates" due to the H₂O:C ratio resembling water.
• Modern definition expands beyond Cn(H₂O)n.
• Serve crucial roles in plants and animals (energy storage, structural support).
• Essential for various industries: sugar, starch, paper, textiles, food processing, pharmaceuticals.
• Found in nucleic acids (2-deoxy-D-ribose).
• Some medicines are carbohydrate-based.

Carbohydrate Classification

• Classified by hydrolysis behavior:
  • Monosaccharides: Simplest sugars, not further hydrolyzable.
  • Oligosaccharides: 2-10 monosaccharide units, mostly hexoses.
  • Polysaccharides: More than 10 monosaccharide units.

Monosaccharides

• Crystalline, water-soluble, sweet-tasting.
• Polyhydroxy aldehydes or polyhydroxy ketones with 3+ carbons.
• Classified by:
  • Number of carbons.
  • Presence of aldehyde or keto group.
• Suffix "-ose" denotes a carbohydrate.
• Simplest examples: glyceraldehyde and dihydroxyacetone (trioses).

Oligosaccharides

• Glycosidic linkages of 2-10 monosaccharide units.
• Sub-classified by number of units: disaccharides, trisaccharides, tetrasaccharides (e.g., stachyose).

Polysaccharides

• Non-sweet, high molecular weight.
• Long chains (hundreds/thousands of monosaccharide units), linear or branched.
• Two types:
  • Homopolysaccharides (homoglycans): One type of monosaccharide.
  • Heteropolysaccharides (heteroglycans): Two or more types of monomers.
• Derived carbohydrates yield monosaccharides + derivatives (sulfate esters, amino-sugars, uronic acids).

Drawing Sugar Molecules

• Three methods:
  • Fischer projection: Linear structure, shows configuration.
  • Haworth projection: Cyclic forms, shows relative stereochemistry.
  • Chair/boat conformational: Represents 3D shapes.

Fischer Projection

• Ball-and-stick models show actual molecular configuration.
• Shows solid and dashed wedges for bond direction.
• Highlights chiral carbon atoms (four different attached groups).

Isomerism

• Isomers: Same molecular formula, different structures/orientation.
  • Structural (constitutional) isomers: Different structures/functional groups.
  • Stereoisomers: Same molecular and structural formula, different spatial arrangements. Number of stereoisomers = 2ⁿ (n = number of chiral centers).
    • Enantiomers (optical isomers): Mirror images, non-superimposable (D and L isomers). Number of enantiomeric pairs = 2ⁿ/2.
    • Diastereomers: Differ in one or more chiral centers, not mirror images. Have different physical/chemical properties.
    • Epimers: Diastereomers differing in stereochemistry at one chiral center.

Enantiomers

• Mirror images, non-superimposable.
• Designated D and L isomers.
• Same chemical/physical properties except optical rotation of plane-polarized light.
• D/L assignment based on the highest-numbered chiral center (configurational atom): OH to the right (D), OH to the left (L).

Optical Activity

• Optically active compounds rotate plane-polarized light (clockwise = dextrorotatory (+), anticlockwise = levorotatory (-)).
• Measured using a polarimeter.
• Most naturally occurring carbohydrates have D-configuration.

Diastereomers

• Differ in one or more chiral centers but are not mirror images.
• Have different physical and chemical properties and names.

Sugar Cyclization

• 5 and 6-carbon sugars spontaneously cyclize to furanose (5-membered ring) or pyranose (6-membered ring) forms.
• Involves formation of hemiacetals (aldose) or hemiketals (ketose).
• Process is spontaneous and reversible.

Haworth Projection

• Represents cyclic forms.
• Rules: OH to right in Fischer → OH down in Haworth; OH to left in Fischer → OH up in Haworth.

Anomers

• New stereoisomers created during ring closure (α- and β- anomers).
• Anomeric carbon: New chiral center formed.

Mutarotation

• Interconversion of α- and β- anomers in solution, reaching equilibrium.
• Changes optical rotation over time.

Physical Properties of Monosaccharides

• Solids at room temperature (crystalline).
• Highly water-soluble due to many OH groups.
• Mostly sweet-tasting.
• Optically active.
• Many exhibit mutarotation.

Monosaccharide Reactions

• Esterification: Formation of phosphate esters (e.g., glucose 6-phosphate).
• Glycoside formation: Linkage of monosaccharides via glycosidic bonds.
• Reduction: Carbonyl group reduction to hydroxyl (forms sugar alcohols/alditols, e.g., sorbitol, erythritol).
• Oxidation: Yields aldonic, aldaric (saccharic), or uronic acids, depending on oxidizing agent. Reducing sugars reduce mild oxidizing agents (Fehlings', Benedict's, Tollens' reagents).
• Reaction with alkalis: Aldose-ketose isomerization and aldose epimerization.
• Formation of amino derivatives: Replacement of a hydroxyl group with an amino group (e.g. forming amino sugars, often at C2).

Explore the fascinating world of carbohydrates, the most abundant organic compounds. This quiz covers their classification, from monosaccharides to polysaccharides, and highlights their importance in various industries and biological processes.