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Monosaccharides Notes

Monosaccharides Notes

15.1 Carbohydrates

  • Definition: Carbohydrates are a major source of energy in our diet.
  • Composition: Composed of carbon (C), hydrogen (H), and oxygen (O).
  • Also Known As: Saccharides or "sugars".
  • Learning Goal: Classify a monosaccharide as an aldose or a ketose, and indicate the number of carbon atoms.

Carbohydrate Formation and Function

  • Production: Carbohydrates such as glucose are produced by photosynthesis in plants.
  • Synthesis: Formed from carbon dioxide (CO2), water (H2O), and sunlight energy.
  • Oxidation in Cells: Carbohydrates are oxidized in living cells producing CO2, H2O, and releasing energy.

Types of Carbohydrates

  • Monosaccharides: Simplest form of carbohydrates.
  • Disaccharides: Composed of two monosaccharides.
  • Polysaccharides: Composed of multiple monosaccharides.

Monosaccharides

  • Structure: Contain several hydroxyl (-OH) groups attached to a chain of 3 to 7 carbon atoms.
  • Classification:
    • Aldoses: Monosaccharides with an aldehyde group.
    • Ketoses: Monosaccharides with a ketone group.
  • Hydroxyl Groups: Present on all carbons except the carbonyl carbon.

Types of Monosaccharides by Carbon Count

  • Triose: 3 carbon atoms.
  • Tetrose: 4 carbon atoms.
  • Pentose: 5 carbon atoms.
  • Hexose: 6 carbon atoms.
    • Aldopentose: 5-carbon sugar with an aldehyde group.
    • Ketohexose: 6-carbon sugar with a ketone group.

15.2 Fischer Projections

  • Key Sugars: Glucose, galactose, and fructose are significant monosaccharides.
  • Learning Goal: Identify and draw D or L configurations of Fischer projections for common monosaccharides.
  • Fischer Projection:
    • Places aldehyde group at the top (most oxidized).
    • Displays -H and -OH on horizontal lines.
    • Achiral -CH2OH group located at the bottom.

D and L Notations

  • The orientation of the -OH on the chiral carbon farthest from the carbonyl group determines the isomer:
    • Left: L-isomer.
    • Right: D-isomer.

Examples of Common Monosaccharides

D-Glucose

  • Structure: Most common hexose.
  • Sources: Found in fruits, vegetables, corn syrup, and honey.
  • Other Names: Also known as dextrose and blood sugar.
  • Functionality: Building block for sucrose, lactose, cellulose, and glycogen.

D-Fructose

  • Structure: Ketohexose (C6H12O6).
  • Characteristics: Sweetest carbohydrate; found in fruit juices and honey.
  • Conversion: Converts to glucose in the body.

D-Galactose

  • Structure: Aldohexose (C6H12O6).
  • Availability: Not found free in nature; obtained from lactose.
  • Distinction: Similar structure to glucose but different arrangement of -OH on carbon 4.

15.4 Haworth Structures

  • Learning Goal: Draw and identify the Haworth structures for monosaccharides.
  • Stability: The most stable forms are five- or six-atom rings (cyclic structures).
  • Formation: Resulting from the reaction of a carbonyl group and a hydroxyl group in the same molecule.

Cyclic Haworth Structures

  • Hexoses: The -OH group on C-5 reacts with the aldehyde or ketone group.
  • D-Isomer: Last CH2OH group drawn above the ring.

Specific Sugar Structures

a-D-Glucose and β-D-Glucose

  • Anomers: Formed by the -OH on C1; draws anomers where:
    • a-anomer: -OH drawn down.
    • β-anomer: -OH drawn up.
  • Solution Behavior: Cyclic structures can open and close; a-D-Glucose converts to β-D-Glucose and back.

Galactose and Fructose

  • Galactose: Haworth structure similar to glucose, with -OH on C4 drawn above the ring.
  • Fructose: Forms a five-atom ring; formed by the -OH group on carbon 5 reacting with carbon 2.

15.5 Chemical Properties of Monosaccharides

  • Sugar Alcohols: Includes D-sorbitol, D-xylitol, etc., used as sweeteners in sugar-free products.
  • Learning Goal: Identify oxidation and reduction products of monosaccharides and classify reducing sugars.

Oxidation of Monosaccharides

  • Cyclic Forms: Mainly exist in cyclic forms in solution; small amounts of open-chain form present.
  • Oxidation Reaction: Aldehyde groups can oxidize to carboxylic acids using oxidizing agents like Benedict’s solution.

Reducing Sugars

  • Definition: Monosaccharides with a carbonyl group that oxidizes to form a carboxylic acid.
  • Reactivity: Respond to Benedict’s reagent (Cu2+) yielding carboxylic acids.
  • Examples: Glucose, galactose, and fructose.
  • Naming: Carboxylic acids formed by replacing -ose with -onic acid.

Reduction of Monosaccharides

  • Conversion: Carbonyl group turns into an alcohol group.
  • Products: Sugar alcohols (ald Itols).
  • Naming Convention: Replace the -ose ending with -itol (e.g., D-Glucose converts to D-Glucitol or sorbitol).

Practice Exercises

  • Identify each as D or L isomer:
    • A. Ribose - B. Threose - C. Fructose
  • Draw the Fischer projection of D-Fructose.
  • Write the products of oxidation and reduction of D-Mannose.