Introduction to Carbohydrates

Carbohydrates Introduction

• Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones
• They are large molecules that produce other compounds upon hydrolysis
• Carbohydrates are an important class of naturally occurring organic compounds
• Examples include glucose, fructose, starch, and cellulose
• They are all composed of carbon, hydrogen, and oxygen
• The general formula is Cm(H₂O)m
• Often referred to as saccharides
• Carbohydrates contain alcoholic hydroxyl groups, aldehyde groups, and ketone groups

Classification of Carbohydrates

• Classified into three major classes based on the number of simple sugar molecules produced by hydrolysis

• 1. Monosaccharides (simple sugars):

  • Single unit carbohydrates
  • Cannot be broken down into simpler carbohydrates upon hydrolysis
  • Example: glucose and fructose
  • Formula: C₆H₁₂O₆
  • No reaction with water

• 2. Oligosaccharides:

  • Made up of 2 to 10 monosaccharide units
  • Disaccharides contain two monosaccharide units
  • Example: sucrose (glucose + fructose)
  • Trisaccharides contain three monosaccharide units
  • Example: raffinose (glucose + fructose + galactose)

• 3. Polysaccharides:

  • Contain more than 10 monosaccharide units
  • One molecule of starch or cellulose upon hydrolysis yields a large number of glucose units
  • Formula: (C₆H₁₀O₅)n +H₂O → nC₆H₁₂O₆

Monosaccharides

• Simplest one-unit sugars
• General formula: CnH₂nOn, where n varies from 3 to 8
• Types: Trioses, tetroses, pentoses, hexoses, heptoses, octoses
  • Specific examples given of each type, including their chemical formula and examples

Glucose

• Most common monosaccharide
• Also known as dextrose
• Naturally occurring glucose is D(+) glucose, fructose is D(-) fructose
• Pentahydroxyhexanal
• Essential constituent of human blood
• Normal blood glucose level: 65-110 mg/dL
• In diabetics, the level is higher than normal
• Occurs in abundance as cane sugar and polysaccharides like starch and cellulose
• White crystalline solid
• Soluble in water, sparingly soluble in ethanol
• Optically active

Physical Properties of Glucose

• White crystalline solid, melting point of 146°C
• Soluble in water, sparingly soluble in ethanol
• Optically active

Naming Simple Monosaccharides

• Functional group = aldehyde
• 6 carbons = hexose, 5 carbons = pentose

Aldohexose Sugars

• Lists specific examples of D-sugars like D-allose, D-altrose, D-glucose, D-mannose, D-idose, D-galactose, and D-talose

Numbering Monosaccharides

• Fischer projection
• Arrange the carbon chain vertically
• Aldehyde at top (number 1)
• Ketone next to top (number 2)

Projections for Representing Glucose

• Haworth projection
• Cyclohexane projection
• Fischer projection

Carbohydrate Structure

• D- or L-sugars: indicate stereochemistry of the lowest chiral carbon
  • D-sugars: hydroxyl group on the lowest chiral carbon points to the right
  • L-sugars: hydroxyl group on the lowest chiral carbon points to the left

Acetal and Ketal Formation

• Ketone + alcohol → hemi-ketal/hemi-acetal → acetal

Ring Formation in Monosaccharides

• The -OH group on C5 is well-placed to react with the C1 aldehyde function
• The resulting 6-membered pyranose ring is relatively unstrained
• -OH groups on C4 and C6 are less favorably oriented

Fischer Projection Form

• Shows different forms of D-glucose (α-D-glucose and β-D-glucose)
• Anomeric carbon: carbon involved in hemiacetal or acetal formation

Chain-Ring Interconversion

• The conversion between chain and ring forms of glucose

α-D-Glucose

• Specific structure details of α-anomer

β-D-Glucose

• Specific structure details of β-anomer

Haworth Projection Formulas for Sugars

• Examples of Haworth projections for sugars

Mutarotation

• Gradual change in specific rotation when glucose is dissolved in water

Monosaccharide Anomers

• Anomeric carbon
• α-anomer: hydroxyl group is down
• β-anomer: hydroxyl group is up
• Anomers undergo mutarotation. Specific examples of α and β forms shown

Chemical Properties of Glucose

• 1. Oxidation

  • Glucose oxidized to gluconic acid with a weak oxidizing agent (e.g., bromine water)
  • Glucose oxidized to glucaric acid with a strong oxidizing agent (e.g., nitric acid)

• 2. Reduction

  • Glucose reduced to D-glucitol (sorbitol) with sodium borohydride

• 3. Acetylation

  • Glucose reacts with acetic anhydride in presence of anhydrous zinc chloride to form penta-acetyl glucose

• 4. Reaction with hydroxylamine

  • Glucose reacts to form glucose oxime

• 5. Reaction with phenylhydrazine

  • Excess phenylhydrazine yields glucosazone

Reactions of Glucose

• Includes details on reaction products
• Shows the reaction mechanism with diagrams

Fructose

• Rotates the polarimeter to the left
• Structures of various forms (a-D-Fructose, b-D-Fructose, a-D-Fructofuranose, β-D-Fructofuranose)
• Reactions summary

Reactions of Fructose

• Includes details on reaction products (reduction, mild oxidation, vigorous oxidation)

Glycosidic Bond

• O-glycosidic bond, example diagrams of methyl α-D-glucopyranoside and methyl β-D-glucopyranoside

Disaccharides

• Examples of sucrose, lactose, and maltose

Polysaccharides

• General overview
• Glycogen
• Cellulose
• Starch
• Specific structure details and roles of each type like glycogen, cellulose, amylose, amylopectin are included, with diagrams