Carbohydrate Chemistry Quiz

Carbohydrate Introduction

• Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones
• They are large molecules that form compounds through hydrolysis
• They are a vital class of naturally occurring organic compounds
• Examples of carbohydrates include glucose (grapes), fructose (honey), starch (potatoes), and cellulose (wood)
• Carbohydrates are composed of carbon (C), hydrogen (H), and oxygen (O)
• The general formula for carbohydrates is C_m(H₂O)_m
• Carbohydrates are often called saccharides
• Carbohydrates are polyfunctional compounds containing alcoholic hydroxyl groups, aldehyde groups, and ketone groups

Classification of Carbohydrates

• Carbohydrates are categorized into three groups based on the number of simple sugars produced by hydrolysis
• 1. Monosaccharides: These are single-unit carbohydrates that cannot be broken down into simpler carbohydrates through hydrolysis. Examples include glucose and fructose (C₆H₁₂O₆)
• 2. Oligosaccharides: These contain 2 to 10 monosaccharide units. Disaccharides (e.g., sucrose) contain two monosaccharides, and trisaccharides (e.g., raffinose) contain three.
  • Sucrose → glucose + fructose
  • Raffinose → C₆H₁₂O₆ + C₆H₁₂O₆ + C₆H₁₂O₆
• 3. Polysaccharides: These contain more than 10 monosaccharide units. A single molecule of starch or cellulose can form a very large number of glucose units through hydrolysis.
  • Starch: made from amylose & amylopectin
  • Glycogen: branched polymer of glucose; used for energy storage in animals
  • Cellulose: linear polymer of glucose; used as structural material in plant cell walls

Monosaccharides

• Monosaccharides are the simplest form of carbohydrates.
• They have the general formula C_nH_2nO_n, where n ranges from 3 to 8.
• Different names are given based on the number of carbons in the molecule (trioses, tetroses, pentoses, hexoses, heptoses, octoses)
• Examples include glyceraldehyde, erythrose, ribose, glucose, and heptulose
• The most common monosaccharides are pentoses and hexoses
• Glucose and fructose are amongst the most important and typical hexoses

Glucose

• Glucose is the most common monosaccharide
• Also known as dextrose, it's a dextrorotatory isomer
• The naturally occurring glucose is D(+) glucose and fructose is D(-) fructose (optical isomers).
• It's a pentahydroxyhexanal
• It is a vital component of human blood.
• Normal blood glucose levels are 65-110 mg/dL
• In diabetic patients, glucose levels increase
• It exists as a solid, soluble in water, sparingly soluble in ethanol, and is optically active

Physical Properties of Glucose

• Glucose is a white crystalline solid
• Melting point (M.P.) is 146°C
• It's soluble in water and sparingly soluble in ethanol
• It exhibits optical activity

Naming Simple Monosaccharides

• Monosaccharides have functional groups like aldehyde or a ketone
• Chiral centers have a set configuration; changing the configuration will yield a different sugar

Aldohexoses

• Examples of aldohexoses include D-allose, D-altrose, D-glucose, D-mannose, D-idose, D-galactose, and D-talose.

Numbering Monosaccharides

• In the Fischer projection, the aldehyde group is placed at the top, and the carbon atoms are numbered sequentially.
• For keto sugars, the keto group is placed second from the top

Projections for representing Glucose

• Haworth projection
• Cyclohexane projection
• Fischer projection

Carbohydrate Structure

• D-sugars have the hydroxyl group on the rightmost chiral carbon
• L-sugars have the hydroxyl group on the leftmost chiral carbon

Acetal and Ketal Formation

• Ketones and alcohols react to form hemiacetals and ketals.
• The resulting product is an acetal

Ring Formation in Monosaccharides

• The -OH group on C5 reacts with C1 to form a 6-membered ring (pyranose).
• The resulting ring is relatively unstrained

Specific Rotation

• Specific rotations of α and β anomers decrease or increase to a stable equilibrium.
• The process of change in optical rotation is called mutarotation

Monosaccharide Anomers

• The anomeric carbon is the carbon that was involved in the hemiacetal/ketal formation.

Chemical Properties of Glucose

• Oxidation: Oxidation with a weak oxidizing agent (bromine water) yields gluconic acid. Oxidation with a strong oxidizing agent (nitric acid) yields glucaric acid.
• Reduction: Reduction with sodium borohydride gives glucitol (sorbitol).
• Acetylation: Reaction with acetic anhydride in the presence of anhydrous zinc chloride forms penta-acetyl glucose.
• Reaction with hydroxylamine: Forms glucose oxime
• Reaction with phenylhydrazine: Forms glucosazone

Chemical Properties of Fructose (Ketose)

• Oxidation similar to glucose but with more complex results
• Reduction similar to glucose, producing sorbitol or mannitol
• Acetylation, hydroxylamine, and phenylhydrazine reactions are also possible.

Glycosidic Bond

• A bond formed between a hemiacetal/ketal functional group of a carbohydrate and another molecule.
  • Methyl-α or -β-D-glucopyranoside as an example

Disaccharides

• Sucrose, Lactose, and Maltose are examples

Polysaccharides

• Starch, Glycogen, and Cellulose are examples

Glycogen

• Major storage form of glucose in animals
• Highly branched
• Linear linkages (α-1,4), branch linkages (α-1,6)

Cellulose

• Found in plants cell walls
• Linear polymer of glucose
• α(1-4) linkages
• Hydrogen bonding creates strength

Starch

• Composed of amylose and amylopectin
• Amylose: α(1→4) linkages
• Amylopectin: α(1→4) linkages; α(1→6) branch points