Carbohydrates

Definition

• Carbohydrates are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O).

• They usually follow the general formula Cₙ(H₂O)ₙ.

• Contain multiple hydroxyl (-OH) groups and a main functional group that is either:

  • An aldehyde (aldose), or

  • A ketone (ketose).

Classification of Carbohydrates

Stereochemistry of Carbohydrates

Chirality and Stereoisomers

• Monosaccharides have multiple chiral centers, so they can exist in many stereoisomeric forms.

• The number of stereoisomers possible = 2ⁿ, where n = number of chiral carbons.

• Most sugars exist as D or L configurations.

Determining D or L form:

• Look at the –OH group on the chiral carbon farthest from the carbonyl (C=O) group.

  • If –OH is on the right → D-sugar.

  • If –OH is on the left → L-sugar.

• The human body primarily uses D-sugars.

Types of Stereoisomers

Drawing Enantiomers:

• Identify chiral centers.

• Flip all chiral configurations (right ↔ left, wedge ↔ dash).

Important Monosaccharides

Glucose

• Most abundant monosaccharide.

• Main source of energy for cells through glycolysis.

• Found in table sugar (sucrose), milk (lactose), glycogen, and cellulose.

• Diabetes: The body struggles to move glucose into cells → high blood sugar.

Galactose

• Found in milk (as part of lactose).

• Converted to glucose in the body using the enzyme epimerase.

Mannose

• Found in cranberries.

• Not easily absorbed by the body.

• Helps prevent UTIs — bacteria attach to mannose and are excreted in urine.

Fructose

• Sweetest naturally occurring monosaccharide.

• Combines with glucose to form sucrose (table sugar).

• Converted into glucose for use by body cells.

Monosaccharide Ring Formation

• In aqueous solutions, monosaccharides rarely exist in linear form; they cyclize to form rings.

• The process converts a Fischer projection (open chain) into a Haworth projection (ring).

Anomers

• When the ring forms, a new chiral center (the anomeric carbon) is created.

• Depending on the position of the first hydroxyl group (–OH) on the first carbon:

  • Alpha (α): –OH points downward.

  • Beta (β): –OH points upward.

Reactions of Carbohydrates

Benedict’s Test

• Detects reducing sugars (aldoses).

• When Copper(II) ions (blue) are added to a sugar solution:

  • The aldehyde group is oxidized, forming a sugar acid.

  • Cu²⁺ → Cu⁺ is reduced, forming brick-red Cu₂O precipitate.

• Positive Test: Formation of red/orange precipitate = reducing sugar present.

Disaccharides

Formation:

• Created through a condensation reaction (water is removed).

• Occurs between two hydroxyl (-OH) groups of monosaccharides.

• The bond formed is a glycosidic bond.

Naming the Bond:

• Based on the carbon numbers and anomeric configuration (α or β) of the linking monosaccharides.

  • Example: α(1→4) glycosidic bond.

Polysaccharides

Functions

• Storage Polysaccharides: store glucose.

• Structural Polysaccharides: provide rigidity and protection.

Carbohydrates and Blood Grouping

• Specific oligosaccharides on red blood cell surfaces determine blood type.

• These sugars form a protective coating (glycoproteins/glycolipids).

• Combinations: AO, BO, AB, OO determine possible blood types.

Quick Recap Summary