Chapter 13: Carbohydrates and Chiral Molecules – Structured Study Notes

Chapter 13: Carbohydrates

13.1 Carbohydrates

Carbohydrates are essential biomolecules that serve as a major source of energy in the human diet. They are composed of carbon, hydrogen, and oxygen atoms and are commonly referred to as saccharides or "sugars."

• Major source of energy: Carbohydrates provide fuel for cellular processes.

• Composition: Made from carbon (C), hydrogen (H), and oxygen (O).

• Photosynthesis: Plants synthesize carbohydrates from CO2, H2O, and sunlight.

• Oxidation in cells: Carbohydrates are oxidized to produce CO2, H2O, and energy.

Photosynthesis equation:

$6CO_2 + 6H_2O + \text{energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Respiration equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

Types of Carbohydrates:

• Monosaccharides: Simplest carbohydrates (single sugar units).

• Disaccharides: Composed of two monosaccharide units.

• Polysaccharides: Contain many monosaccharide units.

Monosaccharides

Monosaccharides are the building blocks of carbohydrates, consisting of three to eight carbon atoms in a chain, with one carbon in a carbonyl group.

• 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

Monosaccharides are classified by the number of carbon atoms:

• Triose: 3 carbon atoms

• Tetrose: 4 carbon atoms

• Pentose: 5 carbon atoms

• Hexose: 6 carbon atoms

Examples:

• Aldopentose: Five-carbon saccharide with an aldehyde group (e.g., ribose).

• Ketohexose: Six-carbon saccharide with a ketone group (e.g., fructose).

Representative Structures

Name	Type	Structure
Glyceraldehyde	Aldotriose	CHO–CHOH–CH2OH
Threose	Aldotetrose	CHO–(CHOH)2–CH2OH
Ribose	Aldopentose	CHO–(CHOH)3–CH2OH
Fructose	Ketohexose	CH2OH–CO–(CHOH)3–CH2OH

Fructose

• IUPAC Name: 1,3,4,5,6-Pentahydroxy-2-hexanone

• Structure: Contains a ketone group at C2 and hydroxyl groups at C1, C3, C4, C5, and C6.

Study Check Example

• A: Aldohexose (six carbons, aldehyde group)

• B: Ketopentose (five carbons, ketone group)

13.2 Chiral Molecules

Chirality and Chiral Molecules

Chirality is a property where an object or molecule cannot be superimposed on its mirror image. This is commonly illustrated by left and right hands.

• Chiral molecules: Have the same number of atoms but are arranged differently in space, resulting in nonsuperimposable mirror images.

• Achiral molecules: Mirror images are superimposable.

Everyday Examples

Object	Chiral or Achiral
Gloves	Chiral
Baseball bats	Achiral
Drinking glasses	Achiral
Shoes	Chiral

Structural Isomers vs. Stereoisomers

• Structural isomers: Same molecular formula, different bonding arrangements.

• Stereoisomers: Same molecular formula and bonding sequence, but different spatial arrangement.

Examples of Structural Isomers

Formula	Isomer 1	Isomer 2
C2H6O	Ethanol (CH3CH2OH)	Dimethyl ether (CH3OCH3)
C3H6O	Propanal (CH3CH2CHO)	Propanone (CH3COCH3)

Chiral Carbon Atoms

A carbon atom is chiral if it is bonded to four different groups. Molecules with at least one chiral carbon atom are chiral and have nonsuperimposable mirror images.

• Enantiomers: Stereoisomers that are nonsuperimposable mirror images.

Achiral Carbon Atoms

If the mirror image of a molecule can be rotated and superimposed, the molecule is achiral.

Drawing Fischer Projections

A Fischer projection is a two-dimensional representation of a three-dimensional molecule, commonly used for carbohydrates.

• Most highly oxidized carbon group is placed at the top.

• Vertical lines represent bonds going back (away from viewer).

• Horizontal lines represent bonds coming forward (toward viewer).

D and L Notations

D and L isomers are assigned based on the position of the –OH group on the chiral carbon farthest from the carbonyl carbon:

• L-isomer: –OH group on the left.

• D-isomer: –OH group on the right.

Study Check Example

• D-Isomer: –OH is on the right.

• L-Isomer: –OH is on the left.

Chemistry Link to Health: Enantiomers in Biological Systems

Enantiomers play a crucial role in biological systems because enzymes and cell surface receptors are chiral. Only one enantiomer of a compound may be biologically active, as the chiral receptor fits only one arrangement of substituents.

• L-dopa: Used to treat Parkinson's disease; converted to dopamine in the brain.

• D-dopa: Not effective for Parkinson's disease.

• Drug researchers now use chiral technology to produce active enantiomers of chiral drugs.

Summary Table: Enantiomers in Health

Compound	Active Enantiomer	Inactive Enantiomer	Application
Dopa	L-dopa	D-dopa	Treatment of Parkinson's disease

Key Takeaways:

• Carbohydrates are vital for energy and biological structure.

• Chirality affects molecular interactions in biological systems.

• Understanding isomerism is essential for drug design and metabolic processes.