Organic Chemistry and Life

Role of Organic Compounds in Biological and Everyday Contexts

Organic chemistry is the study of carbon-containing compounds and their role in living systems and materials. Organic molecules are fundamental to life and technology.

• DNA, RNA: Nucleic acids that store and transmit genetic information.

• Proteins, carbohydrates, and fats: Major classes of biomolecules essential for structure, energy, and metabolism.

• Vitamins: Organic compounds required in small amounts for vital biochemical functions.

• Medicines: Many pharmaceuticals are organic molecules designed to interact with biological systems.

• Clothing: Fabrics such as cotton, wool, and synthetic fibers are organic in nature.

• Plastics, rubber, gasoline: Industrial materials and fuels derived from organic chemistry.

Example: The structure of DNA is based on a sugar-phosphate backbone and nitrogenous bases, all organic molecules.

Development of Organic Chemistry

Historical Milestones in Organic Chemistry

The field of organic chemistry evolved from the study of substances derived from living organisms to the synthesis of organic compounds from inorganic sources.

• Royal purple from mollusks: Early use of organic dyes from natural sources.

• Fermentation of grapes: Biological process producing ethanol, an organic compound.

• 1780s “organic” vs “inorganic”: Initial distinction between compounds from living and non-living sources.

• 1828-1850 synthesis of “organic” chemicals from “inorganic” ones: Friedrich Wöhler synthesized urea from ammonium cyanate, disproving the vital force theory.

• Natural versus synthetic: Organic compounds can be produced both by nature and artificially in the laboratory.

Example: Wöhler’s synthesis of urea:

Structural Theory

Bonding Patterns in Organic Compounds

Organic compounds are characterized by specific bonding patterns based on the valence electrons of their constituent atoms.

• Valence electrons: Carbon forms 4 bonds, oxygen 2, hydrogen and halogens 1 each.

• Carbon bonding: Carbon atoms can bond to other carbons, forming chains and rings.

• Modern definition: August Kekulé (1861) defined organic chemistry as the study of carbon compounds.

Example: Methane () has carbon at the center with four single bonds to hydrogen.

Isomers

Types and Properties of Isomers

Isomers are compounds with the same molecular formula but different arrangements of atoms, leading to distinct properties.

• Constitutional isomers: Atoms are connected differently, resulting in different chemical and physical properties.

• Example: Ethanol () and dimethyl ether () are constitutional isomers.

Additional info: Other types of isomerism include stereoisomerism (same connectivity, different spatial arrangement).

Tetrahedral Shape

Geometry of Methane and Organic Molecules

The tetrahedral geometry is central to understanding the three-dimensional structure of organic molecules.

• Proposed by: J.H. van’t Hoff and J.A. Le Bel (1874).

• Methane (): Carbon forms four equivalent bonds arranged tetrahedrally (bond angle 109.5°).

Example: The tetrahedral shape explains the spatial arrangement of atoms in alkanes.

The Octet Rule

Stability and Electron Configuration

The octet rule states that atoms tend to react to achieve eight electrons in their valence shell, similar to noble gases.

• Ionic vs covalent bonds: Proposed by G.N. Lewis and W. Kossel (1916).

• Stable configuration: Atoms gain, lose, or share electrons to complete their octet.

Example: Oxygen in water shares electrons to achieve an octet.

Ionic Bonds

Formation and Properties of Ionic Compounds

Ionic bonds result from the electrostatic attraction between oppositely charged ions, typically formed by elements with large differences in electronegativity.

• High melting solids

• Soluble in polar solvents

• Conduct electricity in solution

Example: Sodium chloride () is an ionic compound.

Covalent Bonds

Electron Sharing and Molecular Formation

Covalent bonds involve the sharing of electrons between atoms, resulting in the formation of molecules.

• Electron dot formulas: Represent shared electrons as dots.

• Lewis structures: Use lines (dashes) to represent shared pairs (each dash = 2 electrons).

Example: Water () has two covalent bonds between oxygen and hydrogen.

Writing Lewis Structures

Steps for Constructing Lewis Structures

Lewis structures visually represent the arrangement of atoms and electrons in a molecule.

• Valence electrons: For main group elements, the number equals the group number.

• Charge adjustment: Add electrons for negative charges, remove for positive charges.

Example: The Lewis structure for the ammonium ion () removes one electron for the positive charge.

Lewis Structures (Continued)

Achieving Noble Gas Configuration

Atoms in molecules strive to achieve the electron configuration of noble gases through sharing or transferring electrons.

• Hydrogen: Shares 1 electron to achieve 2 (duet rule).

• Carbon: Shares 4 electrons to achieve 8 (octet rule).

• Nitrogen, oxygen, halogens: Share some electrons, others remain as unshared pairs (lone pairs).

Example: Oxygen in water has two lone pairs and two shared pairs.