Common Bonds to Carbon

Bonding Patterns of Key Elements

Organic molecules are primarily composed of carbon, hydrogen, oxygen, nitrogen, and halogens. Understanding their typical bonding patterns is essential for predicting molecular structure and reactivity.

• Carbon (C): Forms 4 bonds (tetravalent)

• Nitrogen (N): Forms 3 bonds (trivalent)

• Oxygen (O): Forms 2 bonds (divalent)

• Hydrogen (H): Forms 1 bond (monovalent)

• Halides (F, Cl, Br, I): Form 1 bond (monovalent)

Example: Methane (CH4) has carbon bonded to four hydrogens.

Hybridization

Types of Hybridization and Their Geometries

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding. The type of hybridization determines the geometry and bond angles of molecules.

• sp3: Tetrahedral geometry, 4 equivalent bonds, bond angle ≈ 109.5° (e.g., methane, CH4)

• sp2: Trigonal planar geometry, 3 equivalent bonds, 1 unhybridized p orbital, bond angle ≈ 120° (e.g., ethene, C2H4)

• sp: Linear geometry, 2 equivalent bonds, 2 unhybridized p orbitals, bond angle ≈ 180° (e.g., ethyne, C2H2)

Bond Length Trend: Bond length decreases as s-character increases:

Molecular Geometries

Electron Pair and Molecular Geometry

The arrangement of electron pairs around a central atom determines the molecular geometry. Lone pairs affect the observed shape.

Resonance Structures

Drawing and Interpreting Resonance

Resonance structures are different Lewis structures for the same molecule, showing delocalization of electrons. They are connected by double-headed arrows.

• Cations: Use 1 curved arrow to show electron movement.

• Anions: Use 2 curved arrows.

• Heteroatoms: Lone pairs on atoms like O or N can participate in resonance.

• Rules: Do not move electrons away from a double bond or break single bonds.

Example: Resonance in the acetate ion (CH3COO-):

(with the negative charge delocalized over both oxygens)

Major and Minor Resonance Contributors

Stability and Contribution to Resonance Hybrid

Not all resonance structures contribute equally to the resonance hybrid. The most stable (major) contributors have full octets, minimal formal charges, and negative charges on more electronegative atoms.

• Structures with complete octets are more stable.

• Neutral structures are more stable than charged ones.

• Negative charges should be placed on more electronegative atoms (e.g., O over N).

Example: In the resonance of formate ion, the structure with the negative charge on oxygen is the major contributor.

Summary Table: Hybridization and Geometry

Practice Problems and Applications

Lewis Structures and Resonance Practice

• Draw resonance structures for SO3 and O3.

• Classify bonds as ionic, covalent, or mixtures (e.g., NaCl is ionic, CO2 is covalent).

• Write condensed and expanded structural formulas for organic molecules.

• Determine the direction and magnitude of dipole moments (e.g., C–Cl is polar, dipole towards Cl).

• Assign formal charges in Lewis structures using the formula:

Example: For the nitrate ion (NO3-), the formal charge on each oxygen can be calculated using the above formula.

Additional info:

• These notes cover foundational concepts in organic chemistry, including atomic structure, bonding, hybridization, resonance, and molecular geometry, which are essential for understanding reactivity and structure in organic molecules.