Organic Molecule Structure and Reactions

Atoms and Bonding

  • Learning Objectives:
    • Understand the quantum mechanical structure of the atom.
    • Know the shapes of quantum mechanical orbitals.
    • Write electron configurations.
    • Draw Lewis structures.

Quantum Mechanical Atom

  • Rutherford's model: Nucleus with protons and neutrons, electrons orbiting.
  • Bohr's model: Electrons exist only at specific energy levels (quantization).
    • Ground state: Lowest energy level.
    • Excited states: Higher energy levels.
  • Wave-particle duality: Electrons behave as both particles and waves.
  • Heisenberg uncertainty principle.

Orbitals

  • Orbitals are regions of space where electrons are likely to be found.
  • Types of orbitals: s, p, d.
    • s orbitals: 1 per energy level.
    • p orbitals: 3 per energy level.
    • d orbitals: 5 per energy level.

Electron Configurations

  • Order of filling orbitals: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, etc.

  • Maximum electron capacity:

    • s orbital: 2 electrons.
    • p orbital: 6 electrons.
    • d orbital: 10 electrons.

  • Examples:

    • Carbon (6 electrons): 1s22s22p21s^22s^22p^2
    • Argon (18 electrons): 1s22s22p63s23p61s^22s^22p^63s^23p^6
    • Iron (26 electrons): [Ar]4s23d6[Ar]4s^23d^6

Valence Electrons

  • Valence electrons are in the outermost orbitals (highest principal quantum number).
  • Atoms interact through valence electrons to form molecules.
  • Carbon has 4 valence electrons and typically forms four bonds.

Lewis Structures

  • Covalent bonds: Sharing of electron pairs between atoms achieving stable octets (8 electrons).
  • Octet rule: Atoms share electrons to complete their octets (especially C, N, O).
  • Expanded octets: Elements like Si, P, S can accommodate more than 8 electrons.

Drawing Lewis Structures – Steps

  • Count valence electrons.
  • Assemble bonding framework with single bonds.
  • Add nonbonding electron pairs to outer atoms (except H).
  • Assign remaining electrons to inner atoms.
  • Minimize formal charges.

Formal Charge

  • Formal charge = (valence electrons on free atom) - (electrons assigned in Lewis structure).
  • Minimize formal charges by converting lone pairs to shared pairs.

Resonance

  • When multiple ways exist to minimize formal charges, resonance structures occur.
  • Resonance hybrid: Average of resonance structures.
  • Examples: nitrate anion, carboxylic acids, amides, benzene.