Jan 14, 2025 Chem 261 Notes

Energetics of Forming Bonds

  • Molecular Formation: As two hydrogen atoms come together, molecular hydrogen (H2) is formed from two single hydrogen atoms (H + H).

  • Energy Diagram: The energy diagram illustrates the interaction of two hydrogen atoms forming a bond, with a bond length of 1Å (angstrom) as the average H-H bond distance.

Molecular Orbitals

LCAO (Linear Combination of Atomic Orbitals)

  • Concept: The combination of atomic orbitals of s-character generates a molecular orbital known as a sigma molecular orbital (σ).

  • Types of Orbitals:

    • Bonding Orbital: 1s

    • Anti-bonding Orbital: s*

  • Inter-nuclear Distance: Denoted as R, representing the distance between the nuclei of the interacting atoms.

Orbital Representation

Hybridization

  • The mixing of atomic orbitals occurs to create hybrid orbitals suitable for bonding.

  • sp3 Hybridization:

    • Forms when there are single bonds among 2nd row elements.

    • Geometry: Tetrahedral, with bond angles approximately 109°.

    • Rotation: Free rotation around single bonds is allowed.

  • Visual Representation: Different representations in structural formulas indicate bond directionality:

    • Solid wedge: Toward you (out of the page)

    • Dashed wedge: Away from you (into the page)

    • Plain line: Undefined geometry or in-plane.

Orbital Types and Bonding

sp2 Hybridization

  • Application: Occurs with double bonds in 2nd row elements.

  • Configuration: Involves three bindings to 2nd row atoms, forming a planar geometry, with bond angles of 120°.

  • Rotation: No free rotation around double bonds due to the required alignment of p orbitals.

  • Bond Types:

    • Sigma Bonds: Formed through overlap of s atomic orbitals,

    • Pi Bonds: Formed through overlap of p atomic orbitals.

sp Hybridization

  • Characteristics: Involves triple bonds with two atoms bonded to carbon.

  • Geometry: Linear, with an angle of 180° between bonding atoms.

  • Bond Composition: Contains one sigma bond and two pi bonds.

  • Example: Acetylene (Ethyne).

Comparison of Hybridization Types

  • sp3 vs sp2:

    • sp3: Tetrahedral geometry, 4 substituents, angles of 109°.

    • sp2: Planar geometry, 3 substituents, angles of 120°.

  • Pi Bonds: In sp2 hybridization, overlap of p orbitals forming pi bonds restricts rotation around double bonds.

Size and Shape of Molecules

  • Determining Factors: Bond lengths and types dictate molecular geometry.

  • Bond Lengths: Average bond length between hydrogen and a 2nd row element is about 1Å.

  • Molecular Representation:

    • Show only valence electrons.

    • Lines represent pairs of bonded electrons.

    • Non-bonding electrons, like lone pairs, may be omitted.

    • Carbon can be depicted by the end of a line or angle instead of its symbol.

Examples of Molecular Representations

  1. Propene (CH3CH=CH2): Illustrates bonding types and geometry.

  2. Steroid Molecules: Exhibit sp3 hybridization throughout.

  3. Tetrahydrofuran (THF): Contains an ether functional group; carbonyl carbon is sp2 hybridized.

  4. Estradiol: A steroid hormone with a planar carbon structure.

  5. Testosterone: Known as a male hormone, demonstrates androgenic properties and various functional groups (alkene, ketone, alcohol).