lecture 1 copy

Ethene and Ethane Bonding

  • Shared electron pair is localized between nuclei (localized bond).

  • Sigma (σ) and pi (π) electron density distinctions:

    • Sigma (σ) electron density between C atoms: Bond length = 1.54 Å.

    • Pi (π) electron density between C atoms: Bond length = 1.33 Å.

Pi Electron Delocalization in Dienes

  • π electrons in double bonds (C1=C2 & C3=C4) are delocalized over four atoms.

  • More electron density is present between central carbon atoms in 1,3-pentadiene compared to typical single bonds.

  • Question: What happens if more than one π bond exists in a molecule? Delocalization is impacted by sp3 hybridization.

Stability of Conjugated Dienes

  • Conjugated dienes have lower heats of hydrogenation, indicating greater stability (29 kJ/mol stabilization).

  • Example bond lengths:

    • Double bond (H2): -255 kJ/mol, bond length = 1.45 Å.

    • Single bond (H2): -226 kJ/mol, bond length = 1.37 Å.

Identification of Delocalization

  • Look for:

    • Double or triple bonds in conjugation.

    • Bonds next to electron-withdrawing (-) or electron-donating (+) groups or lone pairs (:).

Conjugation and Stability

  • Four double bonds that are not conjugated are separated by two single bonds.

  • Tetrahedral (sp3) carbon atoms prevent overlap of p orbitals in double bonds.

Resonance Structures Basics

  • Atoms never move in resonance forms.

  • Only π bonds or lone pairs in p orbitals can be moved.

  • Overall charge must remain constant; bonding framework intact.

Resonance Characteristics

  • Molecules represented by multiple Lewis structures differing in π electrons are termed resonance structures.

  • σ bond electrons are localized, while π electrons are delocalized.

  • The actual structure is a hybrid of these contributors (resonance hybrid).

Requirements for Canonical Forms

  • All forms must be valid Lewis structures; certain structures (e.g., carbon with five bonds) are invalid.

  • Nuclei positions must remain constant across structures; atoms in resonance must lie in a plane.

  • The energy of the actual molecule is lower than any single structure, leading to stabilization.

Guidelines for Stabilizing Criteria

  1. Structures with maximum octets contribute more.

  2. More covalent bonds equate to greater stability.

  3. Increased charge separation decreases stability.

  4. Structures with formal charges are less stable than neutral ones.

  5. Structures with multiple formal charges contribute minimally.

  6. Structures with adjacent like charges are unfavorable.

  7. Negative charges are more stable on electronegative atoms.

  8. Positive charges are best on less electronegative atoms.

Examples of Molecular Structures

  • Various resonance structures illustrated to show electron delocalization and shifting positions in molecular frameworks.

Molecular Orbital (MO) Theory

  • Linear conjugated systems like ethylene and butadiene reveal energetic gaps in HOMO and LUMO levels.

  • Extent of conjugation affects absorption in UV-visible spectra, with more conjugation allowing longer wavelengths absorbed.

Cross Conjugation

  • In cross-conjugated compounds, not all groups are conjugated with each other but lead to altered bond lengths in comparison to non-cross-conjugated bonds.

Hyperconjugation

  • σ-p system interaction via delocalization with p orbitals; stabilizes alkyl carbocations and free radicals.

  • Requires sp2 hybrid carbons and hydrogen on α-carbon; similar to conjugation but effect is smaller in magnitude.

Stability of Carbocations

  • More substituted carbocations are favored due to hyperconjugation and the electron-donating effect of alkyl groups, which stabilize the charge.

To create a quiz, I can suggest some short answer questions based on the notes provided. Here are a few questions you could use:

  1. What type of bond corresponds to the localization of shared electron pairs between two nuclei?

  2. Compare the bond lengths of sigma (σ) and pi (π) bonds between carbon atoms in ethylene.

  3. In a conjugated diene like 1,3-pentadiene, how are π electrons distributed?

  4. What does a lower heat of hydrogenation indicate about the stability of conjugated dienes?

  5. Which types of groups can influence the delocalization in a molecular structure?

  6. Describe the role of resonance structures in molecular stability.

  7. What are the requirements for valid canonical forms in resonance structures?

  8. How does hyperconjugation contribute to the stability of carbocations?

  9. What effect does increased charge separation have on molecular stability?

  10. In terms of stability, which position is preferable for a positive charge in a molecule?

These questions can guide students to review key concepts related to ethene and ethane bonding, the stability of conjugated dienes, resonance structures, and molecular orbital theory.