Lecture 11: Lewis Structures and Related Concepts

Non-Equivalent Resonance Structures

  • Not all resonance structures possess equal significance; one may represent the actual molecule more accurately in terms of energy and stability.

  • Guidelines:

    • Overall resonance structure is an average of all structures.

    • Second-row atoms (B, C, N, O, F) must satisfy the octet rule.

    • In structures where all octets are satisfied:

    • Favor the structure with fewest and smallest formal charges (aligned with electronegativity trends).

    • Structures featuring formal charges of more than ±2 are less significant.

    • Adjacent atoms bearing like charges are unfavorable.

    • Negative charges assigned to the more electronegative (EN) atom are prioritized over charges on less EN atoms.

  • Formal charges must add up to the same total number of formal charges across the structure.

Lewis Structures for NCO− Ion (Cyanate)

  • Total valence electrons = 16 e⁻.

  • The evaluation of different Lewis structures will help identify the most stable configuration that obeys the octet rule. Various contributions to the resonance hybrid will influence the stability of the structure.

Drawing Lewis Structures of Covalent Compounds

Example: Carbon Dioxide (CO₂)

  1. Arrange atoms in probable order: C-O-O or O-C-O.

  2. Identify electronegativities: O = 3.5, C = 2.5.

  3. Place the least electronegative atom (C) as the central atom.

  4. Resulting structure should represent O-C-O.

Steps to Draw Lewis Structures

  1. Determine total valence electrons:

    • 1 Carbon atom x 4 valence electrons = 4 e⁻.

    • 2 Oxygen atoms x 6 valence electrons = 12 e⁻.

    • Total = 16 e⁻.

  2. Form single bonds connecting C to each O:

    • Structure: O : C : O (all valence electrons accounted).

  3. Place electron pairs around the atoms:

    • Needs revisiting to satisfy octets, redistribute so that each atom achieves octet:

    • Result: C::O::C, satisfying the octet rule.

  4. Verify the electron distribution in the final structure where each atom achieves octet.

Lewis Structures and Formal Charge

  • Structure representation for CO₂: O=C=O

  • Formal charges can be calculated using the formula
    ext{Formal Charge} = ext{Valence Electrons} - ext{(Nonbonding Electrons + 1/2 Bonding Electrons)}

  • Contributions of resonance structures will determine major and minor contributors based on formal charges.

Structural Isomers vs. Resonance Structures

  • Example: CH₃NO₂ (Nitromethane) vs. Methyl Nitrite.

  • Key differences:

    • Isomers have identical molecular formulas but differ in the bonding arrangement of atoms.

    • Resonance structures represent the same compound with variations in electronic arrangements, maintaining atomic positions.

  • Overall, isomers are distinct molecules, while resonance structures are representations of the same molecule.

Peptide Bonds

  • Structure of the peptide bond is also known as the amide bond.

  • Resonance in peptide bonds imparts partial double bond character, resulting in restricted rotation around the C-N bond.

  • The planar nature of the peptide bond due to resonance significantly influences protein structure.

Exceptions to the Octet Rule

Incomplete Octets

  1. When the central atom has fewer than eight electrons due to electron deficiency (common in Groups 2A and 3A) such as BeCl₂ and AlBr₃.

  2. Molecules with an odd number of electrons (e.g., NO₂) result in some atoms having fewer than eight electrons, leading to radicals.

Expanded Octets

  • Elements such as phosphorus, sulfur, and chlorine can have expanded octets due to availability of vacant d orbitals, which accommodate extra electrons.

  • These elements can form more than 8 electrons by engaging in additional bonding:

    • Common bonding patterns:

    • Phosphorus typically forms 3-5 bonds.

    • Sulfur typically forms 3-6 bonds.

    • Chlorine can form 3-7 bonds.

  • Specifically, phosphorus can sometimes expand its octet to achieve lower energy states.

Examples of Expanded Octets

  1. Phosphate (PO₄³⁻) with formal charges shown indicating no net charge on phosphorus.

  2. Sulfonic acids and sulfates (like SO₂, SO₃, SO₄²⁻) demonstrate how sulfur can either obey the octet rule or utilize expanded octets.

Biological Significance of Phosphate and Sulfate

  • Phosphorylation impacts various biological molecules and is crucial for the modification of proteins, especially on amino acids like serine, threonine, and tyrosine.

  • Sulfate addition to proteins and carbohydrates functions similarly to enhance biological functions.