Organic Chemistry1.2: Covalent Bonding, Resonance, and Structural Formulas

Covalent BondingKey Concepts of Covalent Bonding

  • Valence Electrons: Electrons in the outermost shell of an atom, crucial for bonding. The number of valence electrons corresponds to the group number in the periodic table.

  • Octet Rule: Atoms share electrons to achieve a full valence shell, mimicking the electron configuration of noble gases. For hydrogen, this means sharing 2 electrons, while for second-row elements, it means sharing 8 electrons.

  • Hydrogen Molecule: The simplest example of covalent bonding, where two hydrogen atoms share their single electrons to form H2.

  • Expansion of Valence Shells: Elements in the third row can share more than 8 electrons due to the availability of d-orbitals, allowing for more complex bonding scenarios.

Types of Nonmetals and Their Compounds

  • Second Row Nonmetals: Key examples include carbon in methane (CH4), nitrogen in ammonia (NH3), oxygen in water (H2O), and fluorine in hydrogen fluoride (HF). Each of these compounds illustrates the principles of covalent bonding and the octet rule.

  • Group III Elements: Boron in borane (BH3) and aluminum in aluminum trihydride (AlH3) demonstrate how elements can form stable compounds with fewer than 8 electrons.

  • Third Row Nonmetals: Elements like phosphorus and sulfur can form stable compounds by sharing more than 8 electrons, utilizing d-orbitals for bonding.

Charged Ions in Covalent Bonding

  • Ammonium Ion (NH4+): Nitrogen shares 8 electrons but contributes only 4 of its own, resulting in a +1 charge due to the loss of one electron.

  • Carbocations (e.g., CH3+): Carbon shares 6 electrons while contributing 3, leading to a +1 charge from the loss of one electron.

  • Carbanions (e.g., CH3-): Carbon shares 8 electrons and contributes 5, gaining an electron and resulting in a -1 charge.

Types of Formulas in Organic Chemistry

  • Molecular Formula: Represents the types and numbers of atoms in a compound, e.g., ethane (C2H6).

  • Condensed Structural Formulas: Provide connectivity information in an abbreviated form, e.g., CH3CH3.

  • Lewis Structures: Offer a detailed representation of bonding, showing types of bonds, lone pairs, and formal charges.

  • Structural Isomers: Molecules with the same molecular formula but different connectivity, highlighting the importance of structure in chemical properties.

Resonance StructuresUnderstanding Resonance

  • Definition of Resonance Structures: Different valid Lewis representations of the same compound, illustrating the dynamic nature of electrons in molecules.

  • Resonance Hybrid: The actual structure of a compound is a hybrid of all possible resonance forms, which cannot be represented by a single Lewis structure.

  • Electron Mobility: Electrons can be 'pushed' around in resonance forms, adhering to specific rules for valid transformations.

Valid Resonance Structures

  • Criteria for Validity: Resonance structures must follow covalent bonding rules, account for total valence electrons, and respect the octet rule as much as possible.

  • Stability Comparison: Stability of resonance forms is assessed based on the number of complete octets, total bonds, placement of negative charges, and charge separation.

Major and Minor Contributors

  • Major Contributor: The most stable resonance structure, typically having all atoms with complete octets and negative charges on the most electronegative atoms.

  • Minor Contributor: Less stable structures that may have incomplete octets or unfavorable charge distributions.

Examples of Resonance Structures

  • Example 1: For CH3CNO, three resonance structures can be drawn, with the hybrid representing the actual compound.

  • Example 2: Acetic acid's acetate ion shows delocalization of negative charge over two oxygen atoms, stabilizing the ion and resulting in bond orders between single and double bonds.

Structural RepresentationsCondensed Structural Formulas

  • Definition: Written without showing all individual bonds, indicating connectivity in a simplified manner.

  • Usage: Atoms bonded to a central atom are listed after it, and identical groups can be represented with parentheses and subscripts.

Line-Angle Formulas

  • Definition: Also known as skeletal structures, where bonds are represented by lines and carbons are implied at line ends or intersections.

  • Hydrogens: Typically not shown for carbons unless significant; other atoms are displayed explicitly.

Importance of Structural Representation

  • Clarity in Communication: Different structural representations help convey complex information about molecular structure and bonding in a clear and concise manner.

  • Application in Reactions: Understanding these structures is crucial for predicting reactivity and interactions in organic chemistry.