Bond Polarity and Covalent Bonds

Ordering Covalent Bonds by Polarity

Covalent bond polarity is determined by the difference in electronegativity between the two atoms involved. The greater the difference, the more polar the bond.

• Electronegativity: A measure of an atom's ability to attract shared electrons in a bond.

• Order of Polarity (most to least polar): C–F > C–O > C–N > C–H > C–C

• Example: The C–F bond is highly polar due to the large electronegativity difference between carbon and fluorine.

Lone Pairs, Formal Charges, and Lewis Structures

Drawing Lone Pairs and Assigning Formal Charges

Lewis structures show all valence electrons, including lone pairs. Formal charge helps identify the most stable resonance forms.

• Lone pairs: Non-bonding pairs of electrons localized on an atom.

• Formal charge formula:

• Application: Add lone pairs to atoms like oxygen and nitrogen, and calculate formal charges to ensure the most stable structure.

Hybridization and Molecular Orbitals

Hybrid Orbitals in Organic Molecules

Hybridization describes the mixing of atomic orbitals to form new, equivalent hybrid orbitals suitable for bonding.

• sp3 hybridization: Tetrahedral geometry, 109.5° bond angles (e.g., methane, CH4).

• sp2 hybridization: Trigonal planar geometry, 120° bond angles (e.g., ethene, C2H4).

• sp hybridization: Linear geometry, 180° bond angles (e.g., acetylene, C2H2).

• Example: In nicotine, lone pairs on nitrogen atoms may reside in sp2 or sp3 orbitals depending on their bonding environment.

Bonding from Atomic Orbital Overlap

Bonds form from the overlap of atomic orbitals. The type of overlap determines the bond type.

• σ (sigma) bond: Head-on overlap of orbitals (e.g., s-s, s-p, or p-p along the internuclear axis).

• π (pi) bond: Side-on overlap of parallel p orbitals.

• σ* and π*: Antibonding orbitals formed from out-of-phase overlap.

• Example: A C–C single bond is a σ bond; a C=C double bond consists of one σ and one π bond.

Condensed and Skeletal Structures

Converting Condensed to Skeletal Structures

Skeletal (line-angle) structures are a simplified way to represent organic molecules, omitting hydrogen atoms bonded to carbons.

• Condensed structure: Shows all atoms in a compact form (e.g., CH3CH2OH).

• Skeletal structure: Each vertex or line end represents a carbon atom; hydrogens on carbons are implied.

• Application: Practice converting between these representations for clarity and efficiency in organic chemistry.

Hybridization in Complex Molecules

Identifying Hybridization in Large Structures

Atoms in complex molecules like Streptomycin A can be assigned hybridization based on their bonding and geometry.

• sp3: Four single bonds or lone pairs (tetrahedral).

• sp2: Double bond or three groups (trigonal planar).

• sp: Triple bond or two groups (linear).

Acidity, Basicity, and Resonance

Ranking Acidity and Basicity

Acidity and basicity are fundamental properties in organic chemistry, influenced by structure, electronegativity, and resonance.

• Acidity: The tendency of a compound to donate a proton (H+).

• Factors affecting acidity: Electronegativity, resonance stabilization, inductive effects, and atom size.

• Ranking example: For halogen acids, HI > HCl > HF in acidity due to bond strength and size.

• Basicity: The tendency of a compound to accept a proton.

• Strongest base: The species with the highest electron density and least stabilization of the lone pair.

Resonance Contributors

Resonance structures depict delocalization of electrons, increasing stability.

• Resonance: The actual structure is a hybrid of all valid resonance contributors.

• Rules: Only move electrons, not atoms; all contributors must be valid Lewis structures.

• Example: Acetate ion has two resonance forms with negative charge on either oxygen.

Acid-Base Reactions and Mechanisms

Drawing Acid-Base Reaction Products

Acid-base reactions involve proton transfer. Curved arrows show electron movement, and equilibrium arrows indicate favored side.

• Bronsted-Lowry acid: Proton donor.

• Bronsted-Lowry base: Proton acceptor.

• Equilibrium: Favors the side with the weaker acid and base (more stable species).

• Example:

Nomenclature and IUPAC Naming

Systematic Naming of Organic Compounds

IUPAC nomenclature provides a standardized way to name organic molecules based on structure.

• Longest carbon chain: Determines the parent name.

• Numbering: Assign lowest possible numbers to substituents.

• Functional groups: Suffixes and prefixes indicate functional groups (e.g., -ol for alcohols, -one for ketones).

• Example: 3-ethoxy-2-methylhexane, 4-isopropylcyclohexanol.

Physical Properties: Boiling Point

Comparing Boiling Points

Boiling point depends on molecular weight, intermolecular forces, and branching.

• Hydrogen bonding: Increases boiling point (e.g., alcohols, amines).

• Branching: More branching lowers boiling point due to decreased surface area.

• Example: Among isomers, the most branched compound typically has the lowest boiling point.

Conformational Analysis

Newman Projections and Dihedral Angles

Newman projections visualize the spatial arrangement of atoms around a bond, useful for analyzing conformational stability.

• Dihedral angle: The angle between two substituents on adjacent carbons, as viewed down the bond axis.

• Staggered conformation: Lower energy, more stable.

• Eclipsed conformation: Higher energy, less stable.

• Example: The anti conformation (180° dihedral angle) is most stable for two methyl groups.

Chair Conformations of Cyclohexane

Cyclohexane adopts chair conformations to minimize strain. Substituents prefer equatorial positions for stability.

• Axial vs. equatorial: Axial positions are parallel to the ring axis; equatorial are around the equator of the ring.

• Bulky groups: Prefer equatorial positions to reduce 1,3-diaxial interactions.

• Example: In cis-1-ethyl-3-methylcyclohexane, the most stable chair places both groups equatorial if possible.