Chemistry: Electronegativity and Bonding Concepts

Electronegativity

• Electronegativity measures an atom's ability to attract electrons. It's a measure of reactivity.
• Electronegativity increases across a period and decreases down a group.
• Electronegativity is affected by the number of protons and electron shells. More protons lead to higher attraction for electrons.

Hydrogen Bonding

• A hydrogen bond is an electrostatic attraction between polar molecules.
• It occurs when a hydrogen atom bonded to a highly electronegative atom (like N, O, or F) is attracted to another nearby highly electronegative atom.
• It's a strong dipole-dipole attraction, not a true chemical bond.

Hybrid Orbitals

• Hybrid orbitals are mixed orbitals formed by combining atomic orbitals.
• Orbital hybridization is the process of combining atomic orbitals.
• If one s and three p orbitals combine to form four equivalent orbitals, the resulting orbitals have one part s character and three parts p character.
• The superscript "3" in sp³ indicates three p orbitals combining with one s orbital.
• Each resulting hybrid orbital has 25% s character and 75% p character.

Substitution Reactions

• Substitution reactions replace one functional group in a molecule with another.
• They are crucial in organic chemistry.
• Substitution reactions are categorized as electrophilic or nucleophilic, depending on the reagent involved.

Addition Reactions

• Atoms or groups attach to a molecule without displacing other atoms/groups in an addition reaction.
• Unsaturated compounds undergo addition reactions.

Elimination Reactions

• Atoms or groups detach from a molecule without displacing others.
• It's the reverse of addition reactions.

Chemical Formulas in Organic Chemistry

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound. (e.g., CH₃ for ethane)
• Molecular Formula: Shows the actual number of each type of atom in a molecule. (e.g., C₂H₆ for ethane)
• Structural Formula: Displays the arrangement of atoms and their connections within a molecule. It shows the structure, and bond order.
• Different types of structural formulas include single, double, and triple bonds.

Representations of Structural Formulas

• Ball-and-stick model: Represents atoms as spheres and bonds as sticks.
• Dash formula: Shows 3D structure using wedges and dashes for bonds coming out of or behind the plane of the paper.
• Condensed formula: Shortened way of writing the molecular structure. (e.g., CH₃CH₂CH₂OH)
• Bond-line formula: A simplified way of drawing structural formulas, where carbon and hydrogen atoms are implied.

Acids and Bases

• Brønsted-Lowry acid: A proton donor.
• Brønsted-Lowry base: A proton acceptor.
• Lewis acid: An electron pair acceptor.
• Lewis base: An electron pair donor.

Nature of Chemical Reactions

• Chemical reactions involve breaking existing bonds and forming new ones.
• Energy is released when a bond is formed and absorbed when a bond is broken. The amount is the same.
• Covalent bonds can break in two distinct ways (discussed in next section).

Breaking Covalent Bonds

• Heterolysis (Heterolytic Fission): Asymmetrical bond breaking; one atom retains both electrons.
• Ions are formed in this process
• Homolysis (Homolytic Fission): Symmetrical bond breaking; each of the atoms retains one electron. Radical reactions.

Heterolysis of Carbon-Carbon Bonds

• Carbocations are formed when a carbon-carbon bond undergoes heterolysis where one of the atoms keeps both electrons.
• Carbanions are formed when a carbon-carbon bond undergoes heterolysis where the other atom keeps both electrons.
• Electronegativity difference between the attached atom and the carbon atom plays a big role.

Types of Reagents

• Nucleophiles: Electron-rich species that donate electrons to share.

  • Can be negatively charged anions (e.g., OH⁻, CN⁻)
  • OR neutral molecules with lone pairs (e.g., H₂O)

• Electrophiles: Electron-poor species that accept electrons.

  • Can be positively charged cations (e.g., H⁺, NO₂⁺)
  • OR neutral molecules that are electron deficient (e.g., FeCl₃, AlCl₃)