Organic Chemistry Basics

Overview

This lecture introduces fundamental concepts in organic chemistry, focusing on bonding, Lewis structures, types of organic molecules, and functional groups.

Atomic Bonding Preferences

• Carbon forms 4 bonds; hydrogen forms 1; beryllium 2; boron 3; nitrogen 3; oxygen 2; halogens (F, Cl, Br, I) 1 bond.
• Elements in the second row (C, N, O, F) prefer 8 valence electrons (octet rule).

Covalent and Ionic Bonds

• Covalent bonds involve shared electrons: nonpolar (equal sharing), polar (unequal sharing), hydrogen bonds (H attached to N, O, or F).
• Ionic bonds involve electron transfer, forming cations (positive) and anions (negative), held by electrostatic attraction.

Electronegativity and Bond Polarity

• A bond is polar if electronegativity difference ≥ 0.5; nonpolar if <0.5.
• Hydrocarbons (C-H bonds) are nonpolar due to small electronegativity difference.

Lewis Structures and Bond Types

• Lewis structures show bonding and lone pairs to satisfy each atom's bonding preference.
• Alkanes: single bonds only (saturated); formula CnH2n+2.
• Alkenes: at least one C=C double bond (unsaturated).
• Alkynes: at least one C≡C triple bond (unsaturated).

Bond Length, Strength, and Composition

• Single bonds (longest, weakest), double bonds (intermediate), triple bonds (shortest, strongest).
• Sigma (σ) bonds: strongest, present in all single bonds.
• Pi (π) bonds: found in double and triple bonds; sigma bonds are stronger than pi bonds.

Bond Order and Hybridization

• Bond order: single = 1, double = 2, triple = 3.
• Hybridization of carbon: 4 groups = sp3, 3 groups = sp2, 2 groups = sp.
• Bond hybridization is based on the connected atoms’ hybridizations (e.g., sp3-s).

Sigma and Pi Bond Counting

• Each single bond = 1 sigma bond.
• Double bond = 1 sigma + 1 pi.
• Triple bond = 1 sigma + 2 pi.

Formal Charges

• Formal charge = valence electrons - (number of bonds + dots).
• Positive charge (carbocation); negative charge (carbanion); radical (odd number of electrons).

Functional Groups and Nomenclature

• Alcohol: –OH group (e.g., ethanol).
• Aldehyde: terminal carbonyl (C=O) with H (e.g., ethanal).
• Ketone: internal carbonyl (e.g., propanone).
• Ether: oxygen between carbons (e.g., dimethyl ether).
• Ester: carbonyl adjacent to oxygen and carbon chain (e.g., methyl ethanoate).
• Carboxylic acid: carbonyl + –OH (e.g., pentanoic acid).

Expanding Condensed Structures

• Methyl groups (CH3) at molecule ends; CH2 in the middle; CH units branch with substituents.

Key Terms & Definitions

• Octet Rule — Atoms prefer 8 electrons in their valence shell.
• Sigma Bond (σ) — Strongest covalent bond, head-on overlap.
• Pi Bond (π) — Weaker side-by-side overlap, found in double/triple bonds.
• Functional Group — Specific group of atoms providing characteristic properties and reactivity.

Action Items / Next Steps

• Practice drawing Lewis structures for given molecular formulas.
• Memorize alkane names up to 10 carbons and their formulas.
• Review functional group structures and naming conventions.