Organic Chemistry Notes CHAPTER 2

Organic Chemistry: Skeletal Structure and Resonance

Chapter Overview

  • Topics Covered:

    • Skeletal Structures

    • Resonance

    • Electronegativity and its trends

    • Valence Electrons and Bonds

    • Functional Groups

    • Acid-Base Chemistry

Part 1: Electronegativity Rules

  • Electronegativity Trends:

    • Increases from left to right across the periodic table.

    • Decreases from top to bottom.

  • Electronegativity Values:

    • Fluorine (F) has the highest value of 4.0.

    • Cesium (Cs) has the lowest value of 0.7.

    • Color Coding:

      • Red: Most electronegative

      • Yellow: Medium electronegativity

      • Green: Least electronegative

Part 2: Valence Electrons and Bonds

  • Common Formal Charges:

    • Table 2.2 Summary:

      • Carbon (C): Valence = 4, Charge = 0

      • Nitrogen (N): Valence = 5, Charge = +1 or -1

      • Sulfur (S): Valence = 6, Charge = -1 or +1

      • Phosphorus (P): Valence = 5, Charge = 0

Part 3: Worked Example - Methylamine

  • Drawing Methylamine:

    • Valences:

      • Hydrogen (H) = 1

      • Carbon (C) = 4

      • Nitrogen (N) = 5

    • Structure: Connect atoms as H3C—NH2.

    • Identify lone pairs: One on nitrogen (N).

    • Dipole Moment: Direction from least electronegative (C) to most (N); represented as μ = 1.31.

Part 4: Skeletal (Bond-Line) Structures

  • Drawing Techniques:

    • Implicit hydrogens and carbons to simplify notation.

    • Example: Caffeine

      • Condensed formula: C8H10N4O2

Drawing Rules:

  • Rule 1: Carbon at each line intersection and end of lines.

  • Rule 2: Implicitly add hydrogen atoms connected to carbons.

  • Rule 3: Show heteroatoms explicitly (atoms besides C and H).

Zigzag Format:

  • Represents bond angles for sp³ and sp² hybridized carbons.

  • Linear geometry indicated for sp-hybridized atoms.

Part 5: Functional Groups

  • Overview of Functional Groups:

    • Hydrocarbons, Oxygen heteroatomic, Nitrogen-based, etc.

    • Naming conventions:

      • Alkane: -ane (e.g., Ethane)

      • Alkene: -ene (e.g., Ethene)

      • Alcohol: -ol (e.g., Ethanol)

      • Aldehyde: -al (e.g., Ethanal)

      • Ketone: -one (e.g., Propanone)

      • Carboxylic Acid: -oic acid (e.g., Ethanoic Acid)

Part 6: Resonance Structures

  • Introduction to Resonance:

    • Represents delocalization of electrons.

    • Key for understanding stability in molecules.

  • Resonance Structures Rules:

    • Individual resonance forms are not real, they differ in electron placement.

    • Forms obey normal valency rules.

    • Resonance hybrids are more stable than individual forms.

Cardinal Rule of Resonance:

  • Pi electrons only.

Part 7: Drawing Resonance Structures

  • Key Considerations:

    • Use square brackets around resonance structures.

    • Draw electron-pushing arrows to indicate electron flow.

  • Important Notes:

    • Never break a single bond while creating resonance forms.

    • Follow the octet rule; temporary configurations with fewer than 8 electrons are allowed, but no more.

Examples of Resonance Patterns

  1. Allylic Lone Pair:

    • Lone pairs adjacent to multiple bonds can appear in resonance structures.

  2. Pi-bonds between Different Atoms:

    • Can stabilize charges through delocalization.

  3. Conjugated Pi-bonds:

    • Common in aromatic compounds, allow for multiple resonance contributions.

Part 8: Acid-Base Chemistry

  • Brønsted–Lowry Definition:

    • Acid: Proton donor (H+).

    • Base: Proton acceptor.

  • Conjugate Acid/Base:

    • Forms when substance donates (acid) or accepts (base) a proton.

General Acid-Base Reaction Format:

  • General Reaction:
    H−A+B<br>ightarrowA−+H−BH-A + B <br>ightarrow A^- + H-B

  • Determining Acid Strength:

- Characterized by pKa values, where lower pKa = stronger acid.

Factors affecting Acid Strength (ARIO):

  1. Atom type carrying the charge.

  2. Resonance stabilization.

  3. Induction from neighboring atoms/groups.

  4. Orbital holding the charge (size and electronegativity differences).

  • Leveling Effect:

    • Solvents have pKa values influencing acid/base reactions.

    • Example: Strong acids react with solvents, making their effect irrelevant.

Lewis Acids and Bases

  • Lewis Definition:

    • Lewis Acid: Electron-pair acceptor.

    • Lewis Base: Electron-pair donor.

Final Considerations

  • Understanding resonance and functional groups is crucial for predicting reactivity in organic mechanisms.

  • Differentiation between Brønsted and Lewis definitions helps explain a broader range of acid-base behavior.