🧪 Core Organic Chemistry Skills

Your Detective Toolkit for Understanding Molecules

Imagine you’re a detective, and molecules are your mystery cases. Every detective needs a toolkit—magnifying glass, fingerprint kit, notebook. In organic chemistry, your toolkit has four essential skills:

1. Degree of Unsaturation – Counting the “missing pieces”
2. Formal Charge – Finding who “owns” the electrons
3. Resonance Structures – Seeing a molecule’s different disguises
4. Curved Arrow Notation – Tracking where electrons travel

Let’s master each one!

🔍 Skill 1: Degree of Unsaturation (DoU)

The Cookie Cutter Analogy

Think of hydrogen atoms like sprinkles on a cookie. A “fully decorated” cookie (saturated molecule) has the maximum sprinkles possible. But sometimes, cookies have fewer sprinkles because:

• There’s a double bond (like a dent in the cookie)
• There’s a ring (cookie folded into a donut shape)
• There’s a triple bond (deep groove)

Degree of Unsaturation tells us how many “dents” or “folds” are missing from our cookie!

The Magic Formula

For a molecule with formula CₐHᵦNᵧOᵨXₑ:

DoU = (2a + 2 + b - c - e) ÷ 2

Where:

• a = number of Carbons
• b = number of Nitrogens (add 1 for each N)
• c = number of Hydrogens
• e = number of Halogens (F, Cl, Br, I)
• Oxygen doesn’t count! (It’s neutral)

Quick Reference Table

Example: What’s in C₆H₆?

DoU = (2×6 + 2 - 6) ÷ 2
DoU = (12 + 2 - 6) ÷ 2
DoU = 8 ÷ 2 = 4

DoU = 4 → This is benzene! (1 ring + 3 double bonds = 4)

Example: C₄H₈O

DoU = (2×4 + 2 - 8) ÷ 2
DoU = (8 + 2 - 8) ÷ 2 = 1

DoU = 1 → One double bond OR one ring!

⚡ Skill 2: Formal Charge Calculation

The Pizza Slice Analogy

Imagine electrons are pizza slices at a party. Every atom “expects” a certain number of slices based on their usual appetite:

• Carbon expects 4 slices
• Nitrogen expects 5 slices
• Oxygen expects 6 slices
• Hydrogen expects 1 slice

Formal charge = What they expect − What they actually got

The Counting Rules

Formal Charge = Valence electrons
              − Lone pair electrons
              − ½(Bonding electrons)

Or simpler:

FC = V − L − B

• V = Valence electrons (from periodic table)
• L = Lone pair electrons (non-bonded, count all)
• B = Bonds (each bond = sharing, so count once)

Example: Hydronium Ion (H₃O⁺)

graph TD
    A["Oxygen in H₃O⁺"] --> B["Valence e⁻ = 6"]
    B --> C["Lone pairs = 2 #40;one pair#41;"]
    C --> D["Bonds = 3"]
    D --> E["FC = 6 − 2 − 3 = +1"]

Oxygen normally has 6 valence electrons. Here it has:

• 2 lone pair electrons
• 3 bonds

FC = 6 − 2 − 3 = +1 ✓

Example: Ammonium Ion (NH₄⁺)

Nitrogen expects 5 electrons:

• Lone pairs = 0
• Bonds = 4

FC = 5 − 0 − 4 = +1 ✓

Example: Hydroxide Ion (OH⁻)

Oxygen in OH⁻:

• Valence = 6
• Lone pairs = 6 (three pairs)
• Bonds = 1

FC = 6 − 6 − 1 = −1 ✓

Quick Tips

🎭 Skill 3: Resonance Structures

The Superhero Costume Analogy

Spider-Man is always Peter Parker, but he can wear different costumes—classic red, black suit, Iron Spider. Same person, different looks!

Molecules work the same way. Resonance structures are different ways to draw the SAME molecule. The electrons can be shown in different positions, but:

• Atoms stay in place (only electrons move)
• The real molecule is a BLEND of all structures
• More structures = more stable molecule

Rules for Drawing Resonance

1. Only move electrons (π bonds and lone pairs)
2. Never break σ bonds (single bonds stay!)
3. Keep the same number of electrons
4. Obey octet rule (mostly—except for some atoms)

Example: Carbonate Ion (CO₃²⁻)

The carbonate ion has THREE resonance structures:

Structure 1:     Structure 2:     Structure 3:
    O⁻              O               O⁻
    ‖               ‖               |
    C               C               C
   / \             / \             / \
  O⁻  O           O⁻  O⁻          O   O⁻
                                   ‖

Reality: All three C-O bonds are IDENTICAL (1.33 bond order)

Example: Benzene

graph LR
    A["Structure A"] <--> B["Structure B"]
    A --> C["Real benzene is the average"]
    B --> C

The two Kekulé structures of benzene show alternating single and double bonds, but the real benzene has all equal bonds!

Ranking Resonance Structures

Not all resonance structures are equal! Better structures:

1. ✅ Have more atoms with octets
2. ✅ Have fewer formal charges
3. ✅ Put negative charge on electronegative atoms
4. ✅ Have no adjacent same charges

➡️ Skill 4: Curved Arrow Notation

The GPS for Electrons

Curved arrows are like GPS directions for electrons. They show:

• WHERE electrons come from (tail of arrow)
• WHERE electrons go (head of arrow)

Two Types of Arrows

The Golden Rules

1. Arrows start from electrons (lone pairs or bonds)
2. Arrows point to atoms or bonds
3. Arrow tail = source (where e⁻ leave)
4. Arrow head = destination (where e⁻ go)

Example: Proton Transfer

    H           H
    |           |
H—O: + H—Cl → H—O—H + :Cl⁻
    ↑
    Arrow from lone pair to H

Step-by-step:

1. Oxygen’s lone pair attacks hydrogen
2. H-Cl bond breaks toward Cl
3. Products: H₃O⁺ and Cl⁻

Example: Resonance Arrow Movement

Moving electrons in carbonate:

    :O:⁻           :O:
     |              ‖
     C    →        C
    / \           / \
   O   O⁻       :O:⁻ O⁻
   ‖

Arrow shows: π electrons from C=O move to become lone pair on oxygen

Common Mistakes to Avoid

❌ Wrong: Arrow pointing FROM atom TO electrons ✅ Right: Arrow pointing FROM electrons TO atom

❌ Wrong: Breaking a single bond without showing where electrons go ✅ Right: Every arrow must show complete electron movement

🎯 Putting It All Together

The Detective’s Checklist

When you see a new molecule:

graph TD
    A["New Molecule"] --> B["Calculate DoU"]
    B --> C["Identify rings & multiple bonds"]
    C --> D["Calculate formal charges"]
    D --> E["Check total charge matches"]
    E --> F["Draw resonance structures"]
    F --> G["Use arrows to show electron movement"]

Practice Problem

Molecule: CH₃NO₂ (Nitromethane)

1. DoU = (2×1 + 2 + 1 - 3) ÷ 2 = 1 → One double bond!

2. Formal Charges:

   • N has 4 bonds, 0 LP → FC = 5 - 0 - 4 = +1
   • One O has 1 bond, 3 LP → FC = 6 - 6 - 1 = −1
   • Other O has 2 bonds, 2 LP → FC = 6 - 4 - 2 = 0

3. Resonance: Two equivalent structures (negative charge on either oxygen)

🚀 You Did It!

You now have four powerful tools in your chemistry detective kit:

Remember: These skills work TOGETHER. Master them, and you’ll solve any molecular mystery! 🔬

“Chemistry is not about memorizing—it’s about understanding patterns. And now, you have the patterns!”