Addition Reactions

🧪 Alkenes: Addition Reactions

The Double Bond Party - Where Atoms Love to Join!

Imagine a double bond between two carbon atoms is like two best friends holding hands with BOTH hands (a double handshake). They’re really close, but they’re so popular that other atoms want to join their friendship circle!

When new atoms arrive, the friends let go of ONE handshake to make room for the newcomers. That’s basically what addition reactions are all about!

🎯 The Big Picture

graph TD
    A["Alkene with C=C"] --> B{Addition Reaction}
    B --> C["New atoms attach"]
    C --> D["Double bond becomes Single bond"]
    D --> E["Product with more atoms!"]

Simple rule: Double bond opens up → New friends join in!

1️⃣ Alkene Hydrogenation

What Is It?

Hydrogenation = Adding hydrogen (H₂) to an alkene

Think of it like this: The double bond is a closed door with two locks. Hydrogen is like a key that opens one lock, and now each carbon gets one hydrogen friend.

The Recipe

• Ingredients: Alkene + H₂ gas
• Secret helper: A metal catalyst (like Pt, Pd, or Ni)
• Result: Alkane (no more double bond!)

Example

    H   H
    |   |
H₂C=CH₂  +  H₂  →  H₃C-CH₃
(ethene)         (ethane)

Real-life use: This is how vegetable oils become solid butter! The double bonds in oils get “filled up” with hydrogen.

2️⃣ Alkene Halogenation

What Is It?

Halogenation = Adding halogens (like Br₂ or Cl₂) to an alkene

Imagine the double bond as a magnet, and halogens are metal pieces that get pulled in and stick on both sides!

The Recipe

• Ingredients: Alkene + Halogen (Br₂ or Cl₂)
• No catalyst needed! (room temperature)
• Result: Dihalide (halogen on each carbon)

Example

    Br  Br
    |   |
H₂C=CH₂  +  Br₂  →  H₂C-CH₂
(ethene)           (1,2-dibromoethane)

Cool test: Brown bromine water turns COLORLESS when you add an alkene. This is how chemists test for double bonds!

3️⃣ The Markovnikov Rule

The Big Question

When adding H-X (like HBr) to an uneven alkene, WHERE does each part go?

The Rule (Easy Version!)

“The RICH get RICHER” 🤑

• The hydrogen (H) goes to the carbon that ALREADY HAS MORE HYDROGENS
• The other part (X) goes to the carbon with FEWER hydrogens

Why This Name?

Vladimir Markovnikov was a Russian chemist who figured this out in 1869!

Example

CH₃-CH=CH₂  +  HBr  →  CH₃-CHBr-CH₃
(propene)              (2-bromopropane)

The H went to the CH₂ (already has 2 H’s → now 3!) The Br went to the CH (only had 1 H → stays at 1!)

4️⃣ Anti-Markovnikov Addition

The Plot Twist! 🔄

Sometimes, we WANT the opposite result - H goes to the carbon with FEWER hydrogens!

How to Do It?

Add peroxides (like ROOR) to the mix. This changes everything!

Example

CH₃-CH=CH₂  +  HBr  +  peroxide  →  CH₃-CH₂-CH₂Br
(propene)                           (1-bromopropane)

Notice: The Br ended up on the END carbon instead!

Memory trick: “Peroxides make things PERVERSE” - they flip the normal rule!

5️⃣ Hydration of Alkenes

What Is It?

Hydration = Adding water (H₂O) to make an alcohol

The double bond is thirsty! It drinks water and becomes an alcohol.

The Recipe

• Ingredients: Alkene + H₂O
• Helper: Acid catalyst (like H₂SO₄)
• Result: Alcohol!

Example

CH₃-CH=CH₂  +  H₂O  →  CH₃-CHOH-CH₃
(propene)              (2-propanol)

Note: This follows Markovnikov’s rule!

• OH goes to the MORE substituted carbon
• H goes to the LESS substituted carbon

6️⃣ Electrophilic Addition

The Master Mechanism! 🎭

This is HOW all these reactions actually work at the atomic level.

What’s an Electrophile?

Electro = electricity lover Phile = friend

An electrophile is an “electron-hungry” species. It sees the electron-rich double bond and thinks: “YUMMY!” 🍽️

The 3-Step Dance

graph TD
    A["Step 1: Electrophile approaches"] --> B["Step 2: Attacks π electrons"]
    B --> C["Forms carbocation intermediate"]
    C --> D["Step 3: Nucleophile attacks"]
    D --> E["Product formed!"]

Example with HBr

1. H⁺ approaches the double bond (it’s electron-hungry!)
2. H⁺ grabs electrons from the double bond → Creates a carbocation (C⁺)
3. Br⁻ attacks the positive carbon → Done!

Why Markovnikov happens: The carbocation forms on the carbon where it’s MOST STABLE (more substituted = more stable)!

7️⃣ Syn and Anti Addition

The Direction Question

When TWO things add to a double bond, do they come from the SAME side or OPPOSITE sides?

Syn Addition = Same Side

Both new atoms approach from the SAME face of the molecule.

Like: Two friends sitting next to each other on a seesaw 🎢

Example: Hydrogenation (H₂ + catalyst)

• Both H atoms come from the SAME side
• They’re delivered by the metal surface together!

Anti Addition = Opposite Sides

The two new atoms approach from OPPOSITE faces.

Like: Two friends on opposite sides of a revolving door 🚪

Example: Halogenation (Br₂)

• The bromine atoms end up on OPPOSITE sides
• There’s a special “bromonium ion” intermediate!

graph TD
    A["Syn Addition"] --> B["Same side approach"]
    B --> C["Both atoms ABOVE or BELOW"]

    D["Anti Addition"] --> E["Opposite side approach"]
    E --> F["One ABOVE, one BELOW"]

🗺️ Quick Summary Map

🌟 The ONE Analogy to Remember

Think of the double bond as a popular kid at school with an extra friendship bracelet to give away:

• Hydrogenation: Twin hydrogen buddies come together (syn) and share the bracelet
• Halogenation: Two halogen siblings approach from opposite doors (anti)
• Markovnikov: The new friend goes where there’s already a big friend group (stability!)
• Anti-Markovnikov: With a special invitation (peroxide), rules change!
• Electrophilic: Everyone loves the popular kid’s extra electrons!

💪 You’ve Got This!

Addition reactions are just atoms joining the double-bond party. The double bond opens up, new atoms join in, and everyone’s happy!

Remember:

• Double bond = extra electrons = attractive to electrophiles
• Markovnikov = rich get richer
• Syn = same side, Anti = opposite sides
• Catalysts and conditions control the outcome!

Now you’re ready to predict products like a chemistry pro! 🧪✨