Alkynes Basics

🔗 Alkynes: The Triple Bond Superheroes

The Story of Three Friends Holding Hands

Imagine you have best friends. Sometimes you hold hands with one friend — that’s a single bond. Sometimes you’re extra close and hold both hands with a friend — that’s a double bond. But what if you’re SO close that you hold hands with BOTH arms AND link your legs together? That’s a TRIPLE BOND — and that’s what makes alkynes special! 🤝🤝🤝

🎯 What Are Alkynes?

Alkynes are hydrocarbons with at least one carbon-carbon triple bond.

Think of it this way:

• Alkanes = Single handshake (C-C)
• Alkenes = Double high-five (C=C)
• Alkynes = Triple super-grip (C≡C)

The Formula Family

Simple Example:

• Ethyne (C₂H₂): The simplest alkyne
• Also called acetylene — the gas welders use!

H—C≡C—H

That’s it! Two carbons holding on with a triple bond, each with one hydrogen.

🏗️ Structure of the Triple Bond

What’s Inside That Triple Bond?

A triple bond is like a bundle of three ropes:

graph TD
    A["🔵 Triple Bond"] --> B["1 Sigma Bond<br/>σ bond"]
    A --> C["2 Pi Bonds<br/>π bonds"]
    B --> D["Head-to-head overlap<br/>Strong & stable"]
    C --> E["Side-by-side overlap<br/>Above & below"]

The Geometry Story

When you triple-grip your friend, you both have to stand in a straight line. There’s no wiggle room!

Why 180°? The two sp orbitals point in opposite directions, making a straight line. The π bonds sit above and below like a cloud around the sigma bond.

Bond Length & Strength

Magic Rule: More bonds = Shorter & Stronger!

Think of it like hugging. The tighter you hug (more bonds), the closer you get!

🌡️ Physical Properties of Alkynes

Are They Like Water or Oil?

Alkynes behave a lot like their cousins (alkanes and alkenes):

1. State of Matter

• First three alkynes (C₂-C₄): Gases 💨
• C₅-C₁₃: Liquids 💧
• C₁₄ and up: Solids 🧊

2. Solubility

• Water? Nope! (Non-polar molecules don’t mix with polar water)
• Oil or organic solvents? Yes! They’re best buddies.

3. Density

• Lighter than water (density < 1 g/cm³)
• They float! 🎈

4. Boiling Points

Pattern: Bigger molecule = Higher boiling point

Why? More atoms = more surface area = stronger London forces = harder to boil!

⚡ Acidity of Terminal Alkynes

The Surprising Sour Side!

Here’s something amazing: Terminal alkynes can act like weak acids!

What’s a Terminal Alkyne? An alkyne with a hydrogen attached to the triple-bonded carbon.

Terminal:     H—C≡C—R    ✅ Has H on triple bond
Internal:     R—C≡C—R    ❌ No H on triple bond

Why Are They Acidic?

Think of sp carbons as GREEDY for electrons:

More s-character = Electrons held closer = Easier to release H⁺

The Acid Strength Ladder

graph TD
    A["💪 MOST ACIDIC"] --> B["Ethyne<br/>HC≡CH<br/>pKa = 25"]
    B --> C["Ammonia<br/>NH₃<br/>pKa = 38"]
    C --> D["Ethene<br/>H₂C=CH₂<br/>pKa = 44"]
    D --> E["Ethane<br/>H₃C-CH₃<br/>pKa = 50"]
    E --> F["🐣 LEAST ACIDIC"]

Making Acetylide Anions

When terminal alkynes lose their H⁺, they form acetylide ions:

HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃

Real-Life Use: These acetylide ions are like chemical LEGO pieces — they can attach to other molecules to build bigger ones!

🧪 From Calcium Carbide: Industrial Acetylene

The Rock That Makes Fire!

Here’s a cool industrial trick to make acetylene:

Step 1: Make Calcium Carbide

CaO + 3C → CaC₂ + CO
(lime + carbon → calcium carbide)

This happens in a super hot electric furnace (2000°C)!

Step 2: Add Water

CaC₂ + 2H₂O → HC≡CH + Ca(OH)₂
(calcium carbide + water → acetylene + lime)

Why This Matters

Acetylene from carbide was HUGE in history!

• Old miners used carbide lamps (water + carbide = light!)
• Welders still use acetylene torches today
• Acetylene burns at 3300°C — hot enough to cut through steel!

graph TD
    A["🪨 Limestone<br/>CaCO₃"] --> B["🔥 Heat"]
    B --> C["CaO<br/>#40;Quicklime#41;"]
    C --> D["+ Carbon<br/>⚡ 2000°C"]
    D --> E["CaC₂<br/>#40;Calcium Carbide#41;"]
    E --> F["+ Water 💧"]
    F --> G["⚡ Acetylene<br/>HC≡CH"]

🔄 Alkyne Dehydrohalogenation

Removing the Extras to Get Triple Bonds!

Dehydrohalogenation = “De” (remove) + “Hydro” (hydrogen) + “Halogenation” (halogen)

It’s like undoing a zipper — you remove H and X (halogen) to create a new bond!

The Recipe

Starting Material: Dihaloalkane (two halogens on neighboring carbons)

What You Need: Strong base (like NaNH₂ in liquid NH₃)

What Happens:

        H Br
        |  |
    H—C—C—H   + 2NaNH₂  →  H—C≡C—H + 2NaBr + 2NH₃
        |  |
        H Br

Step by Step

graph TD
    A["🧪 Vicinal Dihalide<br/>X-C-C-X"] --> B["Add Strong Base<br/>NaNH₂"]
    B --> C["Remove H + X<br/>#40;First elimination#41;"]
    C --> D["Vinyl Halide<br/>C=C-X"]
    D --> E["Add More Base"]
    E --> F["Remove H + X<br/>#40;Second elimination#41;"]
    F --> G["🎯 ALKYNE!<br/>C≡C"]

The Two-Step Dance

Why Strong Base? The vinyl halide (intermediate) is stubborn! You need a very strong base like NaNH₂ to pull off that second elimination.

Real Example

Making Propyne from 1,2-Dibromopropane:

CH₃—CHBr—CH₂Br + 2NaNH₂ → CH₃—C≡CH + 2NaBr + 2NH₃

🎯 Quick Summary

🌟 Why Alkynes Matter

Alkynes aren’t just chemistry textbook stuff — they’re everywhere!

• Welding — Acetylene torches cut through metal
• Pharmaceuticals — Many medicines have alkyne groups
• Plastics — Building blocks for polymers
• Organic Synthesis — Chemists love using alkynes to build complex molecules

You now understand the superheroes of organic chemistry — molecules that hold on with THREE bonds and won’t let go! 🦸‍♂️

Remember: When someone asks about alkynes, think of three friends holding hands in a straight line — strong, close, and inseparable! 🤝🤝🤝