The Reactivity Riddle: Why Alkenes are more Reactive than Alkanes but less than Alkynes?
Unravel the mystery behind hydrocarbon reactivity based on bond strength, electron density, and molecular geometry.
The Fundamental Reactivity Puzzle
In organic chemistry, we observe a fascinating pattern: Alkenes are more reactive than alkanes but less reactive than alkynes. This seems counterintuitive at first - shouldn't stronger bonds mean less reactivity? Let's solve this riddle step by step.
🎯 JEE Importance
This concept appears in electrophilic addition reactions, Markovnikov's rule applications, and stability comparisons - all crucial for JEE Main and Advanced.
🚀 Quick Navigation
1. Bond Strength Analysis
The Bond Strength Paradox
At first glance, bond strength data seems to contradict the reactivity order. Let's examine the actual bond energies:
| Bond Type | Bond Length (Å) | Bond Energy (kJ/mol) | Character |
|---|---|---|---|
| C-C (Alkane) | 1.54 | 347 | Single (σ) |
| C=C (Alkene) | 1.34 | 611 | Double (σ + π) |
| C≡C (Alkyne) | 1.20 | 837 | Triple (σ + 2π) |
💡 Key Insight
While total bond energy increases from single to triple bonds, the π-bond component is actually weaker and more accessible to reactions.
π-bond energy: ~264 kJ/mol (relatively weak compared to σ-bonds)
Breaking Down the Bonds
Alkanes (C-C Single Bond)
- Pure σ-bond - strong, localized electrons
- Electrons are tightly held between nuclei
- Requires high energy to break (347 kJ/mol)
- Mainly undergoes substitution reactions
Alkenes (C=C Double Bond)
- One σ-bond + One π-bond
- π-bond electrons are delocalized above and below molecular plane
- π-bond weaker (∼264 kJ/mol) and more accessible
- Undergoes addition reactions easily
Alkynes (C≡C Triple Bond)
- One σ-bond + Two π-bonds
- Even more electron density available
- π-bonds are weaker and more exposed
- Highest reactivity in addition reactions
2. Electron Density & Accessibility
The Real Reason: Electron Availability
Reactivity is not just about bond strength - it's about how easily electrons can be attacked by electrophiles.
Alkanes
Electrons tightly bound in σ-bonds, difficult to access
Alkenes
π-electrons exposed and available for attack
Alkynes
Maximum π-electron density, most accessible
Molecular Geometry & Electron Exposure
Alkane Geometry
- Tetrahedral geometry around each carbon
- σ-bonds directed along internuclear axis
- Electron cloud between nuclei - well protected
- Electrophiles cannot easily access electrons
Alkene Geometry
- Trigonal planar geometry
- π-bond electron cloud above and below molecular plane
- Electrons exposed and accessible to electrophiles
- Perfect for electrophilic addition reactions
Alkyne Geometry
- Linear geometry
- Two perpendicular π-bond clouds around σ-bond axis
- Maximum electron exposure from all sides
- Even more accessible to electrophiles than alkenes
3. The Complete Reactivity Picture
Reactivity Order: Alkynes > Alkenes > Alkanes
Evidence from Chemical Reactions
Reaction with Bromine Water
- Alkanes: No reaction (requires UV light)
- Alkenes: Decolorizes bromine water quickly
- Alkynes: Decolorizes bromine water fastest
Reaction Rate: Alkyne > Alkene > Alkane
Reaction with KMnO₄ (Baeyer's Test)
- Alkanes: No reaction
- Alkenes: Decolorizes slowly, forms glycol
- Alkynes: Decolorizes rapidly (except terminal alkynes)
Electrophilic Addition Reactions
- Alkanes: Substitution only (requires catalyst)
- Alkenes: Addition occurs readily (Markovnikov)
- Alkynes: Addition occurs most readily
🎯 JEE Trick Question Alert!
Exception: Terminal alkynes ($\ce{R-C≡C-H}$) are LESS reactive than alkenes toward electrophilic addition due to the sp-hybridization making carbon less electronegative.
Remember: Internal alkynes > Alkenes > Terminal alkynes > Alkanes
4. Practice Problems
Test Your Understanding
Problem 1: Arrange in increasing order of reactivity toward electrophilic addition: Ethane, Ethene, Ethyne
Problem 2: Why does ethene decolorize bromine water faster than ethane but slower than ethyne?
Problem 3: Explain why terminal alkynes are an exception to the general reactivity order.
Problem 4: Which would react fastest with HBr: CH₄, CH₂=CH₂, or HC≡CH? Explain.
JEE Advanced Tip
For terminal alkynes, the acidity (due to sp-hybridization) makes them reactive toward strong bases, but less reactive toward electrophiles compared to internal alkynes and alkenes.
📋 Quick Reference Guide
Reactivity Factors
- Bond Strength: π-bonds weaker than σ-bonds
- Electron Density: More π-bonds = more electrons
- Accessibility: π-electrons exposed to attack
- Geometry: Linear > Planar > Tetrahedral exposure
Key Comparisons
- Alkanes: Only substitution reactions
- Alkenes: Electrophilic addition (Markovnikov)
- Alkynes: Fastest electrophilic addition
- Exception: Terminal alkynes less reactive
Memory Aid
"More π-bonds = More Problems for Electrophiles"
More π-bonds means more electron density exposed, making it easier for electrophiles to attack.
🎯 Exam Strategy
Remember: Alkynes (except terminal) > Alkenes > Alkanes for electrophilic addition
Always check if it's a terminal alkyne - they're special cases!
Understand WHY the reactivity order exists - don't just memorize
In descriptive answers, explain both bond strength AND electron accessibility
Mastered Hydrocarbon Reactivity?
Continue your organic chemistry journey with reaction mechanisms and named reactions