Carbon & Tetravalency
Carbon has 4 valence electrons (2s² 2p²). Ground state: divalent. Excited state: tetravalent by promotion of one 2s electron to 2pz.
Tetrahedral Geometry: Four valencies directed towards corners of a regular tetrahedron. Bond angle = 109°28' (Le Bel & Van’t Hoff).
Hybridisation in Organic Compounds
Hybridisation: Mixing of atomic orbitals to form new equivalent hybrid orbitals.
Types of Hybridisation
| Type | Orbitals Used | Unused p-orbitals | σ bonds | π bonds | Bond Angle | Geometry | % s-character |
|---|---|---|---|---|---|---|---|
| sp³ | 1s + 3p | Nil | 4 | 0 | 109.5° | Tetrahedral | 25% |
| sp² | 1s + 2p | 1 | 3 | 1 | 120° | Trigonal Planar | 33.33% |
| sp | 1s + 1p | 2 | 2 | 2 | 180° | Linear | 50% |
Determination of Hybridisation
Method 1: π-bond count
| Number of π-bonds | Hybridisation |
|---|---|
| 0 | sp³ |
| 1 | sp² |
| 2 | sp |
Method 2: Electron Pair (ep) Method
ep = bp + lp
bp = number of atoms attached
lp = 0 (if π-bond, +ve charge, odd e⁻), lp = 1 (if –ve charge)
| ep | Hybridisation |
|---|---|
| 2 | sp |
| 3 | sp² |
| 4 | sp³ |
All C atoms: sp³ hybridised
All C atoms: sp² hybridised
Applications of Hybridisation
Size of Hybrid Orbitals
s-character ↑ → size ↓
sp > sp² > sp³
Electronegativity
s-character ↑ → electronegativity ↑
sp > sp² > sp³ > p
sp-hybrid C: electronegative
sp³-hybrid C: electropositive
Bond Length in Hydrocarbons
| Bond | Length (Å) | Bond | Length (Å) |
|---|---|---|---|
| C–H (sp³) | 1.112 | C–C (sp³) | 1.54 |
| C–H (sp²) | 1.103 | C=C (sp²) | 1.34 |
| C–H (sp) | 1.08 | C≡C (sp) | 1.20 |
Bond Strength
| Bond | Energy (kcal/mol) | Bond | Energy (kcal/mol) |
|---|---|---|---|
| C–H (sp³) | 104 | C–C (sp³) | 80–90 |
| C–H (sp²) | 106 | C=C (sp²) | 122–164 |
| C–H (sp) | 121 | C≡C (sp) | 123–199 |
Acidity of Hydrocarbons
Acidity ∝ % s-character
pKa: 25 < 44 < 50
Order: HC≡CH > CH₂=CH₂ > CH₃–CH₃
Steric Effect
Bulky groups cause mechanical interference → slow reaction.
Easily forms carbocation due to 3 bulky groups.
Forms carbocation to release strain.
Bulky groups prevent planarity → inhibit resonance.
Electronic Displacement in Covalent Bonds
Types of Effects
Polarisation (Permanent):
- Inductive
- Mesomeric
Polarisability (Temporary):
- Electromeric
- Inductomeric
- Hyperconjugative
JEE Main Weightage
3–4 questions expected from hybridisation, bond length, acidity, steric effects.