Inductive, resonance (mesomeric), electromeric and hyperconjugation effects move electron density; they decide the stability of carbocations, carbanions and free radicals and the strength of acids and bases.
How it's asked: Ranking carbocation stability, acid strength of substituted acids and phenols, counting hyperconjugative structures, identifying electrophiles and nucleophiles, and +R/−R groups.
Why this is true
A carbocation is an sp² carbon with an empty p-orbital and only six electrons, so anything that pushes electron density into it spreads the positive charge and stabilises it. Alkyl groups do this in two ways: by the +I effect through σ-bonds, and more powerfully by hyperconjugation, in which an adjacent C–H σ-bond overlaps with the empty p-orbital. A tertiary carbocation has the most α-hydrogens around it (nine in (CH₃)₃C⁺), so stability runs 3° > 2° > 1° > CH₃⁺; allyl and benzyl cations gain extra stability because the charge is delocalised by resonance. Acidity follows the stability of the conjugate base: electron-withdrawing (−I, −R) groups pull density away from the negative charge of a carboxylate or phenoxide and make the acid stronger, with the inductive effect fading rapidly with distance. Carbanions, being electron-rich, follow exactly the opposite order to carbocations.
Key points
Key formulae
Hyperconjugative structures
count = number of α-hydrogens
Acid strength link
stronger acid ⇔ more stable conjugate base ⇔ lower pKa
Carbocation order
3° > 2° > 1° > CH₃⁺
Reactive intermediates
| Intermediate | Hybridisation | Electrons on C | Stability order |
|---|---|---|---|
| Carbocation (R₃C⁺) | sp², planar | 6 (empty p) | 3° > 2° > 1° > CH₃⁺ |
| Free radical (R₃C•) | sp², planar | 7 | 3° > 2° > 1° > CH₃• |
| Carbanion (R₃C⁻) | sp³, pyramidal | 8 | CH₃⁻ > 1° > 2° > 3° |
Common traps
- Applying the inductive order to carbanions in the same direction as carbocations — alkyl groups destabilise carbanions.
- Ignoring distance: the −I effect of Cl at the α-carbon is much stronger than at the γ-carbon.
Worked example
How many hyperconjugative structures can be drawn for the isopropyl cation, (CH₃)₂CH⁺, and for the tert-butyl cation?
High-yield
Carbocation: 3° > 2° > 1° (hyperconjugation + I); EWG strengthens acids; hyperconjugative structures = α-H count.
Quick check
Q1.Which is the strongest acid?
Q2.Which of these is an electrophile?