Organic Chemistry · Alkenes: Structure and Reactivity
Carbocation Structure and Stability
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A carbocation A carbon with three bonds and a positive charge; sp2, planar, empty p orbital. Full entry → is a carbon atom with only three bonds and a positive charge — six valence electrons instead of eight. It is the most important reactive intermediate in introductory organic chemistry: it appears in electrophilic additions to alkenes (Topic 7), in SN1 and E1 reactions (Chapter 11), in Friedel–Crafts chemistry (Chapter 16), and throughout biochemistry, where allylic and benzylic cations are key intermediates in terpene and steroid biosynthesis.
Two structural features define a carbocation: it is sp2-hybridized and planar (three σ bonds at 120°), with an empty p orbital perpendicular to that plane. Everything about its reactivity follows from that empty orbital. This topic covers the geometry, the stability ordering (tertiary > secondary > primary > methyl), the electronic reasons (hyperconjugation Delocalization of adjacent C–H (or C–C) σ electrons into the empty p orbital of the cation. Full entry → and induction), the resonance stabilization of allylic and benzylic cations, and the practical consequences for mechanisms.
Why this matters
Carbocation stability is the hidden engine behind three major selectivity rules. It explains Markovnikov's rule (the proton adds to give the more stable cation), the Zaitsev rule in eliminations (the more substituted alkene forms via the more stable cation or transition state), and the rate order of SN1/E1 reactions (tertiary halides react fastest because the cation forms fastest). It also predicts rearrangements: whenever a 1,2-shift Migration of H or an alkyl group from an adjacent atom to the cationic carbon. Full entry → can convert a less stable cation into a more stable one, rearranged products appear (Topic 11). Understanding why cations are stabilized — not just the ranking — lets you predict reactivity for structures you have never seen.
The college version
Core Concepts
Geometry: sp2, planar, empty p orbital
The carbocation carbon is bonded to three groups and has no lone pairs, so it adopts trigonal planar geometry: three σ bonds in one plane at 120°, and one empty p orbital perpendicular to the plane. It is sp2-hybridized — the same hybridization as an alkene carbon, but without the π bond to fill the p orbital. Planarity matters for two reasons: it maximizes overlap for electron donation by neighboring groups (hyperconjugation), and it leaves the empty orbital exposed to nucleophilic attack from either face.
The stability ordering
Carbocations are stabilized by electron-donating alkyl groups, in the order:
tertiary > secondary > primary > methyl
Each additional alkyl group lowers the cation's energy substantially (roughly 20–40 kJ/mol per group in typical solution-phase comparisons; exact values are medium-dependent). More substituted is more stable — the same direction as radicals. Experimentally the ordering is confirmed by solvolysis A substitution reaction where the solvent is the nucleophile. Full entry → rates (tertiary halides ionize far faster than primary) and by hydride-transfer equilibria.
Why alkyl groups stabilize: hyperconjugation
The main electronic reason is hyperconjugation: the C–H (and C–C) σ bonds of adjacent alkyl groups overlap with the cation's empty p orbital, delocalizing electron density into it. The more adjacent C–H bonds, the more σ → p donation and the more stable the cation — a tertiary cation has up to nine adjacent C–H bonds, a methyl cation none. This is the same phenomenon that stabilizes more-substituted alkenes (Topic 6), viewed from the cation's perspective.
The inductive contribution
Alkyl groups are also weakly electron-donating by induction: they push electron density through the σ framework toward the electron-poor carbon, because carbon is slightly more electronegative than hydrogen and the alkyl group "donates" through the polarized C–C bonds. Induction is a real but smaller effect than hyperconjugation; together they account for the full stability ordering.
Resonance: allylic and benzylic cations
Some cations are stabilized far beyond the alkyl-group trend by resonance. An allylic cation A cation on a carbon adjacent to a C=C; charge delocalized by resonance. Full entry → (CH2=CH–CH2⁺) has two resonance forms:
CH2=CH-CH2+ ⟷ CH2+-CH=CH2
A benzylic cation A cation on a carbon attached to a benzene ring; charge delocalized into the ring. Full entry → (C6H5–CH2⁺) delocalizes the charge into the ring through several forms. Resonance-stabilized cations outrank ordinary tertiary cations. Practical consequence: HX addition to 1,3-dienes and styrene-type alkenes gives products reflecting the delocalized cation, and benzylic/allylic halides ionize readily in SN1 reactions.
Consequences for mechanisms
- Markovnikov selectivity (Topic 8): the proton lands to create the most stable cation.
- SN1/E1 rates (Chapter 11): rate-determining step is cation formation, so tertiary substrates react fastest; primary substrates essentially cannot go through cations.
- Rearrangements (Topic 11): cations rearrange by 1,2-hydride or 1,2-alkyl shifts whenever the product cation is more stable.
- Carbocation stability vs. leaving group ability: a stable cation means a good leaving group can depart; hydroxide and alkoxide are poor leaving groups precisely because the resulting cations would be very unstable.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Carbocation stability | carbanion stability | The order is reversed: alkyl groups stabilize cations (donate) but destabilize anions (the lone pair is pushed back). |
| Hyperconjugation | resonance | Hyperconjugation uses σ (C–H/C–C) electrons; resonance uses π electrons. Both delocalize charge, but from different orbitals. |
| Allylic cation | vinylic cation | Allylic (charge on carbon next to C=C) is resonance-stabilized; vinylic (charge on a C=C carbon) is very unstable and does not form in ordinary reactions. |
| Benzylic position | aromatic ring carbon | Benzylic = carbon attached to the ring; the ring carbons themselves carry no charge in benzylic cation resonance forms. |
| More substituted cation | more stable cation | They coincide for saturated cations, but resonance (allylic/benzylic) can make a less-substituted cation more stable. |
| Carbocation geometry | carbocation charge | The cation is sp2/planar because it has an empty p orbital — not because it is "flat like an alkene" in the sense of a π bond. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A carbocation is a carbon that lost one of its four "friends" and now has an empty seat. If a carbon has many neighbors leaning over to fill that seat, it feels steadier. A carbon at the center of three other carbons has the most neighbors helping — it's the happiest, most stable seat. And if the empty seat sits right next to a double bond, the whole row of electrons can shuffle over to help, like a crowd passing a message down a line.
Worked example
Example 1 — Rank these carbocations by stability. Consider (CH3)3C⁺ (tertiary), (CH3)2CH⁺ (secondary), CH3CH2⁺ (primary), CH2=CH–CH2⁺ (allylic), and C6H5–CH2⁺ (benzylic). Answer: The saturated order is tertiary > secondary > primary, but the allylic and benzylic cations are resonance-stabilized and outrank the saturated tertiary cation. Reasonable order: benzylic ≈ allylic > tertiary > secondary > primary. Rationale: resonance spreads the charge over two (allylic) or several (benzylic) atoms, which is a much larger stabilization than alkyl donation.
Example 2 — Count hyperconjugative contributors. The number of adjacent C–H bonds available for σ → p donation is a quick proxy for stability. For methyl cation (CH3⁺): zero adjacent carbons, zero C–H donors. For ethyl cation (CH3CH2⁺): one adjacent carbon with 3 C–H bonds → 3 donors. For isopropyl cation ((CH3)2CH⁺): two adjacent carbons, 3 + 3 = 6 donors. For tert-butyl cation ((CH3)3C⁺): three adjacent carbons, 3 × 3 = 9 donors. Write the pattern:
donors = 3 × (number of adjacent carbons)
So tertiary (9) > secondary (6) > primary (3) > methyl (0), matching the stability order. The correlation is a useful shortcut, not the whole story (C–C bonds donate too, and geometry matters).
Example 3 — Predict which cation forms from 2-methylpropene. When 2-methylpropene ((CH3)2C=CH2) is protonated, the proton can add to either alkene carbon. Adding to the CH2 carbon leaves the charge on the internal carbon: (CH3)3C⁺, tertiary. Adding to the internal carbon leaves a primary cation on CH2. Which forms? Answer: The tertiary cation (CH3)3C⁺ forms, because it is far more stable — this is Markovnikov's rule in action, and it is why HBr addition to 2-methylpropene gives tert-butyl bromide.
Example 4 — Is the allylic cation stable enough to form? 3-Chloro-1-butene (CH2=CH–CH(Cl)–CH3) ionizes in polar solvent to give the allylic cation CH2=CH–CH⁺–CH3. Draw the two resonance forms and explain why this cation forms even though it is secondary. Answer: The cation is secondary, but it is allylic: the resonance forms CH2=CH–CH⁺–CH3 ↔ CH2⁺–CH=CH–CH3 delocalize the charge, stabilizing the ion enough that SN1-type ionization is facile. The same resonance explains why allylic halides are among the most reactive in SN1/E1 chemistry.
Key takeaways
- Carbocation: carbon with 3 bonds, + charge, sp2, trigonal planar, empty p orbital.
- Stability order: tertiary > secondary > primary > methyl; more alkyl groups = more stable.
- Hyperconjugation (σ C–H → empty p orbital) is the main stabilization; induction contributes weakly.
- Allylic and benzylic cations are resonance-stabilized and can beat ordinary tertiary cations.
- Carbocation stability drives Markovnikov's rule, Zaitsev's rule, SN1/E1 rates, and rearrangement chemistry.
- A methyl cation (CH3⁺) is the least stable common cation; primary cations rarely form in solution.
- Stable cations can be isolated as salts only with special delocalization (e.g., tropylium, trityl).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Describe the hybridization, geometry, and key orbital of a carbocation.
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sp2-hybridized, trigonal planar (three σ bonds at 120°), with one empty p orbital perpendicular to the plane.
Write the stability order for saturated carbocations and give the two electronic reasons.
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Tertiary > secondary > primary > methyl. Reasons: hyperconjugation (σ C–H donation into the empty p orbital) and the weak inductive electron donation of alkyl groups.
Why is a benzylic cation more stable than a tertiary alkyl cation?
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The benzylic cation delocalizes the positive charge into the aromatic ring through resonance (several equivalent forms), which spreads the charge over many atoms — a much larger stabilization than alkyl-group donation.
How does the number of adjacent C–H bonds correlate with carbocation stability?
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More adjacent C–H bonds = more σ → p donation = more stable cation; the count is 3 × (number of adjacent carbons), giving 9 > 6 > 3 > 0 across tertiary to methyl.
Why do tertiary alkyl halides undergo SN1 reactions much faster than primary ones?
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SN1 rate is set by the rate-determining ionization step, which forms the carbocation; tertiary cations are much more stable (lower activation energy for ionization) than primary cations, so the rate is far higher.
Predict the carbocation formed when 1-butene (CH3CH2CH=CH2) is protonated under Markovnikov conditions.
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Protonation at the terminal CH2 carbon gives the secondary cation CH3CH2CH⁺CH3 (Markovnikov), which is more stable than the primary cation that would result from protonation at the internal carbon.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- carbocation
- A carbon with three bonds and a positive charge; sp2, planar, empty p orbital.
- hyperconjugation
- Delocalization of adjacent C–H (or C–C) σ electrons into the empty p orbital of the cation.
- inductive effect
- Electron donation/pull through σ bonds due to electronegativity differences.
- allylic cation
- A cation on a carbon adjacent to a C=C; charge delocalized by resonance.
- benzylic cation
- A cation on a carbon attached to a benzene ring; charge delocalized into the ring.
- 1,2-shift
- Migration of H or an alkyl group from an adjacent atom to the cationic carbon.
- solvolysis
- A substitution reaction where the solvent is the nucleophile.
Sources & references
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