Chemistry · General Principles Of Organic Chemistry · NEET
A carbocation is a carbon atom carrying a positive charge because it lost one bonding pair of electrons. It has only 6 electrons around it (a sextet), not the usual 8. Example: CH₃⁺ (methyl cation). Because it is short of electrons, it is very reactive and wants electrons badly. NCERT earlier called it a 'carbonium ion' — same thing.
The order is 3° (tertiary) > 2° (secondary) > 1° (primary) > CH₃⁺ (methyl). In NCERT symbols: (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺. More alkyl groups attached to the positive carbon = more stable. Remember this exact order — NEET asks 'most stable carbocation' almost every year.
A tertiary carbocation has 3 alkyl groups attached to the positive carbon; a secondary has only 2. Alkyl groups push electron density toward the positive carbon by two effects: the +I (inductive) effect and hyperconjugation (electrons of nearby C–H bonds spread onto the empty orbital). More alkyl groups means more of this help, so the positive charge is spread out and the ion is more stable. NEET 2020 tested exactly this — the answer was hyperconjugation.
Hyperconjugation is the sideways overlap of the electron pair of an adjacent C–H (alpha C–H) bond with the empty p-orbital on the positive carbon. It lets those bonding electrons partly fill the empty orbital, spreading the positive charge. The more alpha (α) C–H bonds around the cation, the more hyperconjugation, the more stable. This is why a cation with more alkyl groups (more α C–H) wins.
The positively charged carbon is sp² hybridised and the three groups around it lie flat in one plane (planar), with about 120° angles. There is an empty unhybridised p-orbital sitting perpendicular to that plane. This flat shape is why SN1 reactions give both mirror-image products (racemic mixture) — the nucleophile can attack the flat cation from either face.
A carbocation is already electron-poor. An electron-withdrawing group (like -NO₂) pulls electrons AWAY through its -I effect, making the positive carbon even more electron-starved and less stable. The -I effect weakens fast with distance, so the farther the -NO₂ is from the positive carbon, the less it hurts. NEET 2018 and 2023 both used this idea: the cation with -NO₂ farthest away is the most stable.
In benzylic (next to a benzene ring) and allylic (next to a C=C) carbocations, the empty p-orbital overlaps with the neighbouring π electrons, so the positive charge is spread (delocalised by resonance) over several atoms. Sharing the charge over many atoms makes it much more stable — often more stable than a normal tertiary cation. This is why benzylic halides react fastest in SN1 (NEET 2024).
A tertiary butyl carbocation is more stable than the secondary butyl carbocation because of which one of the following?
The most stable carbocation among the following is:
Which of the following carbocations is expected to be the most stable? (Each is a ring cation bearing an -NO₂ substituent, with the positive charge at varying positions relative to -NO₂.)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
3° > 2° > 1° > methyl, i.e. (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺. Benzylic and allylic cations are even more stable because of resonance.
Yes. The positive carbon has only 6 electrons (a sextet) instead of 8, so it is electron-deficient and highly reactive. It behaves as an electrophile, attracting electron-rich nucleophiles.
A carbocation has a positive carbon with 6 electrons and is electron-deficient (electrophile). A carbanion has a negative carbon with 8 electrons including a lone pair and is electron-rich (nucleophile). Their stability orders are opposite.
Many organic reactions (SN1, E1, acid dehydration, Friedel-Crafts, electrophilic addition to alkenes by Markovnikov) go through a carbocation intermediate. The reaction always chooses the path that forms the MOST STABLE carbocation, so this one idea answers many questions.
The positive carbon is sp² hybridised and planar (flat, ~120° bond angles) with an empty p-orbital perpendicular to the plane.
