Chemistry · General Principles Of Organic Chemistry · NEET
A carbanion is a carbon atom that carries a negative charge and a lone pair of electrons. The carbon has only 6 electrons of its own bonds plus the extra pair, so it is electron-rich. Its stability order is opposite to a carbocation: methyl and primary carbanions are the MOST stable, tertiary is the LEAST. Memory hook: a carbanion HATES electron-pushing groups, so more alkyl groups make it WORSE.
A carbanion carbon is pyramidal (three bonds + one lone pair, sp3, like NH3). Stability order is the reverse of a carbocation: CH3(−) > 1° > 2° > 3°, because electron-donating alkyl groups add charge onto an already negative carbon. More s-character (sp > sp2 > sp3) also stabilises it.
Your doubts, answered
What is a carbanion in chemistry?
A carbanion is a species where a carbon atom has a full negative charge and a lone pair of electrons. That carbon makes only three bonds (trivalent) and keeps the fourth electron pair to itself. Because it has extra electrons, a carbanion is electron-rich and acts as a nucleophile (it attacks positive centres).
What is the difference between a carbanion and a carbocation?
A carbocation has a POSITIVE charge and only 6 electrons around carbon (it is short of electrons, electron-poor, an electrophile). A carbanion has a NEGATIVE charge and a lone pair (electron-rich, a nucleophile). Their stability orders are exactly opposite: alkyl groups stabilise a carbocation but destabilise a carbanion.
Why is a 3 degree (tertiary) carbanion the least stable?
Alkyl groups push electrons toward the carbon (+I effect). A carbanion is already loaded with a negative charge, so pushing MORE electron density onto it makes it worse. More alkyl groups = more crowding of electrons = less stable. So the order is CH3(-) > 1 degree > 2 degree > 3 degree.
Is a carbanion a nucleophile or an electrophile?
A carbanion is a nucleophile. It has a lone pair and a negative charge, so it is electron-rich and looks for positive centres to attack. This is the opposite of a carbocation, which is an electron-poor electrophile.
What is the shape of a carbanion?
A carbanion carbon is sp3 hybridised in the simple case, with three bonds and one lone pair. This gives a pyramidal shape, like ammonia (NH3). The lone pair sits in one sp3 orbital and pushes the three bonds down into a pyramid.
Why does more s-character make a carbanion more stable?
s-orbitals are closer to the nucleus than p-orbitals. When the orbital holding the lone pair has more s-character, the electrons are held nearer the positive nucleus, which lowers energy and stabilises the negative charge. So an sp carbanion (50% s) is more stable than sp2 (33%) which is more stable than sp3 (25%). This is why terminal alkyne C-H is the most acidic.
⚠️ The NEET trap ✗ Assuming tertiary carbanion is the most stable because tertiary carbocation is the most stable. ✓ For carbanions the order is REVERSED. Methyl and primary carbanions are most stable; tertiary is least stable, because electron-donating alkyl groups pile more electron density onto an already negative carbon. 🧠 Carbocation loves alkyl groups; carbanion hates them. Flip the order every time you see a negative carbon.
Real NEET questions
2016
The pair of electrons in the given carbanion, CH3-C≡C(−), is present in which of the following orbitals?
A · 2p
B · sp3
C · sp2
D · sp ✓
Solution: The terminal carbanion carbon forms one sigma bond and is part of a triple bond (two pi bonds), and it holds one lone pair. It has two sigma/lone-pair electron domains (one sigma bond + one lone pair), so the carbon is sp hybridised. The lone pair therefore sits in an sp hybrid orbital. Answer: (D) sp.
2017
Which one is the correct order of acidity?
A · CH2=CH2 > CH3-CH=CH2 > CH3-C≡CH > CH≡CH
B · CH≡CH > CH3-C≡CH > CH2=CH2 > CH3-CH3 ✓
C · CH≡CH > CH2=CH2 > CH3-C≡CH > CH3-CH3
D · CH3-CH3 > CH2=CH2 > CH3-C≡CH > CH≡CH
Solution: Acidity of a C-H bond rises with the s-character of the carbon, because more s-character stabilises the carbanion left behind. Order of s-character: sp (alkyne, 50%) > sp2 (alkene) > sp3 (alkane). Among alkynes, terminal CH≡CH (two acidic H) beats CH3-C≡CH. So the order is CH≡CH > CH3-C≡CH > CH2=CH2 > CH3-CH3. Answer: (B).
2018
In the Reimer-Tiemann reaction (C6H5-O(−)Na(+) + CHCl3 + NaOH → salicylaldehyde), the electrophile involved is:
A · Dichloromethyl anion, CHCl2(−)
B · Formyl cation, CHO(+)
C · Dichloromethyl cation, CHCl2(+)
D · Dichlorocarbene, :CCl2 ✓
Solution: CHCl3 first loses a proton to base to form the trichloromethyl carbanion CCl3(−). This carbanion then loses a Cl(−) to give the reactive electrophile dichlorocarbene, :CCl2, which attacks the phenoxide ring. Answer: (D) dichlorocarbene.
Solved General Principles Of Organic Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Methyl > 1 degree (primary) > 2 degree (secondary) > 3 degree (tertiary). Electron-donating alkyl groups destabilise the negative carbon, so fewer alkyl groups mean a more stable carbanion. This is the reverse of the carbocation order.
Is a carbanion sp3 or sp2 hybridised?
A simple alkyl carbanion is sp3 hybridised and pyramidal, with the lone pair in an sp3 orbital. But if it is next to a triple bond (like an alkynide) it can be sp, and if resonance is involved it can be sp2 to allow the charge to spread out.
What stabilises a carbanion?
Electron-withdrawing groups (-I, -M) stabilise a carbanion by pulling away the extra negative charge. Resonance that spreads the charge (as in the carboxylate or enolate ion) and higher s-character of the orbital also stabilise it. Electron-donating alkyl groups do the opposite and destabilise it.
Why is a carbanion a nucleophile?
Because it has a lone pair and a negative charge, a carbanion is electron-rich. It donates that pair to positive or electron-poor centres, which is the definition of a nucleophile.
