Structure of the Carbonyl Group: sp2 Hybridisation and Why C=O Is Polar
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
In the carbonyl group (C=O), the carbon is sp2 hybridised. This makes it flat (trigonal planar) with bond angles of about 120°. The C=O bond is polar because oxygen is more electronegative and pulls the shared electrons toward itself, so the carbon becomes slightly positive (electrophilic) and the oxygen becomes slightly negative (nucleophilic). Memory hook: "sp2 = flat like a table (120°); O is greedy, so C is positive and gets attacked."
The carbonyl carbon is sp2 hybridised, giving a flat (trigonal planar) shape with ~120° bond angles. Oxygen pulls the electrons, so carbon is δ+ (electrophilic) and oxygen is δ- (nucleophilic). Resonance with the dipolar form (C⁺–O⁻) explains the high polarity and large dipole moment.
Your doubts, answered
Is the carbonyl carbon sp2 or sp3 hybridised?
It is sp2. The carbonyl carbon forms three sigma bonds (to two other atoms and using its three sp2 orbitals). These three sigma bonds lie in one flat plane. The leftover p-orbital of carbon overlaps side-by-side with a p-orbital of oxygen to form the pi bond of the C=O double bond. A quick rule for NEET: any carbon in a double bond is sp2. So carbonyl carbon is always sp2, never sp3.
What are the bond angles around the carbonyl carbon?
About 120°. Because the carbon is sp2 and trigonal planar, the three groups attached to it spread out evenly in a flat triangle. So all bond angles (like H-C-H, H-C=O) are close to 120°. This is the same shape as a BF3 molecule. Real values can shift a little from 120° when the attached groups have different sizes, but NEET expects you to write approximately 120°.
Why is the C=O bond polar?
Because oxygen is much more electronegative than carbon. Electronegativity means the power to pull shared electrons. Oxygen pulls the electrons of the C=O bond toward itself. So oxygen gets a partial negative charge (δ-) and carbon gets a partial positive charge (δ+). This uneven sharing makes the bond polar and gives aldehydes and ketones a real dipole moment. This is why they are more polar than ethers.
Why is carbonyl carbon called electrophilic and oxygen nucleophilic?
Because the C=O bond is polar. The carbon is electron-poor (δ+), so it wants electrons. Any species that likes positive charge is electrophilic, so carbon acts as a Lewis acid and is attacked by nucleophiles. The oxygen is electron-rich (δ-) with two lone pairs, so it is a nucleophilic centre and Lewis base. This is the whole reason nucleophilic addition happens at the carbon in the next chapters.
How does resonance explain the high polarity of C=O?
NCERT shows two structures. Structure A is the normal neutral form C=O. Structure B is a dipolar (charge-separated) form where the pi electrons shift fully onto oxygen, giving carbon a full + and oxygen a full − (C⁺–O⁻). The real carbonyl is a resonance mix of both. Because structure B contributes, the carbonyl is more polar than a simple double bond and has a large dipole moment.
Is the whole carbonyl group flat or just the carbon?
The carbon and the three atoms directly bonded to it all lie in one plane. So the local shape at the carbonyl carbon is planar. In formaldehyde (HCHO), all four atoms (2 H, C, O) lie in one flat plane. This flat, open shape is also why nucleophiles can attack the carbon easily from top or bottom.
⚠️ The NEET trap ✗ Thinking the carbonyl carbon is sp3 (because carbon usually makes 4 bonds), or thinking the carbon is the negative/nucleophilic end. ✓ Carbonyl carbon is sp2 (it is in a double bond), trigonal planar with ~120° angles. Oxygen is δ- and nucleophilic; carbon is δ+ and electrophilic. 🧠 Double bond → sp2, always. Oxygen is greedy → O is minus, C is plus. Nucleophiles hunt the plus carbon.
Real NEET questions
NEET 2019
The compound that is most difficult to protonate is:
A · HCHO
B · CH3CHO
C · CH3COCH3
D · C6H5CHO ✓
Solution: Protonation happens on the carbonyl oxygen, so the compound whose oxygen has the least electron density is hardest to protonate. Electron-donating alkyl groups (+I effect) push electron density onto the oxygen, so acetone > acetaldehyde > formaldehyde in ease of protonation. In benzaldehyde (C6H5CHO), the phenyl ring pulls electron density away from the C=O by conjugation. This lowers the oxygen's electron density and makes it the least basic, so it is the hardest to protonate. This question directly tests that C=O polarity and oxygen basicity depend on the attached groups.
ReNEET 2026
Arrange in increasing order of polarity: A. CH3CH2OCH2CH3 (diethyl ether), B. CH3CH2OH (ethanol), C. CH3COCH3 (acetone), D. CH3COOH (acetic acid).
A · A < B < C < D
B · C < A < D < B
C · C < A < B < D
D · A < C < B < D ✓
Solution: Compare the polar power of each. Diethyl ether (A) has only a weak C-O-C dipole and no O-H, so it is the least polar. Acetone (C) has a strongly polar C=O carbonyl but no O-H, so it is more polar than ether. Ethanol (B) has an O-H that can hydrogen bond, making it more polar than acetone. Acetic acid (D), with the -COOH group, is the most polar. So the order is A < C < B < D. Note how the polar carbonyl of acetone places it above non-polar ether, showing why aldehydes and ketones are polar.
NEET 2022
Statement I: Boiling points of aldehydes and ketones are higher than hydrocarbons of comparable molecular mass because of weak molecular association due to dipole-dipole interactions. Statement II: Boiling points of aldehydes and ketones are lower than alcohols of similar molecular mass due to the absence of H-bonding.
A · Both Statement I and Statement II are correct ✓
B · Both Statement I and Statement II are incorrect
C · Statement I is correct but Statement II is incorrect
D · Statement I is incorrect but Statement II is correct
Solution: The polar C=O group lets aldehyde and ketone molecules attract each other by dipole-dipole forces. These forces are stronger than the weak forces in non-polar hydrocarbons, so their boiling points are higher (Statement I correct). But aldehydes and ketones have no O-H or N-H, so they cannot form intermolecular hydrogen bonds. Alcohols can, so alcohols boil higher (Statement II correct). Both statements are correct, so the answer is A. This shows the direct effect of carbonyl polarity on physical properties.
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the hybridisation of carbon and oxygen in a carbonyl group?
The carbonyl carbon is sp2 hybridised because it forms three sigma bonds and one pi bond. The oxygen is also usually taken as sp2, holding two lone pairs. For NEET, the key point is that the carbon is sp2 and trigonal planar.
What is the shape and bond angle of the carbonyl group?
The shape around the carbonyl carbon is trigonal planar (flat triangle). The bond angles are approximately 120°, the same as expected for any sp2 carbon like in BF3.
Why do aldehydes and ketones have a dipole moment?
Because the C=O bond is polar. Oxygen pulls the bonding electrons toward itself, so oxygen is δ- and carbon is δ+. This charge separation gives a permanent dipole moment, and resonance with the dipolar form makes it even larger.
Which part of the carbonyl gets attacked by a nucleophile?
The carbon. Since carbon is electron-poor (δ+, electrophilic), electron-rich nucleophiles attack the carbon. The oxygen is δ- and nucleophilic, so it is not attacked by nucleophiles.
Is the carbonyl carbon more like a Lewis acid or a Lewis base?
The carbonyl carbon is a Lewis acid (electrophilic centre) because it is electron-deficient. The carbonyl oxygen, with lone pairs and negative charge, is a Lewis base (nucleophilic centre).
