Medicneet Logo

Nucleophilic Addition to Carbonyl: Why Aldehydes React Faster Than Ketones

Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET

In a carbonyl group (C=O), the carbon is slightly positive, so a nucleophile attacks the carbon and the pi electrons shift to oxygen. Aldehydes react FASTER than ketones for two reasons: aldehydes have less crowding (steric) around the carbon, and they have fewer electron-pushing alkyl groups, so their carbon is more positive and more open to attack. Memory hook: "Aldehyde = A for Attack easily; Ketone = K for Krowded and calm."
Nucleophilic Addition to the Carbonyl GroupCOδ+δ−flat, sp2 carbonNu:→Nu—C—O⁻sp3 alkoxide→H⁺Nu—C—OHfinal productAldehyde faster than Ketone: less crowding + more δ+ on carbon
The nucleophile (Nu) attacks the partially positive carbonyl carbon; pi electrons shift to oxygen forming an alkoxide, which picks up H+ to give the addition product. The flat sp2 carbon becomes sp3. Aldehydes react faster than ketones because they have less steric crowding and a more positive carbon.

Your doubts, answered

Why does the nucleophile attack the CARBON of C=O and not the oxygen?

Oxygen is more electronegative than carbon, so it pulls the shared electrons toward itself. This makes the carbon partially positive (delta plus) and the oxygen partially negative (delta minus). A nucleophile is electron-rich and negative-loving, so it goes to the positive centre, which is the carbon. The pi electrons then move up to oxygen, making a negatively charged alkoxide, which finally grabs a proton to give the product. So the rule is simple: nucleophile to carbon, electrons to oxygen.

Why are aldehydes more reactive than ketones towards nucleophilic addition?

There are TWO reasons and NEET tests both. (1) Steric (space) reason: an aldehyde has one H and one alkyl group on the carbonyl carbon, while a ketone has two bigger alkyl groups. The two groups in a ketone block the nucleophile from reaching the carbon. (2) Electronic reason: alkyl groups push electrons (+I effect) toward the carbonyl carbon and reduce its positive charge. A ketone has two such groups, so its carbon is less positive and less attractive to a nucleophile. Aldehyde has only one, so its carbon stays more positive. Both reasons make aldehydes react faster.

What is the correct reactivity order of common carbonyl compounds?

HCHO (formaldehyde) > CH3CHO (acetaldehyde) > CH3COCH3 (acetone) > CH3COC2H5. Formaldehyde is the most reactive because it has NO electron-pushing alkyl group and no crowding, so its carbon is the most positive and most open. As you add and enlarge alkyl groups, reactivity keeps falling. Remember: more alkyl groups = slower nucleophilic addition.

Does the nucleophilic addition change the sp2 carbon to sp3?

Yes. In the carbonyl group the carbon is sp2 hybridised and flat (planar). When the nucleophile adds to the carbon, the C=O pi bond breaks and a new single bond forms. The carbon now has four single bonds, so it becomes sp3 hybridised and tetrahedral. This shape change is the whole point of ADDITION: a flat double bond becomes a 3D single-bonded carbon.

Why do electron-withdrawing groups make a carbonyl MORE reactive?

Electron-withdrawing groups (like -Cl, -NO2, -CF3) pull electron density AWAY from the carbonyl carbon. This makes the carbon even more positive (more delta plus), so nucleophiles attack faster. It is the opposite of alkyl groups, which push electrons in and slow the reaction down. So: electron-withdrawing = faster addition; electron-donating (alkyl) = slower addition.

⚠️ The NEET trap
✗ Ketones are more reactive because they have two carbon groups and are 'bigger and stronger' molecules.
✓ Aldehydes are more reactive. The two alkyl groups in a ketone crowd the carbon (steric) and push electrons in to lower its positive charge (electronic), so the nucleophile attacks the aldehyde carbon faster.
🧠 'Bigger molecule' does NOT mean 'more reactive'. For nucleophilic addition, fewer and smaller groups on the C=O carbon = faster reaction. Aldehyde wins.

Real NEET questions

NEET 2016 Phase 1

The product formed by the reaction of an aldehyde with a primary amine is:

A · Schiff base ✓
B · Ketone
C · Carboxylic acid
D · Aromatic acid
Solution: A primary amine (R-NH2) is a nucleophile because of the lone pair on nitrogen. Its nitrogen attacks the partially positive carbonyl carbon of the aldehyde (nucleophilic addition), giving an unstable addition product. This then loses a water molecule (elimination) to form a carbon-nitrogen double bond, >C=N-R. Such an imine made from a carbonyl compound and a primary amine is called a Schiff base. So the answer is (A). This is a classic addition-then-elimination reaction of the carbonyl group.
NEET 2020

The reaction between acetone (CH3COCH3) and methylmagnesium chloride (CH3MgCl) followed by hydrolysis gives:

A · tert-butyl alcohol ✓
B · isobutyl alcohol
C · isopropyl alcohol
D · sec-butyl alcohol
Solution: A Grignard reagent supplies a carbanion (CH3 with a negative charge), which is a strong nucleophile. It adds to the partially positive carbonyl carbon of acetone (nucleophilic addition). The oxygen becomes a negative alkoxide, and after hydrolysis with water it becomes -OH. The carbon that was C=O now carries three CH3 groups and one OH, giving (CH3)3C-OH, which is tert-butyl alcohol (2-methylpropan-2-ol). Rule: Grignard + ketone always gives a tertiary alcohol. Answer (A).

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.

See all 44 Aldehydes, Ketones And Carboxylic Acid NEET PYQs ›
Next concept: Cyanohydrin, Acetal, Imine, Oxime and 2,4-DNP →Keep learning — 2 minFeeling ready? Solve the Aldehydes, Ketones And Carboxylic Acid NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

Is nucleophilic addition the main reaction of aldehydes and ketones?

Yes. Because the carbonyl carbon is electron-poor and the group is unsaturated (has a pi bond), aldehydes and ketones mainly undergo nucleophilic ADDITION. This is different from alkenes, whose C=C is electron-rich and undergoes electrophilic addition. Knowing this difference is a common NEET point.

Which is the single most reactive carbonyl compound for NEET?

Formaldehyde, HCHO. It has two hydrogen atoms and no alkyl group on the carbon, so there is no steric crowding and no electron-donating effect. Its carbonyl carbon is the most positive, so nucleophiles attack it fastest.

Do aromatic aldehydes react faster or slower than aliphatic ones?

Slower. In benzaldehyde (C6H5CHO), the benzene ring donates electron density into the carbonyl by resonance, lowering the positive charge on the carbon. So aromatic aldehydes are generally less reactive to nucleophilic addition than simple aliphatic aldehydes like acetaldehyde.

What happens to the hybridisation during the reaction?

The carbonyl carbon changes from sp2 (flat, planar) to sp3 (tetrahedral) once the nucleophile adds. This shape change is a direct sign that an addition reaction has happened.

Why is this concept important for NEET?

Almost every named reaction of aldehydes and ketones (cyanohydrin, imine, oxime, aldol, Grignard) starts with the same nucleophilic addition step. Master the mechanism and reactivity order once, and you can predict products for many questions instead of memorising each reaction separately.

Related concepts:
Structure of the Carbonyl Group: sp2 Hybridisation and Why C=O Is PolarCyanohydrin, Acetal, Imine, Oxime and 2,4-DNP: Addition Product DerivativesGrignard Reagents with Aldehydes, Ketones and CO2Aldol and Cross Aldol Condensation ReactionsWhat Are Aldehydes, Ketones and Carboxylic Acids?
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Aldehydes, Ketones And Carboxylic Acid PYQs