Almost every named carbonyl reaction is one mechanism with a different nucleophile — here hydride, from NaBH4, gives an alcohol.
Acetaldehyde + hydride → ethanol. Every addition in this article follows the same skeleton — structures drawn live.
1. The Carbonyl Carbon Is Electrophilic Because C=O Is Polarized
Oxygen's greater electronegativity pulls the C=O electrons toward itself, leaving carbon partially positive (δ+); that sp2, trigonal-planar carbon is electron-poor and sterically open on both faces.
2. The Nucleophile Adds to Give a Tetrahedral Alkoxide, Then Protonation Gives the Product
The nucleophile donates to the δ+ carbon and the π electrons shift onto oxygen, giving a tetrahedral alkoxide R2C(–Nu)–O− that is then protonated to the product.
Because aldehydes and ketones have no leaving group on the carbonyl carbon, the intermediate keeps the nucleophile — this is addition, not substitution.
3. Aldehydes React Faster Than Ketones Because They Are Less Hindered and More Electrophilic
An aldehyde's single alkyl group crowds the carbon less and donates less electron density than a ketone's two, so it stays more accessible and more δ+ — giving the order formaldehyde > aldehyde > ketone.
4. Hydride Reagents (NaBH4, LiAlH4) Reduce the Carbonyl to an Alcohol
Hydride (H−) adds to give an alcohol — primary from an aldehyde, secondary from a ketone; NaBH4 is mild and selective, while LiAlH4 is far stronger but reacts violently with water.
Acetone is reduced to isopropanol (a secondary alcohol).
5. Grignard and Organolithium Reagents Add a Carbon and Build a New C–C Bond
Grignards (RMgX) and organolithiums (RLi) deliver a carbanion-like carbon that forms a new C–C bond, growing the skeleton and giving an alcohol; being strong bases, they tolerate no acidic O–H or N–H.
Acetaldehyde + methylmagnesium bromide, then H3O+ → isopropanol; a new C–C bond forms at the carbonyl carbon.
6. Cyanide Adds to Give a Cyanohydrin
Cyanide (−C≡N) adds one carbon to give a cyanohydrin — an –OH and –C≡N on the same carbon — in a reversible equilibrium that favors reactive, unhindered carbonyls.
Acetaldehyde + cyanide → its cyanohydrin (2-hydroxypropanenitrile).
7. Nitrogen Nucleophiles Add, Then Eliminate Water to Give an Imine
A primary amine adds, then the nitrogen lone pair expels the –OH as water to replace C=O with C=N — an imine (Schiff base), formed best at mildly acidic pH ~4–5.
Acetaldehyde + methylamine → an imine (C=N), with loss of water.
8. Summary
Polarized, open C=O carbon · nucleophile adds → tetrahedral alkoxide → protonation · aldehydes > ketones · hydride → alcohol · Grignard/RLi → new C–C bond · cyanide → cyanohydrin · amine → imine · one mechanism, different nucleophiles.
Worked example
- A Grignard reagent is a carbanion equivalent — a strong nucleophile (the CH3 carbon is δ−).
- It attacks the electrophilic (δ+) carbonyl carbon; the C=O π electrons collapse onto oxygen, giving a magnesium alkoxide.
- Aqueous acid in the second step protonates the alkoxide to the neutral alcohol.
- Count the carbons on the new alcohol carbon: two methyls from acetone + one from the Grignard + OH = a 3° alcohol.
Answer. 2-methylpropan-2-ol (tert-butanol) — a tertiary alcohol.
Quiz yourself
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Oxygen is more electronegative and pulls electron density away from carbon, so the carbon bears the partial positive charge (δ+). A nucleophile seeks electron-poor centers, so it attacks the δ+ carbon; the displaced π electrons then rest comfortably on the electronegative oxygen as an alkoxide.
It is the alkoxide formed when the nucleophile adds to the sp2 carbon, converting it to an sp3 center bearing four groups (R2C(–Nu)–O−). Aldehydes and ketones have no leaving group on the carbonyl carbon (just H or alkyl), so the intermediate simply gets protonated and keeps the nucleophile — addition, not substitution.
Two reasons. Sterically, acetaldehyde's carbonyl carbon carries one H (less crowding) versus acetone's two methyls. Electronically, acetone's two electron-donating alkyl groups quench more of the carbon's δ+ character than acetaldehyde's single methyl, so acetaldehyde is both more accessible and more electrophilic.
NaBH4 delivers a hydride (H−), so it only reduces C=O to C–OH without changing the carbon count. A Grignard delivers a carbon nucleophile, forming a new C–C bond and adding carbon(s) to the skeleton — turning an aldehyde into a secondary alcohol and a ketone into a tertiary alcohol.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.