Nucleophilic Addition and the Tetrahedral Intermediate
The carbonyl group is polar and flat, which makes it the most attacked functional group in organic chemistry. Every reaction of it starts the same way, and what happens after the first step depends on whether the carbon carries a group that can leave.
What a learner can do afterwards
- Draws the addition step with arrows and identifies the tetrahedral intermediate
- Explains why aldehydes react faster than ketones using both electronic and steric arguments
- Predicts whether the intermediate collapses back to a carbonyl or is protonated to an alcohol
- Explains how acid catalysis and base catalysis each speed the same addition by different means
1 · Read
The carbonyl carbon carries a partial positive charge because oxygen pulls the shared electrons toward itself. The group is flat, with trigonal planar geometry, so nucleophiles can approach from above or below. Every reaction starts the same way: arrows run from the nucleophile to the carbon while the pi bond lifts onto oxygen.
That first attack gives a tetrahedral alkoxide intermediate. What follows depends on the cargo. Aldehydes and ketones carry no leaving group, so the intermediate grabs a proton and the alcohol stands. Acyl compounds carry one, so the carbonyl reforms and substitution results instead.
Aldehydes outrun ketones for two cooperating reasons. Electronically, each alkyl group feeds density into the carbon, so ketones with two donors look less inviting. Sterically, the second alkyl group blocks the approach path. Acid catalysis protonates the oxygen to invite attack, while base catalysis strengthens the nucleophile.
Adding cyanide to a ketone builds a new stereocentre at the attacked carbon. Attack from either face is equally likely, so you get both mirror forms. Name the tetrahedral intermediate in your mechanism and always show the protonation step.
A polar flat carbon invites attack, the intermediate decides the family, and aldehydes beat ketones twice over.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.