Carbonyl Chemistry · Section 47 of 64

Aldehydes & ketones

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The carbonyl group is the most important functional group in organic chemistry. It is electrophilic at carbon, nucleophilic at oxygen, acidifies the hydrogens next to it, and is the pivot around which most carbon–carbon bond formation happens. The next three chapters are all carbonyl chemistry, and they all rest on the structure described here.

Structure: a polarized, planar, sp² carbon

The carbonyl carbon is sp² hybridized and trigonal planar, with a pi bond to oxygen built the same way as an alkene's — except that oxygen is far more electronegative than carbon.

That difference polarizes the pi bond heavily. Carbon carries a substantial δ+ and oxygen a substantial δ−, and the situation is captured by two resonance structures: the neutral C=O form, and a charge-separated form with a full positive charge on carbon and a full negative on oxygen. By the ranking rules of Module 2 the second is a minor contributor — but it is a real one, and it is the one that predicts the reactivity.

Some numbers: a C=O is about 123 pm long and worth roughly 178 kcal/mol, substantially stronger than a C=C at 147. That strength is why carbonyl formation is so often the thermodynamic sink that drives a reaction — and why the keto form dominates the keto–enol equilibrium.

Two sites, two kinds of reactivity

from aboveedge-onThe carbonyl carbon is sp², so the whole group lies in one plane — and both of its faces are open.
Formaldehyde from two angles. The flatness matters twice over: it is what leaves the carbon reachable from either side, and it is why addition to a carbonyl gives a racemic mixture unless something else in the molecule distinguishes the two faces. Every mechanism in Modules 9 to 11 starts by attacking this carbon perpendicular to that plane.
RROCOne functional group, two opposite personalitiesδ+δ−Nu−nucleophiles attack the CARBONH+acids protonate the OXYGEN— which makes the carbon evenmore electrophilic than beforeThe carbon is sp², so the group is FLAT,and both of its faces are wide open —which is why addition to a carbonyl makesa racemic mixture unless something stops it.
The carbonyl group is both electron-poor and electron-rich, at different atoms, and keeping them straight is most of carbonyl chemistry. Oxygen has pulled the shared electrons toward itself, so it carries the lone pairs and the partial negative charge and is what an acid grabs. The carbon is left short, and is what a nucleophile attacks. The two are not alternatives — protonating the oxygen is the standard way of making the carbon more attackable.Nearly every carbonyl reaction in Modules 9, 10 and 11 is one of these two moves, or both in sequence: attack the carbon, or protonate the oxygen to make the carbon a better target first.

A carbonyl is electrophilic at carbon and nucleophilic at oxygen, and both matter. Nucleophiles attack the carbon; acids and Lewis acids coordinate to the oxygen's lone pairs. The second is not a side reaction — protonating the oxygen is how a carbonyl gets activated toward attack, and it is the first step of most acid-catalyzed carbonyl mechanisms.

Aldehydes are written RCHO, with at least one hydrogen on the carbonyl carbon; ketones are RCOR′, with two carbon groups and no hydrogen. That single structural difference is the whole basis of their different reactivity, and it also makes aldehydes oxidizable to carboxylic acids while ketones are not — there is no C–H left on a ketone's carbonyl carbon to remove.

Aldehydes are more reactive than ketones

an aldehydeRHOCone alkyl group donating inand a small hydrogen out of the wayMORE reactivea ketoneRROCtwo alkyl groups donating in, so thecarbon is less δ+ — and both are bulkyLESS reactive
The single most useful reactivity comparison in carbonyl chemistry, and it has two independent causes that happen to agree. An alkyl group donates electron density, which makes the carbon less hungry; an alkyl group is also physically larger than a hydrogen, which makes the carbon harder to reach. A ketone has two of them and an aldehyde one. The hydration numbers put a figure on it: formaldehyde, with no alkyl groups at all, is essentially completely hydrated in water, while acetone barely bothers.Two effects, both pointing the same way. Electronically, each alkyl group pushes electron density toward the carbon and takes the edge off its δ+. Sterically, each one is in the way of an incoming nucleophile. The hydration equilibria measure it: formaldehyde is 99.9% hydrated in water, acetone about 0.2%.

Two effects point the same way.

Sterics. A ketone's two carbon substituents crowd the carbonyl carbon more than an aldehyde's one substituent plus a small hydrogen, making the ketone harder to approach — the same logic as SN2 hindrance in Module 6. And there is a second, less obvious steric cost: the carbon rehybridizes from sp² to sp³ during attack, compressing bond angles from 120° to 109.5° and pushing the substituents closer together. That costs more when the substituents are large.

Electronics. Alkyl groups donate electron density by hyperconjugation and induction, partially cancelling the carbon's positive character. A ketone's two donors cancel more than an aldehyde's one.

The ordering that results — formaldehyde > aldehyde > ketone — holds throughout the next two chapters and extends naturally to the carboxylic acid derivatives of Module 10.

Hydration equilibria prove the trend quantitatively

Water adds reversibly to a carbonyl to give a gem-diol, and where that equilibrium sits is a direct measurement of how electrophilic the carbonyl is.

CompoundAlkyl groupsKeq (hydration)
H₂C=O0~2000
CH₃CHO1~1.4
(CH₃)₂C=O2~0.0014
Cl₃CCHO0, plus 3 Cl~10⁴

Each added alkyl group costs roughly three orders of magnitude — hard, direct evidence for the trend. The last row makes the opposite point: chloral's three electron-withdrawing chlorines pull density away inductively and make the carbonyl more electrophilic, so much so that chloral hydrate is a stable, isolable crystalline solid.

Electron-withdrawing groups near a carbonyl increase its reactivity; electron-donating groups decrease it. This single principle organizes all of Modules 9 and 10 — it is why an acid chloride is ferocious, why an amide is sluggish, and why a conjugated enone is less reactive at the carbonyl than an isolated ketone. When comparing two carbonyls, count what the neighbours are doing to the carbon's electron density.

Conjugation and the alpha carbon

Two structural features reach beyond the carbonyl carbon itself.

When a C=O is conjugated with a C=C, the pi system extends over four atoms and the carbonyl is stabilized — and, as Module 2 showed, a resonance contributor places positive charge on the far beta carbon. That is why conjugated enones can be attacked at either position, and it is the basis of conjugate addition in Module 11.

And the hydrogens on the carbon adjacent to a carbonyl — the alpha hydrogens — have a pKa around 20, some thirty units more acidic than an ordinary alkane C–H, because the resulting anion is delocalized onto the oxygen. Module 11 is built entirely on this.

Recognizing them in the lab

Carbonyls are among the easiest groups to identify spectroscopically, which is worth knowing before Module 14. The IR C=O stretch is a strong, sharp absorption at 1700–1750 cm⁻¹ and is essentially unmistakable — a ketone near 1715, an aldehyde near 1730, conjugation lowering both by about 30 cm⁻¹. An aldehyde also shows a characteristic ¹H NMR signal at 9–10 ppm, further downfield than almost anything else. In ¹³C, the carbonyl carbon appears at 190–220 ppm, far from everything else in the spectrum.

What carries forward

Everything in the next five chapters. Nucleophilic addition is the immediate sequel; acyl substitution in Module 10 is the same first step with a different second one; enolate chemistry in Module 11 exploits the alpha hydrogens; imine and enamine formation connects to Module 12. Getting the polarization and the reactivity ordering solid here pays off repeatedly.