Start with two familiar molecules. Acetone is the solvent in nail-polish remover. Acetaldehyde is the compound your liver makes first when it breaks down ethanol. Both contain a carbon joined to an oxygen by a double bond, C=O. That unit is the carbonyl group, and its carbon is the carbonyl carbon.
What else sits on the carbonyl carbon sorts these compounds into two families. In acetaldehyde, one of the two other groups is a hydrogen. That makes it an aldehyde. In acetone, both other groups are carbons. That makes it a ketone.
The carbonyl group is the most important functional group in organic chemistry. Its carbon is electrophilic (electron-poor, so it attracts electron pairs). Its oxygen is nucleophilic (it has lone pairs to donate). It makes the hydrogens on the neighboring carbon unusually acidic. And many of the most useful reactions that build carbon–carbon bonds happen at it. This chapter, Carboxylic Acids & Derivatives and Enolate Chemistry all rest on the structure set out below.
Structure: a polarized, planar, sp² carbon
Take formaldehyde, the smallest carbonyl compound. Its carbon has three groups around it: two H and the O. Three groups means sp² hybridization, as in an alkene carbon. The three groups spread out in one flat plane, about 120° apart. That shape is called trigonal planar.
The leftover p orbital on carbon stands at right angles to that plane. It overlaps side by side with a p orbital on oxygen to make the π bond. This is the same construction as the π bond of an alkene.
The difference from an alkene is oxygen. Oxygen is far more electronegative than carbon, so it pulls the shared electrons toward itself. The π bond is polarized: its electrons sit nearer oxygen. Carbon ends up with a substantial partial positive charge (δ+) and oxygen with a substantial partial negative charge (δ−).
Two resonance structures describe this. The first is the ordinary neutral C=O. In the second, the π pair has moved entirely onto oxygen, leaving a full + on carbon and a full − on oxygen. By the ranking rules of Drawing Molecules & Moving Electrons, the charge-separated structure is a minor contributor. It is still a real one, and it is the one that predicts the reactivity.
Some numbers. A C=O bond is about 123 pm long and takes roughly 178 kcal/mol to break, well above the 147 kcal/mol of a C=C. That strength explains why forming a C=O so often drives a reaction forward: the product with the C=O is usually lower in energy. It is also why a carbonyl compound normally wins over its enol isomer, the form with a C=C and an O–H instead of the C=O. Enolate Chemistry takes up that equilibrium.
Two sites, two kinds of reactivity
Because carbon is δ+ and oxygen is δ−, the two atoms react with opposite partners. A nucleophile (an electron-pair donor) attacks the carbon. An acid, or a Lewis acid, bonds to one of oxygen's lone pairs.
The second move is not a side reaction. Once the oxygen is protonated, it carries a full + and pulls even harder on the π electrons. That leaves the carbon more electron-poor and easier to attack. Protonating the oxygen is how a carbonyl gets activated, and it is the first step of most acid-catalyzed carbonyl mechanisms. Nearly every carbonyl reaction in the chapters ahead is one of these two moves, or both in sequence. The next section, Nucleophilic addition, follows them through to products.
The flat shape matters here. Take butan-2-one, whose carbonyl carbon carries a CH₃ and a CH₂CH₃. A nucleophile can reach that carbon from either face of the plane. If it adds from in front of the page, the new group ends up on a wedge; from behind, on a hash. The two products are mirror images, and each face is equally easy to reach, so they form in equal amounts. That 50:50 mix of mirror images is a racemic mixture. Addition to a carbonyl gives a racemic mixture whenever it creates a stereocenter at that carbon, unless something else in the molecule or the reagent makes one face easier to reach than the other.
The nucleophile does not come straight down onto the plane, at 90° to the C=O. It comes in along a line about 107° from the C→O direction, tilted away from oxygen. This is called the Bürgi–Dunitz angle.
The reason is a compromise. The nucleophile has to put its electron pair into π*, the empty antibonding partner of the π bond. π* is bigger on carbon than on oxygen, and overlap with its lobe on carbon is best close to 90°. But the filled π bond and oxygen's lone pairs repel the incoming pair and push it back, away from oxygen. The balance between the two is about 107°.
The tilt is also the economical path. In the tetrahedral product, the angle between the oxygen, the carbon and the new group will be about 109.5°. An approach at 107° is already close to that, so the nucleophile arrives almost where its bond will end up. As the bond forms, the groups already on carbon (R, R′ and the oxygen) fold back, away from the nucleophile, and the carbon becomes tetrahedral.
Aldehydes are more reactive than ketones
Compare acetaldehyde and acetone. Each has one CH₃ on the carbonyl carbon. The aldehyde's second group is an H; the ketone's is a second CH₃. Toward the same nucleophile, acetaldehyde reacts faster and more completely. Two separate effects point the same way.
Sterics means crowding. A CH₃ takes up much more room than an H. On its tilted path, the nucleophile passes close to the groups on carbon, so two CH₃ groups get in its way more than one CH₃ and an H. This is the same crowding that slows SN2 at a hindered carbon in Substitution & Elimination.
There is a second, less obvious steric cost. During the attack, carbon changes from sp² to sp³. The angles between its groups close from 120° to about 109.5°, which pushes the groups already there closer together. That costs more when those groups are large.
Electronics means the δ+ itself. An alkyl group such as CH₃ donates a little electron density to the carbon it is attached to. It does this through its C–H bonds (hyperconjugation, the same effect that steadies a carbocation) and through the σ bond (induction). That donation partly cancels the carbonyl carbon's δ+. A ketone has two donors and an aldehyde one, so the ketone's carbon is less electron-poor.
Formaldehyde has no alkyl group at all, so it heads the list. The order is formaldehyde > other aldehydes > ketones, and it holds throughout this chapter. The same two questions, how crowded the carbon is and how electron-poor, also rank the acid derivatives of Carboxylic Acids & Derivatives.
Hydration equilibria prove the trend quantitatively
Dissolve acetaldehyde in water and some of it takes on a water molecule. An H goes to the oxygen and an OH to the carbon. The product has two OH groups on one carbon. It is called a hydrate, or a gem-diol (gem for "twin": both OH on the same carbon). The mechanism comes later in this chapter; here the point is how far the reaction goes.
The reaction is reversible, so it settles at an equilibrium. Its constant, Keq = [hydrate] / [carbonyl compound] (water is the solvent, so its concentration is folded in), measures how electrophilic the carbonyl is. The more the carbonyl carbon wants a nucleophile, the further the equilibrium lies toward the hydrate.
| Compound | Alkyl groups | Keq (hydration) |
|---|---|---|
| H₂C=O | 0 | ~2000 |
| CH₃CHO | 1 | ~1.2–1.4 |
| (CH₃)₂C=O | 2 | ~0.0014 |
| Cl₃CCHO | 1 (CCl₃, which withdraws electrons instead of donating) | ~10⁴ |
In water, formaldehyde is about 99.9% hydrate. Acetaldehyde is just over half hydrate (K ≈ 1.2–1.4). Acetone is only about 0.1% hydrate. Each added alkyl group costs roughly three powers of ten in Keq: hard, direct evidence for the trend.
The last row makes the opposite point. Chloral, Cl₃CCHO, has one carbon group on its carbonyl carbon, like acetaldehyde, but that group is CCl₃, not CH₃. Chlorine is strongly electronegative, so the three chlorines pull electron density away through the σ bonds. CCl₃ withdraws electrons where CH₃ donates them. The carbonyl carbon becomes more electron-poor, so much so that chloral hydrate is a stable crystalline solid you can isolate and put in a bottle.
Rank acetone, benzaldehyde, acetaldehyde and chloral (Cl₃CCHO) from most to least reactive. Justify each step with the two effects above.
Start with the aldehyde/ketone split. Acetone is the only ketone: two methyls, both donating, both in the way. It goes last unless something outranks the split, and nothing here does.
Now compare the three aldehydes by what their one other group does. Acetaldehyde's methyl donates weakly, by induction and hyperconjugation. Chloral's CCl₃ does the opposite: its three chlorines pull electrons away and leave the carbon markedly more electron-poor.
Benzaldehyde carries a benzene ring, and the ring's π electrons are conjugated with the C=O. In one resonance contributor, a ring π pair shifts toward the carbonyl carbon and the positive charge moves into the ring. That spreads out the carbon's δ+ and steadies the starting material far more than a methyl does, so benzaldehyde sits below acetaldehyde.
Answer: chloral > acetaldehyde > benzaldehyde > acetone. The four comparisons used only two ideas, in this order: count the groups on the carbonyl carbon, then ask what each one does to that carbon's electron density.
One caution about ranking an aryl aldehyde against a ketone. The split holds against ordinary open-chain ketones like acetone, but it is not a law. Cyclohexanone adds nucleophiles unusually fast for a ketone. Its flat sp² carbon forces some eclipsing strain on the ring, and going to sp³ relieves that strain rather than creating crowding. In several standard comparisons it beats benzaldehyde. Rank an aldehyde over an open-chain ketone with confidence. Rank a stabilized aldehyde against a particular ring ketone only if the question gives you data.
Conjugation and the alpha carbon
Two structural features reach beyond the carbonyl carbon. Both need names for the neighboring carbons. The carbon bonded directly to the carbonyl carbon is the alpha (α) carbon. The next one out is the beta (β) carbon.
Take but-3-en-2-one, CH₂=CH–CO–CH₃. Its C=C sits right next to the C=O, so the two π bonds are conjugated: the π system runs over four atoms, and the carbonyl is stabilized. A compound like this, with a C=C conjugated to the C=O of a ketone, is an enone. As How Reactions Happen showed, one resonance contributor puts a + on the β carbon. So a nucleophile can attack an enone at either of two places: the carbonyl carbon or the β carbon. Attack at the β carbon is conjugate addition, and it returns in Enolate Chemistry.
The hydrogens on an α carbon are the alpha hydrogens. In acetone they have a pKa around 20. An ordinary alkane C–H has a pKa near 50, so an α hydrogen is some thirty units more acidic. The reason is the anion left behind: its negative charge is shared between the α carbon and the oxygen by resonance. Enolate Chemistry is built on this.
Where they come from
You have already met three ways to make a carbonyl, all in Alkenes & Alkynes.
- Hydrate an alkyne, Markovnikov. Propyne with water, sulfuric acid and a mercury(II) salt gives acetone. The first product is an enol, which rearranges on its own to the ketone (a tautomerization).
- Hydroborate an alkyne, anti-Markovnikov. Propyne with a bulky borane (R₂BH), then basic hydrogen peroxide, puts the oxygen on the end carbon instead. The enol there rearranges to an aldehyde, propanal.
- Cut an alkene with ozone. Ozonolysis splits a C=C in two and turns each end into a C=O. A mild second step (a workup) such as dimethyl sulfide, (CH₃)₂S, keeps an aldehyde end from being oxidized further to a carboxylic acid. 2-Methylbut-2-ene gives acetone and acetaldehyde.
Two more routes come later. Oxidizing an alcohol gives a ketone from a secondary alcohol, and an aldehyde from a primary alcohol if the oxidation stops there; if it does not, the product is a carboxylic acid. Oxidation & Reduction sets this out, and the section on oxidizing an aldehyde, at the end of this chapter, explains why water decides where it stops. Friedel–Crafts acylation, which builds a ketone directly onto a benzene ring, belongs to Aromatic Chemistry.
Recognizing them in the lab
Carbonyls are among the easiest groups to spot with instruments. Three signals are worth knowing: the IR band you met in IR & Mass Spectrometry, and two NMR signals that NMR Spectroscopy, later in the course, explains.
- Infrared (IR). IR spectroscopy records which frequencies of infrared light a molecule's bonds absorb as they stretch, in units of cm⁻¹. The C=O stretch is a strong, sharp absorption at about 1680–1750 cm⁻¹ and is hard to mistake: a ketone near 1715, an aldehyde near 1730. Conjugation lowers both by about 30 cm⁻¹.
- ¹H NMR. This method reports each kind of hydrogen as a signal at a position measured in ppm. The H on an aldehyde's carbonyl carbon appears at 9–10 ppm, further along the scale than almost any other hydrogen.
- ¹³C NMR. The same idea for carbons. The carbonyl carbon appears at 190–220 ppm, far from everything else in the spectrum.
What carries forward
Nucleophilic addition, the next section, is the immediate sequel. Acyl substitution in Carboxylic Acids & Derivatives starts with the same first step and follows it with a different second one. Enolate Chemistry exploits the alpha hydrogens. Imine and enamine formation, later in this chapter, connects to Amines. A firm grip on the polarization and the reactivity order here pays off in every one of them.