Subjects and lessons to explore.

Law

Organic Chemistry 2 · Carbonyl Chemistry

Aldehydes and Ketones: Preparation and Nomenclature

7 min read
12 key termsReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Aldehydes and ketones are carbonyl compounds built around a carbon–oxygen double bond, C=O. An has at least one hydrogen on the carbonyl carbon (R-CHO); a has two carbon groups (R-C(=O)-R'). The polar carbonyl makes the carbon electrophilic, which underlies nearly all carbonyl reactivity. They are prepared mainly by oxidizing alcohols, cleaving alkenes by , acylating arenes, and hydrating alkynes.

Why this matters

Formaldehyde (methanal) is a tissue preservative and sterilizing agent in histology and embalming, while acetone is a common laboratory solvent and one of the "ketone bodies" produced during fat metabolism (elevated in diabetic ketoacidosis). Many pharmaceuticals and steroid hormones contain ketone groups, and acetophenone and benzophenone derivatives appear widely in medicinal chemistry. Recognizing an aldehyde versus a ketone in the lab dictates which oxidant or reductant is safe and appropriate.

The college version

1. Structure and Polarity of the Carbonyl Group

The carbonyl carbon is sp2 hybridized and trigonal planar (bond angles near 120°). Its two resonance forms — the neutral C=O and the charge-separated C+-O− — show the carbon carries partial positive character. This polarity gives aldehydes and ketones significant dipole moments and moderately high boiling points, and it makes the carbon the electrophilic center attacked by nucleophiles.

2. Nomenclature

IUPAC rules: for an acyclic aldehyde, name the longest chain containing the carbonyl carbon and replace the final "-e" of the alkane with "-al"; the carbonyl carbon is always C1, so no locator is needed (e.g., butanal, CH3CH2CH2CHO). For a ketone, replace "-e" with "-one" and number so the carbonyl gets the lowest locator (e.g., 2-pentanone, CH3COCH2CH2CH3). Common names include formaldehyde (methanal), acetaldehyde (ethanal), propionaldehyde, butyraldehyde, acetone (propanone), and benzaldehyde. Cyclic and aromatic ketones include cyclohexanone, acetophenone (methyl phenyl ketone), and benzophenone (diphenyl ketone).

3. Relative Reactivity: Aldehydes vs. Ketones

Aldehydes react faster toward nucleophilic addition for two reasons. Sterically, the aldehyde carbonyl is less crowded (one H, one R) than a ketone (two R groups). Electronically, alkyl groups are electron-donating, so a ketone's two alkyl groups reduce the carbon's positive character more than an aldehyde's single alkyl group. Aromatic ketones (e.g., acetophenone) are further stabilized by conjugation with the ring.

How it works

  1. A is a carbon doubly bonded to oxygen.
  2. Oxygen's greater electronegativity polarizes the bond, making carbon electrophilic.
  3. Aldehydes have a hydrogen on the carbonyl carbon; ketones have two carbon groups.
  4. IUPAC names use "-al" (aldehyde) and "-one" (ketone) suffixes.
  5. Primary alcohols oxidize to aldehydes; secondary alcohols oxidize to ketones.
  6. Ozonolysis of an alkene installs carbonyls at both former double-bond carbons.
  7. builds aryl ketones from acid chlorides and arenes.
  8. Alkyne hydration gives ketones (Markovnikov); terminal alkynes give methyl ketones.
  9. In water, carbonyls equilibrate with gem-diol hydrates.
  10. Aldehydes are more electrophilic (and more reactive) than ketones.

Common confusions

Do not confuseWithDifference
AldehydeKetoneAldehyde has an H on the carbonyl carbon; ketone has two carbon groups
AldehydeCarboxylic acidAldehyde is -CHO; the acid is -COOH (one more oxygen)
"-al" suffix"-ol" suffix"-al" = aldehyde; "-ol" = alcohol
KetoneEsterKetone is R-C(=O)-R; ester is R-C(=O)-OR
HydrateHemiacetalHydrate adds water (two OH); hemiacetal adds one alcohol (one OH, one OR)

Memory aids

"Aldehydes Are At the End (H); Ketones Keep Carbon on Both sides." For naming: "-al" has one "l," like the single H an aldehyde carries; "-one" has "one" carbon group on each side.

Quick review

Topic Recap

Aldehydes and ketones share the polarized C=O group but differ by hydrogen versus carbon substitution at the carbonyl carbon. Nomenclature uses "-al" and "-one" suffixes, with common names still in wide use. They are prepared by , ozonolysis, Friedel-Crafts acylation, and alkyne hydration, and they equilibrate with hydrates in water. Aldehydes outpace ketones in reactivity for steric and electronic reasons.

Knowledge Check

  1. Classify CH3CH2COCH3 and name it.
  2. Give the and the of CH3CHO.
  3. Which is more reactive toward nucleophilic addition, butanal or 2-butanone? Why?
  4. What product forms when 2-butene undergoes ozonolysis with reductive workup?
  5. Why does acetone have a lower boiling point than 1-propanol despite a similar molar mass?

Answers and Rationales

  1. 2-Butanone (ethyl methyl ketone). It is a ketone because the carbonyl carbon bonds to two carbon groups, and numbering gives the carbonyl the lowest locator (C2).
  2. Ethanal; common name acetaldehyde. Two carbons with an aldehyde group.
  3. Butanal (an aldehyde) is more reactive. It is less hindered, and its carbonyl carbon is more electrophilic because a ketone's second alkyl group donates electron density and adds crowding.
  4. Two molecules of acetaldehyde (ethanal). Ozonolysis cleaves the C=C and installs a C=O at each former alkene carbon.
  5. Acetone cannot hydrogen-bond to itself; it interacts only by weaker dipole–dipole forces. 1-Propanol hydrogen-bonds through its O–H, raising its boiling point.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine the carbonyl group as a seesaw with oxygen on one end and carbon on the other. Oxygen pulls electrons much harder, so the double bond's electrons spend more time near oxygen. This leaves oxygen slightly negative and carbon slightly positive — a permanent tug-of-war that makes carbon a magnet for electron-rich molecules (nucleophiles). An aldehyde is this magnet with an easy-to-reach handle (a hydrogen); a ketone is the same magnet with two bulky carbon groups crowding it.

A useful comparison: both are like cash registers accepting "payment" from nucleophiles, but the aldehyde's register is wide open (less crowding, a more positive carbon) while the ketone's sits behind two bulky displays (two alkyl groups) that slow customers down.

This comparison stops being exact because the "crowding" is not purely physical. A ketone's two alkyl groups also donate electron density toward the carbonyl carbon, partly canceling its positive charge — a real electronic effect, not just blockage. The aldehyde–ketone gap is a blend of sterics and electronics, not a single cause.

Simple Example

Propanal (CH3CH2CHO) versus acetone (CH3COCH3). Both carry a C=O, but propanal has a hydrogen on the carbonyl carbon, so it is an aldehyde and is reduced more readily. Acetone has two methyl groups and no carbonyl hydrogen, so it is a ketone and reacts more slowly.

Worked example

Nomenclature and preparation need no curved-arrow mechanism, but the polarity logic and the preparative routes follow a clear order:

  1. Identify the carbonyl carbon. Locate the C=O and decide whether it is bonded to hydrogen (aldehyde), to two carbons (ketone), or to an aryl ring (aromatic ketone).
  2. Assign polarity. Draw the resonance contributor C+-O− to see the partial positive charge on carbon; this one picture predicts reactivity throughout the later chapters.
  3. Trace each preparation to its source. Oxidation of a primary alcohol gives an aldehyde; a secondary alcohol gives a ketone; ozonolysis of an alkene installs a C=O at each former alkene carbon; Friedel-Crafts acylation delivers an aryl ketone; alkyne hydration gives a ketone (a terminal alkyne gives a methyl ketone).
  4. Check the hydration equilibrium. In water, aldehydes and ketones equilibrate with their hydrates (gem-diols). The equilibrium lies further right for aldehydes and electron-poor ketones — the same polarity that drives nucleophilic addition.

Key takeaways

  • High yield: Aldehydes have an H on the carbonyl carbon; ketones have two carbon groups.
  • High yield: IUPAC "-al" = aldehyde, "-one" = ketone; number the carbonyl as low as possible.
  • High yield: Aldehydes are more reactive than ketones (steric + electronic reasons).
  • Common names to memorize: formaldehyde, acetaldehyde, acetone, benzaldehyde, acetophenone, benzophenone.
  • Primary alcohol + PCC (mild oxidation) → aldehyde; secondary alcohol → ketone; overoxidation of aldehydes to acids is a classic trap.
  • Ozonolysis (O₃, then reductive workup) cleaves alkenes; carbonyls land where the alkene carbons were.
  • Terminal alkyne hydration gives a methyl ketone (Markovnikov).
  • Aromatic ketones are stabilized by conjugation with the ring.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Recognize the carbonyl functional group and distinguish aldehydes from ketones by structure.
  • Name aldehydes and ketones using both IUPAC and common nomenclature, including cyclic and aromatic examples.
  • Describe the principal preparative routes: alcohol oxidation, alkene ozonolysis, Friedel-Crafts acylation, and alkyne hydration.
  • Explain carbonyl polarity, the hydration equilibrium, and why aldehydes are generally more reactive than ketones.

Key vocabulary

Aldehyde
Carbonyl with at least one H on the carbonyl carbon (R-CHO)
Ketone
Carbonyl with two carbon groups on the carbonyl carbon
Carbonyl group
The C=O functional group
Carbonyl polarity
Unequal sharing of π electrons, carbon partially positive
IUPAC name
Systematic name: "-al" (aldehyde), "-one" (ketone)
Common name
Traditional name (formaldehyde, acetone, benzaldehyde)
Aromatic/cyclic ketone
Ketone in or attached to a ring (cyclohexanone, acetophenone)
Alcohol oxidation
Removing H₂ from an alcohol to give a carbonyl
Ozonolysis
Oxidative cleavage of an alkene to carbonyl products
Friedel-Crafts acylation
Introducing an acyl group onto an arene
Hydration equilibrium
Reversible addition of water to give a gem-diol
Structural recognition
Spotting aldehyde vs. ketone from a structure

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.