Carbonyl Compounds

Organic compounds containing the carbonyl group (C=O), including aldehydes and ketones. They have the general formula:

$$C_nH_{2n}O$$

Structure

The carbonyl carbon is sp² hybridized with trigonal planar geometry:

  • Carbon-oxygen double bond (one σ, one π)
  • Polar bond (oxygen is more electronegative)
  • Partial positive charge on carbon, partial negative on oxygen
C=O
CC=O
CC(=O)C
c1ccccc1C=O
O=C1CCCCC1
CC(=O)c1ccccc1

Aldehydes vs Ketones

Feature Aldehydes Ketones
General formula R-CHO R-CO-R'
Carbonyl position Terminal Internal
Oxidation Easily oxidized to carboxylic acids Resistant to oxidation
Reactivity More reactive Less reactive
IUPAC suffix -al -one
Example (SMILES) CCC=O (propanal) CCC(=O)C (butan-2-one)

Nomenclature

Aldehydes

  • Replace -e with -al
  • Numbering always starts at carbonyl carbon (carbonyl carbon always has the 1-position)
  • Choose the longest chain containing the functional group
  • Substituent locations denoted with the corresponding number
  • Cyclic / aromatic aldehydes: If attached to a ring, add ‘carbaldehyde’ to the cyclic compound name. For aromatic aldehydes, benzaldehyde is more commonly used than benzenecarbaldehyde.
  • Examples: methanal (formaldehyde, C=O), ethanal (acetaldehyde, CC=O), 3-methylbutanal (isovaleraldehyde, CC(C)CC=O)

[!note] Functional group priority If a compound has two functional groups, the one with lower priority is indicated by its prefix (as substituent). A ketone oxygen in a compound that also contains an aldehyde is indicated by the prefix ‘oxo’ (e.g., 4-oxohexanal, O=CCCCC(=O)C). The prefix ‘formyl’ (–CH=O) is used when aldehyde is the lower-priority group.

Ketones

  • Replace -e with -one
  • Number the chain from the terminal closer to the functional group to give the carbonyl the lowest possible locant (minimum at the 2nd carbon for acyclic ketones)
  • In cyclic ketones, the carbonyl is assumed to be at the 1-position
  • Examples: propanone (acetone / dimethyl ketone, CC(=O)C), butan-2-one (CCC(=O)C), cyclohexanone (O=C1CCCCC1)

Phenyl-substituted Ketones

Common names are used for some phenyl-substituted ketones: the number of carbons (other than those of the phenyl group) is indicated by substituting “-ophenone” for “-ic acid” in the corresponding carboxylic acid name.

Systematic name Common name SMILES
phenylethanone acetophenone / methyl phenyl ketone CC(=O)c1ccccc1
1-phenyl-1-butanone butyrophenone / phenyl propyl ketone CCCC(=O)c1ccccc1
1-phenyl-2-butanone — CC(=O)Cc1ccccc1

Physical Properties

  • Dipole-dipole interactions: Higher boiling points than alkanes/ethers
  • No H-bonding: Lower boiling points than alcohols
  • Solubility: Lower members soluble in water

Reference comparison (similar molecular weight):

Compound Type Boiling point
CH₃CH₂CH₂OH (propan-1-ol) Alcohol 97.4 °C
CH₃COCH₃ (propanone) Ketone 56 °C
CH₃CH₂CHO (propanal) Aldehyde 49 °C
CH₃CH₂OCH₃ (ethyl methyl ether) Ether 10.8 °C

Preparation

Aldehydes

  • Oxidation of primary alcohols — must use a mild oxidizing agent: PCC (Pyridinium chlorochromate) in CH₂Cl₂ is the only reagent that stops at the aldehyde. Strong oxidizing agents (K₂Cr₂O₇ / H₂SO₄, CrO₃ / H₂SO₄, KMnO₄ / H⁺) over-oxidize to carboxylic acids.
  • Ozonolysis of alkenes — cleavage of C=C with O₃ / H₂O / Zn yields aldehydes (from monosubstituted alkene carbons)
  • Hydroformylation of alkenes

Ketones

  • Oxidation of secondary alcohols — any oxidizing agent (K₂Cr₂O₇ / H₂SO₄, CrO₃ / H₂SO₄, KMnO₄ / H⁺, or PCC/CH₂Cl₂) converts secondary alcohols to ketones without over-oxidation
  • Ozonolysis of alkenes — cleavage of C=C with O₃ / H₂O / Zn yields ketones (from disubstituted alkene carbons)
  • Friedel-Crafts acylation — aromatic ketones prepared from benzene + acyl chloride (RCOCl) in presence of AlCl₃ (Lewis acid catalyst)
  • Hydration of alkynes (Markovnikov)

Reactions

1. Nucleophilic Addition

General mechanism: Nu⁻ attacks electrophilic carbonyl carbon, followed by protonation.

With HCN (Cyanohydrin formation)

  • Product: α-hydroxynitrile
  • Base-catalyzed
  • One carbon extension

With Grignard Reagents

  • Aldehydes → Secondary alcohols
  • Ketones → Tertiary alcohols

With Alcohols (Acetal/Ketal formation)

  • Hemiacetal/hemiketal intermediate
  • Acetals/ketals as protecting groups

With NaHSO₃

  • Addition compounds (crystalline solids)
  • Purification technique

2. Addition-Elimination with Nitrogen Nucleophiles

Reagent Product Test
Hydroxylamine (NH₂OH) Oxime -
Hydrazine (NH₂NH₂) Hydrazone -
2,4-Dinitrophenylhydrazine 2,4-DNP derivative Orange/red ppt (positive test)
Primary amine (RNH₂) Imine (Schiff base) -

3. Oxidation/Reduction

Oxidation (Aldehydes only)

  • Tollens' test: [Ag(NH₃)₂]⁺ → Ag (silver mirror)
  • Fehling's/Benedict's test: Cu²⁺ → Cu₂O (brick-red precipitate)
  • Schiff's test: Pink color

Reduction

  • Aldehydes → Primary alcohols
  • Ketones → Secondary alcohols
  • Reagents: NaBH₄ (mild), LiAlH₄ (strong), catalytic H₂

4. Special Reactions

Aldol Condensation

  • Two molecules of carbonyl compound
  • α-H required
  • β-hydroxy carbonyl → α,β-unsaturated carbonyl (on heating)

Cannizzaro Reaction

  • Non-enolizable aldehydes (no α-H)
  • Disproportionation: One molecule oxidized, one reduced

Haloform Reaction

  • Methyl ketones (CH₃-CO-)
  • Iodoform test: Yellow CHI₃ precipitate

Identification Tests Summary

Test Aldehydes Ketones
Tollens' Silver mirror Negative
Fehling's Brick-red ppt Negative
Benedict's Brick-red ppt Negative
2,4-DNP Orange/red ppt Orange/red ppt
Iodoform Positive if CH₃CHO Positive if methyl ketone
Schiff's Pink color Negative

Carbonyl Compounds in Nature

Natural Products

Many biologically important molecules contain aldehyde or ketone groups:

COc1cc(C=O)ccc1O

O=CC=Cc1ccccc1

CC12CCC(CC1=O)C2(C)C

CC1=CC[C@@H](C(C)C)CC1=O
  • Aldehydes generally have pungent odors
  • Ketones tend to smell sweet

Carbohydrates as Carbonyl Compounds

Monosaccharides (simple sugars) are carbonyl compounds that cannot be hydrolyzed further.

Classification

Class Carbonyl Type Example Carbon Count
Aldose Aldehyde Glucose Hexose (6C)
Ketose Ketone Fructose Hexose (6C)

D and L Configuration

  • Based on the configuration of the chiral carbon furthest from the carbonyl group
  • D-sugar: OH on the right in Fischer projection (bottom chiral center)
  • L-sugar: OH on the left in Fischer projection
  • D forms predominate in nature; D-glucose and L-glucose are enantiomers

Glucose (Aldohexose)

O=C[C@H](O)[C@@H](O)[C@H](O)[C@H](O)CO
  • Polyhydroxyaldehyde
  • In solution, forms a cyclic hemiacetal via intramolecular nucleophilic attack of the C₅ hydroxyl on the C₁ aldehyde
  • Reducing sugar: the hemiacetal ring can open to expose the free aldehyde, which is oxidized by Tollens' or Benedict's reagent

Fructose (Ketohexose)

O=C(CO)C(O)C(O)C(O)CO
  • Polyhydroxy ketone
  • Forms a cyclic hemiacetal via intramolecular attack of the C₅ hydroxyl on the C₂ ketone
  • Also a reducing sugar (hemiacetal-reducing sugar)

Reducing vs Non-Reducing Sugars

Reducing sugars (positive Tollens'/Benedict's test):

  • All monosaccharides with a hemiacetal group (glucose, fructose)
  • Ketoses are converted to aldoses under alkaline conditions, then oxidized

Non-reducing sugars (negative Tollens'/Benedict's test):

  • Sugars in acetal form (e.g., glycosides, disaccharides like sucrose)
  • No free hemiacetal hydroxyl; cannot open to reveal a carbonyl
Test Reducing Sugar Non-Reducing Sugar
Tollens' Silver mirror No reaction
Benedict's Brick-red precipitate No reaction

Examples with SMILES

CCC=O
CCC(=O)C
CC(Br)C=O
CC(Cl)CC=O
CC(C)CC=O
O=CCCCCC=O
CC1CCCCC1C=O
c1ccccc1C=O
CC(=O)C
CCC(=O)CCC
O=C1CCCCC1
CC(=O)C(=O)C
CC(=O)CC(=O)C
CC(=O)CC=CC
CC(=O)c1ccccc1
CCCC(=O)c1ccccc1
CC(=O)Cc1ccccc1
CC(C)(C)C1CCCC(=O)C1

Related Topics

Sources