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