Aldehyde rule
The carbonyl carbon carries hydrogen.
That distinguishes formaldehyde from a ketone such as acetone, where carbon groups flank the carbonyl.
Molecular compound · aldehyde
Formaldehyde
CH₂O
Formaldehyde is CH₂O, the simplest aldehyde. The carbonyl carbon is bonded to two hydrogens, giving H₂C=O as the defining molecular structure.
Ontology rule: formaldehyde is the compound. Formalin is an aqueous formulation containing formaldehyde-derived species, typically with stabilizing additives; it is not a synonym for a pure CH₂O sample.
Essentials
The first structural fact is H₂C=O.
One carbonyl carbon is double-bonded to oxygen and bonded to two hydrogens. That makes formaldehyde the simplest aldehyde.
Detail
The carbonyl center is approximately trigonal planar.
Three electron-domain directions surround the carbonyl carbon: two C–H bonds and one C=O bonding axis. A flat 2D drawing therefore matches the local molecular geometry unusually well.
Carbonyl polarity
Oxygen is electron-rich relative to carbon.
This polarization makes the carbonyl carbon electrophilic and the oxygen a hydrogen-bond acceptor.
Understand
In water, formaldehyde participates in hydration chemistry.
Water can add reversibly across the carbonyl to form methanediol. Real aqueous formaldehyde therefore should not be imagined as only isolated H₂C=O molecules floating unchanged.
Formalin
A commercial solution is a mixture.
Concentration, water content and stabilizers belong to the product description, not to the molecular formula CH₂O.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | CH₂O |
|---|---|
| CAS Registry Number | 50-00-0 |
| PubChem CID | 712 |
Advanced
Push past the first model without losing the simple picture.
Is formalin the same compound as formaldehyde?
No. Formaldehyde is CH₂O; formalin is an aqueous formulation containing formaldehyde-derived species.
Why does formaldehyde hydrate readily?
Its small, strongly polarized carbonyl is highly accessible to nucleophilic water.
Does the formula CH₂O by itself show an aldehyde?
No. The formula counts atoms; the H₂C=O connectivity identifies the aldehyde structure.
Constituent elements