Physical and Bond Properties

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Like a C=C double bond, the carbonyl group C=O consists of two molecular orbitals, one σ and one π.

Physical and bond properties of carbonyl compounds

σ and π bonds of the carbonyl group and the two lone pairs of oxygen in sp2 orbitals

The difference is that the oxygen is not bonded to any other atom and has two lone pairs of electrons in sp² orbitals.

The higher electronegativity of oxygen polarizes the C=O bond, which determines its reactivity.

Animation of the charge density of formaldehyde — Charge density of formaldehyde: green shows regions with partial positive charge (the carbon and the hydrogens), and magenta those with partial negative charge (the oxygen)

Structural parameters of acetaldehyde (angles of 121°, 114° and 125°; C=O 1.20 Å, C–H 1.12 Å and C–C 1.50 Å) and its charge-separated resonance form — Structural features of acetaldehyde

The experimentally measured bond angles and distances are compatible with sp² hybridization of the two atoms forming the C=O bond.

The charge-separated resonance form explains the electron deficiency of the carbon in a C=O double bond. This is an important difference from the C=C bond.

The higher electronegativity of oxygen polarizes the C=O bond, which determines its reactivity and, very importantly, that of its adjacent position (α).

Reactive centers of the carbonyl group: electrophilic carbon, nucleophilic and basic oxygen, and acidic α hydrogens