Understanding Hybridization in Allene (C₃H₄)

Allene has the structure H₂C=C=CH₂, with two double bonds between the three carbon atoms. The central carbon is bonded to two other carbons, and each terminal carbon is bonded to two hydrogen atoms.

Step 1: Determine the Hybridization of Terminal Carbons

Each terminal carbon (C1 and C3) is part of a double bond (one σ and one π bond) and bonded to two hydrogen atoms. The steric number (number of atoms bonded + lone pairs) is 3 (2 H atoms + 1 C atom), so hybridization is sp².

Steric number calculation: $\mathrm{SN}=3\Rightarrow \mathrm{sp}{}^2$

Step 2: Determine the Hybridization of the Central Carbon

The central carbon (C2) is bonded to two other carbons via double bonds. Each double bond consists of one σ bond, so C2 has two σ bonds. The steric number is 2, indicating sp hybridization.

Steric number: $\mathrm{SN}=2\Rightarrow \mathrm{sp}$

Step 3: Verify with Molecular Geometry

Terminal carbons (sp²) are trigonal planar with ~120° bond angles. The central carbon (sp) is linear with 180° bond angle between the two terminal carbons, consistent with allene's structure.

Final Answer

The hybridizations are sp and sp² (central carbon is sp, terminal carbons are sp²).

Related Topics

• Hybridization Theory
• Molecular Geometry
• Steric Number and Hybridization
• Cumulated Dienes

Key Formulae and Theory

Hybridization is determined by steric number (SN):

$\mathrm{SN}=\text{number of σ bonds + number of lone pairs}$

For SN=2: sp hybridization (linear geometry)

For SN=3: sp² hybridization (trigonal planar geometry)

For SN=4: sp³ hybridization (tetrahedral geometry)

In allene, the central carbon has no lone pairs and two σ bonds (SN=2), while terminal carbons have three σ bonds each (SN=3).