In the Lewis model, the two bonds in a double bond look...

Name: Problem 15, Chapter 10: Chemistry A Molecular Approach
Uploaded: 2020-06-25T12:33:25-08:00
Duration: 1 min 38 s
Channel: Lottie Adams
Description: In the Lewis model, the two bonds in a double bond look identical. However, valence bond theory shows that they are not. Describe a double bond according to valence bond theory. Explain why rotation is restricted about a double bond but not about a single bond.

In the Lewis model, the two bonds in a double bond look identical. However, valence bond theory shows that they are not. Describe a double bond according to valence bond theory. Explain why rotation is restricted about a double bond but not about a single bond.

Key Concepts

Orbital Hybridization

Orbital hybridization describes the mixing of atomic orbitals to form new, equivalent hybrid orbitals that can arrange in specific geometries. In the case of alkenes, sp² hybridization provides the directional orbitals necessary for forming a sigma bond while leaving an unhybridized p orbital available for pi bond formation.

Valence Bond Theory

Valence bond theory explains chemical bonding as the overlap of atomic orbitals from different atoms, producing localized electron pairs that form bonds. It emphasizes how specific types of orbital interactions result in the observed bonding characteristics, such as bond strength and geometry.

Sigma Bond

A sigma bond is formed by the head-on overlap of atomic orbitals, such as sp² hybrid orbitals in alkenes. This bond creates a strong electron density along the axis connecting the two nuclei. In both single and double bonds, the sigma component is responsible for the primary framework and stability of the bond.

Pi Bond

A pi bond results from the side-to-side overlap of unhybridized p orbitals. In a double bond, the pi bond exists in addition to the sigma bond and is oriented above and below the plane of the molecule. This bond type contributes additional bonding but requires a fixed alignment of the orbitals to maintain its overlap.

Restricted Rotation

Rotation about a double bond is restricted because while the sigma bond can accommodate rotation, the pi bond relies on the parallel alignment of p orbitals. Any rotation would disrupt this alignment, effectively breaking the pi bond and leading to a significant energy barrier. In contrast, a single bond, composed solely of a sigma bond, allows for free rotation without such disruption.

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