Why is the measured H-C-O bond angle for CH2O larger than what is predicted by VSEPR

Why is the measured H-C-O bond angle for CH2O larger than what is predicted by VSEPR?

The Correct Answer and Explanation is :

The measured H-C-O bond angle in formaldehyde (CH₂O) is larger than what is predicted by the Valence Shell Electron Pair Repulsion (VSEPR) theory due to the influence of multiple bonding and differences in electron density distribution. While VSEPR theory provides a basic framework for predicting molecular shapes and bond angles based on electron pair repulsion, it does not account for the nuances introduced by resonance, bond strength, and orbital hybridization.

Explanation:

1. VSEPR Prediction: According to VSEPR theory, the geometry around the central atom (C) in CH₂O is determined by the number of regions of electron density. Formaldehyde has three regions of electron density around carbon: two single bonds (C-H) and one double bond (C=O). These regions arrange themselves in a trigonal planar geometry with an ideal bond angle of approximately 120°.
2. Electron Density and Bonding Effects:

• The C=O double bond has higher electron density compared to the C-H single bonds. This increased electron density causes stronger repulsion between the C=O bond and the C-H bonds.
• To minimize this repulsion, the C-H bonds are pushed slightly closer together, leading to an increase in the H-C-O bond angle beyond the ideal 120°.

1. Hybridization: The carbon atom in formaldehyde is sp² hybridized, which leads to a planar structure. The sp² orbitals create bonds that ideally have 120° angles. However, the delocalization of electron density in the π bond of the C=O group enhances the repulsion, causing the H-C-O bond angle to stretch.
2. Experimental Observation: Measurements show that the H-C-O bond angle in CH₂O is approximately 121-122°. This deviation reflects the combined influence of the high electron density of the C=O bond and the need to reduce repulsion.

In summary, the larger measured bond angle arises from the electron density and repulsion associated with the C=O double bond, which VSEPR alone does not fully capture.