What is the approximate H-O-C bond angle in the glycinium cation [VISUAL]? — Bonding Chemistry Question

STEPS:

1. Identify the central atom of the bond angle:
In the $\text{H-O-C}$ group of the glycinium cation, the oxygen ($\text{O}$) atom is the central atom, covalently bonded to one hydrogen ($\text{H}$) atom and one carbon ($\text{C}$) atom.

2. Determine the number of electron domains on the oxygen atom:
The oxygen atom involved in this single-bonded configuration has two bonding pairs (one to $\text{H}$ and one to $\text{C}$) and retains two nonbonding lone pairs of electrons. This gives the oxygen atom a total of four electron domains.

3. Apply VSEPR theory to find the electron-domain geometry:
According to Valence Shell Electron Pair Repulsion (VSEPR) theory, four electron domains will orient themselves in a roughly tetrahedral arrangement to minimize electrostatic repulsion.

4. Estimate the bond angle based on molecular geometry:
The ideal bond angle for a perfect tetrahedral geometry is $109.5^\circ$. Because nonbonding lone pairs exert greater repulsive forces than bonding pairs, they compress the adjacent $\text{H-O-C}$ bonding domains, pushing the angle to be slightly less than $109^\circ$.

5. Select the closest multiple-choice option:
Comparing the options, $105^\circ$ is close to $109^\circ$ and represents this slight compression from the ideal tetrahedral angle. This makes Option C the correct answer.

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WHY_OTHERS_WRONG:

• Option A is incorrect ($180^\circ$): A $180^\circ$ angle represents a linear domain arrangement (such as $sp$ hybridization) which is only observed when there are two electron domains surrounding a central atom, not four.
• Option B is incorrect ($120^\circ$): A $120^\circ$ angle is characteristic of a trigonal planar electron-domain arrangement (such as the adjacent $sp^2$ hybridized carbonyl carbon atom). Oxygen has four domains rather than three, making this angle too large.
• Option D is incorrect ($90^\circ$): A $90^\circ$ angle is not observed for period 2 central atoms with tetrahedral electron domains because of the strong Coulombic repulsions between the four concentrated electron clouds.