Question

If methanal, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{O}\), were protonated to give \(\mathrm{H}_{2} \mathrm{C}=\mathrm{OH}\), would you expect the \(\mathrm{C}=\mathrm{O}-\mathrm{H}\) angle to be closer to \(180^{\circ}, 120^{\circ}, 109^{\circ}\), or \(90^{\circ}\) ? Explain.

Short answer

The \( \mathrm{C}=\mathrm{O}-\mathrm{H} \) angle should be close to \( 120^{\circ} \).

Step-by-step solution

Identify Protonation Effect

Protonating methanal, \( \mathrm{H}_{2} \mathrm{C}=\mathrm{O} \), leads to the formation of \( \mathrm{H}_{2} \mathrm{C}=\mathrm{OH} \). This changes the structure from a simple aldehyde to a protonated derivative, affecting the bond angles around the carbon atom.

Determine Hybridization

In \( \mathrm{H}_{2} \mathrm{C}=\mathrm{OH} \), the carbon atom is still involved in a double bond with oxygen. The hybridization of carbon in such a scenario usually remains \( sp^2 \), which corresponds to an ideal bond angle of \( 120^{\circ} \).

Consider Resonance and Bond Lengths

Protonation introduces a positive charge on the oxygen, increasing the \( \mathrm{C-OH} \) bond polarity. This may induce a slight deviation in the angle, but since resonance and bond length factors maintain \( sp^2 \) hybridization, the angle remains close to \( 120^{\circ} \).

Conclusion on Bond Angle

Given the \( sp^2 \) hybridization and relative resonance stability, the \( \mathrm{C}=\mathrm{O}-\mathrm{H} \) angle in \( \mathrm{H}_{2} \mathrm{C}=\mathrm{OH} \) after protonation is expected to be close to \( 120^{\circ} \).

Key concepts

Bond Angles

When we talk about bond angles, we are referring to the angle formed between three atoms across at least two bonds. In a molecule like protonated methanal \( \mathrm{H}_2\mathrm{C}=\mathrm{OH} \), the main determinant of the bond angles is the hybridization of the atoms involved, here specifically the carbon atom.

• The carbon atom has an \( sp^2 \) hybridization due to its double bonds.
• This type of hybridization typically forms angles of approximately \( 120^{\circ} \).

Hydrogen atoms bonded to the carbon do not significantly alter this angle, as they mostly follow the hybridization pattern set by carbon. Protonation of methanal slightly modifies the electronegativity distributions, but as long as the carbon retains an \( sp^2 \) hybridization, the bond angle will largely remain close to this ideal value. Understanding bond angles is crucial for predicting the shape and physical properties of molecules, which in turn affects their reactivity and how they interact with others.

Protonation

Protonation occurs when a proton (\( \mathrm{H}^+ \)) is added to a molecule, altering its structure and properties. In the case of methanal \( \mathrm{H}_2\mathrm{C} = \mathrm{O} \), protonation leads to the formation of a positively charged species \( \mathrm{H}_2\mathrm{C}=\mathrm{OH}^+ \). This addition impacts:

• Bond angles: The addition of a proton at the oxygen changes the electronic distribution but not the hybridization of the carbon, which primarily dictates the angle.
• Resonance structures: Protonation increases the resonance possibilities and delocalization of the positive charge across the oxygen.

Practical effects of protonation can include changes in reactivity, stability, and interactions with other molecules. Observing how protonation affects bond angles helps us understand its role in physical and chemical behavior.

Resonance

Resonance refers to the representation of a molecule through multiple structures, which depict the delocalization of electrons within the molecule. This is particularly relevant in protonated species.

• When methanal is protonated, the positive charge can be distributed between the oxygen and carbon atoms.
• This delocalization helps stabilize the molecule even with the additional positive charge.

Although resonance can affect bond angles by altering electron density, the \( sp^2 \) hybridization maintains the \( \mathrm{C}=\mathrm{OH} \) angle close to \( 120^{\circ} \). Resonance allows molecules to adapt structurally without sacrificing stability, crucial in conjugated systems and polyatomic ions.

Chemical Structure

Chemical structure defines the spatial arrangement of atoms in a molecule. It determines:

• Bond lengths and angles
• The molecule's dimensional shape
• Reactivity and intermolecular interactions

In methanal and its protonated form \( \mathrm{H}_2\mathrm{C}=\mathrm{OH} \), the hybridization and presence of double bonds create a planar structure. The geometry of structures affects not only chemical behaviors but also physical properties like boiling points, solubility, and even the color of compounds.Understanding chemical structure helps chemists predict how molecules will act during reactions and interactions in broader chemical processes.