Question

Make atomic-orbital models of ethanoic acid and ethanol and of the ethanoate anion and ethoxide anion. Show how these models can be used to explain the greater acidity of ethanoic acid relative to ethanol.

Short answer

Ethanoic acid is more acidic than ethanol due to the resonance stabilization of its conjugate base, the ethanoate anion.

Step-by-step solution

Understanding the Structures

First, identify the chemical structures of ethanoic acid (CH3COOH) and ethanol (C2H5OH), as well as the corresponding anions: ethanoate (CH3COO-) and ethoxide (C2H5O-). These molecules have different functional groups that affect their acidity.

Atomic Orbital Hybridization

Determine the hybridization of the carbon atoms in both ethanoic acid and ethanol. In ethanoic acid, the carbonyl group ( C=O ) is sp² hybridized, while in ethanol, the carbon atoms are sp³ hybridized.

Delocalization in Ethanoate Anion

For the ethanoate anion, the negative charge on the oxygen is delocalized over the oxygen atoms through resonance, making the structure more stable. Draw resonance structures to illustrate this delocalization.

Comparison with Ethoxide Anion

Unlike the ethanoate anion, the ethoxide anion has the negative charge localized on the oxygen atom. This lack of resonance stability makes the ethoxide anion less stable compared to the ethanoate anion.

Explaining Relative Acidity

Explain that acidity is related to the stability of the conjugate base (anion). The greater stability of the ethanoate anion, due to resonance, makes ethanoic acid more acidic compared to ethanol, whose conjugate base (ethoxide) lacks this stabilizing feature.

Key concepts

Atomic Orbital Hybridization

Atomic orbital hybridization is a key concept in understanding the structure of organic molecules. It describes the mixing of atomic orbitals to form hybrid orbitals, which can form sigma bonds or pi bonds. In ethanoic acid, the carbon atom bonded to the oxygen in the carbonyl group ( C=O ) is sp² hybridized. This hybridization leads to a planar structure, helping delocalize the electrons.
In contrast, the ethanol molecule, specifically the carbon atoms bonded to the hydroxyl group, are sp³ hybridized. This results in a tetrahedral geometry, limiting electron interaction and delocalization compared to sp² hybridization. Understanding these differences in hybridization is critical because they influence the acidity and stability of the molecules.

• sp² hybridized carbons allow more planar, stable structures.
• sp³ hybridized carbons result in a less electron-delocalized structure.

Resonance Structures

Resonance structures play a crucial role in the stability of anions and acids. These are different ways of arranging electrons in a molecule, which depict the delocalization of electrons. In ethanoate anion derived from ethanoic acid, the negative charge can be spread over two oxygen atoms through resonance. This sharing makes the anion more stable than if the charge were localized.
Drawing resonance structures involves showing all possible distributions of the electrons. This concept implies that the actual structure of the anion is a hybrid of all the possible structures, which lowers its energy and increases its stability.

• Resonance increases anion stability by distributing charge.
• The anion appears as a hybrid of all possible resonance structures.

Conjugate Base Stability

Conjugate base stability is an essential concept to determine the acidity of a compound. The more stable the conjugate base, the stronger the corresponding acid. In comparing ethanoic acid and ethanol, ethanoic acid is a stronger acid because its conjugate base, the ethanoate anion, is more stable due to resonance.
Conversely, ethanol's conjugate base, the ethoxide anion, lacks this resonance stabilization and holds the negative charge more localized on a single oxygen. This lack of charge delocalization leads to decreased stability, thus making ethanol a weaker acid than ethanoic acid.

• Stable conjugate bases lead to stronger acids.
• Ethanoate anion's resonance contributes to its stability.

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and dictate the chemistry of the molecule. In ethanoic acid, the key functional group is the carboxyl group ( -COOH ), which is responsible for its acidic properties. The carbonyl portion ( C=O ) and the hydroxyl group ( -OH ) together facilitate the acid's ability to donate a proton.
In ethanol, the hydroxyl group ( -OH ) is the main functional group. Although it can also donate a proton, the absence of the carbonyl group means less stabilization for the resulting anion. Thus, the acidity is somewhat reduced compared to ethanoic acid.

• Carboxyl group enhances acidity by stabilizing the anion.
• The presence and nature of functional groups affect molecular properties.

Anion Stability

Anion stability is crucial for understanding why some acids are stronger than others. A stable anion forms when negative charges are spread across the molecule, reducing repulsive interactions. Ethanoate anion benefits from this stability, as its charge is not localized. Instead, it is resonant between two oxygen atoms, decreasing energy and increasing stability overall.
The ethoxide anion in ethanol, however, does not have this advantage. Its negative charge stays localized on one oxygen atom, resulting in less stability. This simple principle underlies why ethanoic acid is more acidic than ethanol: a more stable conjugate base leads to a stronger acid.

• Delocalization of charge increases anion stability.
• Stable anions contribute to the corresponding acid's strength.