Draw the 2 resonance structures of propanoate: 10. (5 points) Draw the dimer structure of propanoic acid and explain why it readily forms
The Correct Answer and Explanation is:
Question 10 Answer
1. Resonance Structures of Propanoate (6 points)
The propanoate ion (C₂H₅COO⁻) is the conjugate base of propanoic acid (C₂H₅COOH). The negative charge on the carboxylate group is delocalized over the two oxygen atoms, leading to resonance.
Resonance Structures:
Structure 1:
O⁻
||
CH3-CH2-C
\
O
Structure 2:
O
||
CH3-CH2-C
\
O⁻
These two resonance structures show the delocalization of the negative charge between the two oxygen atoms. The actual structure is a resonance hybrid of these, with both C–O bonds being equal in length and the negative charge spread evenly between the oxygens.
2. Dimer Structure of Propanoic Acid (5 points + explanation)
Propanoic acid molecules can form dimers through hydrogen bonding between their carboxylic acid groups. In this dimer, each molecule acts as both a hydrogen bond donor and acceptor.
Dimer Structure:
O H H O
|| | | ||
CH3-CH2-C---O•••••O---C-CH2-CH3
| |
H O
Each acid donates a hydrogen bond from its –OH to the carbonyl oxygen of the other molecule.
Explanation
The propanoate ion exhibits resonance due to the presence of a carboxylate group, where the negative charge is shared equally between two electronegative oxygen atoms. In the resonance structures, one structure has a double bond between the carbon and one oxygen, with the other oxygen carrying the negative charge; the second structure swaps these roles. The resonance stabilization makes the carboxylate ion more stable and helps explain why carboxylic acids like propanoic acid are acidic—they readily donate a proton because the resulting anion is stabilized.
On the other hand, propanoic acid forms dimers in nonpolar solvents or in the gas phase due to strong intermolecular hydrogen bonding between the hydroxyl hydrogen of one molecule and the carbonyl oxygen of another. This interaction is so strong that it results in the formation of a cyclic dimer, where each molecule acts as both a hydrogen bond donor and acceptor. These dimers are particularly stable because the hydrogen bonds form a nearly planar ring structure, reinforcing the interaction.
This tendency to form dimers is common among carboxylic acids and significantly affects their boiling points and solubility. For example, because of dimerization, carboxylic acids often have much higher boiling points than alcohols or ketones of similar molecular weight. In polar solvents (like water), the dimer breaks apart and hydrogen bonds instead form between the acid and water molecules, allowing for good solubility in water.
In summary, the resonance in the propanoate ion stabilizes the conjugate base, and the ability of propanoic acid to form dimers through hydrogen bonding demonstrates a unique and important property of carboxylic acids.
