"Vinegar is sour."
Kana grimaced.
Toma laughed. "That's the anger of carboxylic acid."
"Anger?"
"It releases protons. Aggressive."
Rei corrected. "Rather than attack, just letting go."
"But why is carboxylic acid acidic?" Kana asked.
"The structure of the carboxyl group -COOH," Rei opened a notebook.
"Double bond between C and O, with OH attached."
Toma assembled a model. "This OH's H comes off easily."
"Why?"
"Resonance stabilization," Rei drew a diagram.
"When H⁺ leaves, it becomes -COO⁻. This anion is stable."
"Stable?"
"The negative charge is distributed between two O atoms. Resonance structure."
Kana drew in her notebook. "The two O are equivalent?"
"Yes. Not fixed to either, but oscillating between them."
Toma supplemented. "That's why the anion is stable and H⁺ is easily released."
"Compared to other acids?"
Rei gave an example. "Alcohol's OH doesn't easily release H⁺."
"Why?"
"Alkoxide ion RO⁻ has negative charge concentrated on one O. Unstable."
"Can't resonate."
"That's why alcohols are weak acids, carboxylic acids are strong acids."
Kana understood. "Resonance is the key."
Milia came in. "Talking about pKa?"
"pKa?"
"The logarithm of acid dissociation constant. An indicator of acid strength."
Rei wrote an equation. "HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA]"
"pKa = -log Ka. Smaller means stronger acid."
"What's acetic acid's pKa?"
"About 4.8. A weak acid, but reliably dissociates."
Toma compared. "Hydrochloric acid has pKa about -7. Almost completely dissociates."
"Acetic acid is gentle?"
"The anger of carboxylic acid is modest."
Kana laughed. "Modest anger."
Milia proposed an experiment. "Let's measure pH of different carboxylic acids."
Acetic acid, formic acid, benzoic acid. Dipped pH paper in each.
"All acidic but different strengths!" Kana observed.
"Substituent effects," Rei explained.
"Electron-withdrawing groups lower pKa. Acidity strengthens."
"Why?"
"Because the anion becomes more stable."
Toma gave an example. "Trichloroacetic acid. Three Cl withdraw electrons."
"pKa is about 0.5. A strong acid."
"Conversely, with electron-donating groups?"
"Acidity weakens. The anion destabilizes."
Kana thought. "Then can carboxylic acid's acidity be adjusted?"
"By changing the structure," Milia nodded.
Rei continued. "Also important in living organisms. Like amino acid carboxyl groups."
"Amino acids?"
"They have both. Carboxyl group and amino group."
"Amphoteric."
"Depending on pH, they release or receive protons."
Kana was impressed. "They're flexible."
Toma said quietly. "The anger of carboxylic acid is controlled."
"Only when needed, it releases H⁺."
Milia supplemented. "That's why it's usable in life. Not too violent."
Rei looked out the window. "Modest anger is beautiful."
Kana wrote in her notebook. "The modest anger of carboxylic acid."
"Chemistry's emotion."
The three lined up test tubes. Carboxylic acid quietly releases protons. The anger is gentle, and therefore enduring.