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Why carboxylic acids are more acidic than alcohols?

Why carboxylic acids are more acidic than alcohols?

Carboxylic acids are more acidic than alcohols. Carboxylic acids react with Na, NaOH, Na2CO3 and NaHCO3. But alcohols only reacts with Na. Reson is, carboxylate anion is more stable than alkoxide anion.


Now we look stability of these carboxylate anion and alkoxide anion.


How do we know carboxylic acids are more acidic than alcohols?

  1. Carboxylic acid reacts with Na, NaOH. But alcohols only reacts with Na.
  2. Carboxylic react with Na2CO3 and NaHCO3. But alcohols doesn't.
  3. In same concentration solutions of carboxylic acids and alcohols, pH of carboxylic acid is low.
  4. Ka of ethanol = 10-18 moldm-3 , Ka of ethanoic acid = 1.8 * 10-5 moldm-3



Now, we look why carboxylic acids are more acidic than alcohols.

The main reason is the stability of anions of alcohols and carboxylic acids.


Carboxylic acid solution

The anion forming, when H is removed from carboxylic acid is called as carboxylate anion.

carboxylic acid gives carboxylate anion

Carboxylate anion can show resonance structures. Therefore carboxylate anion can spread it's negative charge by carbonyl group. So that brings a stability to the carboxylate anion and equilibrium goes to the right side.

Resonance structures of carboxylate anion

resonance structures of carboxylate anion


Alcoholic solution

The anion forming, when H is removed from alcohol is called as alkoxode anion.

alcohol alkoxide ion

Alkokside anion can't show resonance structures. Therefore stability of alkoxide is low. So equilibrium goes to the left side.



What is more acidic, benzoic acid or acetic acid?

Benzoic acid is much acidic than acetic acid. Benzoate ion is more stable than acetate ion.



Questions

Benzoate ion is more basic than acetate ion. Is this true?

False. Acetate ion is more basic than benzoate ion. Compare Kb values of acetate ion and benzoate ion. Kb value of benzoate ion is greater than acetate ion.


Kb values of benzoate ion and acetate ion

  1. Benzoate ion: 1.6 * 10-10 mol dm-3
  2. Acetate ion: 5.5 * 10-10 mol dm-3


why is carboxylic acid more acidic than phenol?

Ka value of carboxylic acid is higher than phenol. Due to close oxygen atoms hydrogen atom in the -OH group is highly positively charged than -OH group in the phenol.


why are alchols weaker acids than cooh?

Alcohols form alkoxide anion in the water. Carboxylic acids (-COOH) form carboxylate anions. From, these two anions, carboxylate anion has more stability in the aqueous solutions. So dissociation of carboxylic acid shifts to the right side in the reversible reaction. But, for alcohols, dissociation is not successful as -COOh groups.



Lewis structures

P2O5 lewis structure OH- lewis structure Ammonium ion (NH4+) lewis structure H2CO3 lewis structure