Carboxylic Acids: Acidity

Explain acidity of carboxylic acids

• O-H bond breaks to give a H⁺ proton and -RCO₂^- carboxylate ion

• hence, carboxylic acids are weak(partially dissociates) Bronsted acids(proton donour)

Order the following in increasing acidity/increasing Ka/decreasing pKa

• aliphatic alcohols

• phenol

• water

• ethanoic acid

aliphatic alcohols<water<phenol<ethanoic acid

Why are carboxylic acids more acidic thatn phenols and aliphatic alcohols?

• negative charge is delocalised equally over two highly electronegative oygen atoms

• negative charge is more dispersed

• making carboxylate ion more stabilised

• carboxylic acid is more likely to get deprotonated

• higher Ka, lower pKa, more acidic

• wheras negative charge on phenoxide ion is less delocalised because

• carbon atoms do not bear negative charge as well as two electronegative oxygen atoms in carboxylate ion

reactivity of aliphatic alcohols with

• Na

• NaOH(aq) [strong base]

• Na₂CO₃(aq) OR NaHCO₃(aq) [weak base]

• Na: ✅

• NaOH(aq) [strong base]: ❌

• Na₂CO₃(aq) OR NaHCO₃(aq) [weak base]: ❌

reactivity of phenols with

• Na

• NaOH(aq) [strong base]

• Na₂CO₃(aq) OR NaHCO₃(aq) [weak base]

• Na: ✅

• NaOH(aq) [strong base]: ✅

• Na₂CO₃(aq) OR NaHCO₃(aq) [weak base]: ❌

reactivity of phenols with

• Na

• NaOH(aq) [strong base]

• Na₂CO₃(aq) OR NaHCO₃(aq) [weak base]

• Na: ✅

• NaOH(aq) [strong base]: ✅

• Na₂CO₃(aq) OR NaHCO₃(aq) [weak base]: ✅ (most acidic compared to the other two)

Why are carboxylic acids able to react with sodium carbonate or sodium hydrogencarbonate?

• Based on acid-base equilibrium, equilibrium will always lie to the right side— the side of the weaker acid and weaker base

• Stronger acids are able to protonate the conjugate base of a weaker acid

• carboxylic acids are stronger acids than carbonic acid

• carboxylic acid is able to protonate carbonate ion(-CO₃^2-) or hydrogencarbonate ion(-HCO₂^- )

Explain effect of electron-donating substitutuents on acidity of carboxylic acids

• e- donating groups intensify the negative charge on the carboxylate ion

• destabilising the ion

• carboxylic acid less likely to deprotonate

• decreasing acidity, decreasing Ka, increasing pKa

Explain effect of electron-withdrawing substitutuents on acidity of carboxylic acids. How does electronegativity of halogen in the group affect the acidity?

• e- withdrawing groups disperse the negative charge on the carboxylate ion

• stabilising the ion

• carboxylic acid more likely to deprotonate

• decreasing acidity, decreasing Ka, increasing pKa

• more electronegative the halogen atom in the electron-withdrawing group

• exert stronger inductive effect and disperse the negative charge more

As number of electron-withdrawing group increases, the acidity of carboxylic acid decreases/increases

• as number increases

• negative charge of carboxylate ion is more dispersed

• carboxylate ion is more stable and more likely to deprotonate

• increasing acidity, higher Ka, lower pKa

As the fruther the electron-withdrawing group is to the carborxylate group, the higher/lower the acidity of carboxylic acid

• less indcuctive effect is exerted

• negative charge of carboxylate ion is less dispersed

• carboxylate ion is less stable and less likely to deprotonate

• lower acidity, lower Ka, higher pKa