Acidity of Carboxylic Acids: Effect of EWG and EDG, Alcohol Vs Phenol

Acidity of Carboxylic Acids: What actually is vinegar? What is it like to eat? What is the origin of its flavour? Vinegar is an aqueous solution of ethanoic acid or acetic acid. It has a sour taste. The reason is the presence of H+ ions. What is the name of the ethanoic acid functional group? The functional group carboxylic acid (–COOH) is found in ethanol.

This post will teach you about carboxylic acid acidity and the factors that influence it, such as substituents and halogens. In terms of acidity, you’ll also compare carboxylic acid to alcohol and phenol.

What are Carboxylic Acids?

Carbon compounds that contain carboxyl group \(\left( {{\rm{ – COOH}}} \right)\) are called carboxylic acids. The carboxyl group contains a carbonyl group \(( > {\rm{C}} = {\rm{O}})\) attached to a hydroxyl group \(\left( {{\rm{ – OH}}} \right){\rm{,}}\) and hence it is named carboxyl.

Carboxylic acids may be classified as aliphatic or aromatic depending upon the nature of the group to which the \({{\rm{ – COOH}}}\) group is attached. When the \({{\rm{ – COOH}}}\) group is attached to an \({\rm{H}}\) atom or to an alkyl group, the carboxylic acid is called an aliphatic carboxylic acid. They are represented by the general formula \({\rm{RCOOH}}{\rm{.}}\)

Example: Methanoic acid (Formic acid)\({\rm{ – HCOOH,}}\) Ethanoic acid (Acetic acid)\({\rm{ – C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH,}}\) etc.

When the \({\rm{ – COOH}}\) group is attached to an aryl group, the carboxylic acid is called an aromatic carboxylic acid. They are represented by the general formula \({\rm{ArCOOH}}{\rm{.}}\)

Example: Benzoic acid \({\rm{ – }}{{\rm{C}}_6}{{\rm{H}}_5}{\rm{COOH,}}\) toluic acid \({\rm{ – C}}{{\rm{H}}_3}{{\rm{C}}_6}{{\rm{H}}_4}{\rm{COOH,}}\) etc.

Acidic Nature of Carboxylic Acid

Carboxylic acids are acidic in nature. These are more acidic than water, alcohol, and phenols but less acidic than mineral acids like sulphuric acid \(\left( {{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} \right),\) hydrochloric acid \(\left( {{\rm{HCl}}} \right){\rm{,}}\) nitric acid \(\left( {{\rm{HN}}{{\rm{O}}_{\rm{3}}}} \right){\rm{,}}\) etc.

In aqueous solutions, carboxylic acids undergo ionization to form carboxylate anion \(\left( {{\rm{RCO}}{{\rm{O}}^{\rm{ – }}}} \right)\) and hydronium ion \(\left( {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}} \right)\) In such solutions, these ions exist in equilibrium with the molecular form of the acid.

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The carboxylate ion thus formed shows the following resonance hybrid structures.

Ionisation constant/dissociation constant/acidity constant of an acid

When an acid is dissolved in water, it ionizes as-

\({{\rm{K}}_{{\rm{eq}}}}{\rm{ = }}\frac{{\left[ {{\rm{RCO}}{{\rm{O}}^{\rm{ – }}}} \right]\left[ {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right]}}{{\left[ {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right][{\rm{RCOOH}}]}}\)

\({{\rm{K}}_{\rm{a}}}{\rm{ = }}{{\rm{K}}_{{\rm{eq}}}}\left[ {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right]{\rm{ = }}\frac{{\left[ {{\rm{RCO}}{{\rm{O}}^ – }} \right]\left[ {{{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }} \right]}}{{[{\rm{RCOOH}}]}}\)

Where \({{\rm{K}}_{\rm{eq}}}\) is the equilibrium constant, and \({{\rm{K}}_{\rm{a}}}\) is the ionization constant of the acid or dissociation constant of the acid.

The dissociation constant is generally called the acidity constant because it measures the relative strength of an acid.The stronger the acid, the higher the value of \({{\rm{K}}_{\rm{a}}}.\)

The dissociation constant of acid can also be expressed in terms of \({\rm{p}}{{\rm{K}}_{\rm{a}}},\) which is defined as

\({\rm{p}}{{\rm{K}}_{\rm{a}}}{\rm{ = \, – log}}{{\rm{K}}_{\rm{a}}}\)

A stronger acid will have a higher \({{\rm{K}}_{\rm{a}}}\) value but a smaller \({\rm{p}}{{\rm{K}}_{\rm{a}}},\) while a weaker acid will have a smaller \({{\rm{K}}_{\rm{a}}}\) value but a higher \({\rm{p}}{{\rm{K}}_{\rm{a}}}\) value. In simple words, the larger the value of \({\rm{p}}{{\rm{K}}_{\rm{a}}},\) the weaker is the acid and vice versa.

Effect of Substituents on Acidic Strength of Acids

The nature of the substitute affects the stability of the carboxylate ion and hence affects the acidity of carboxylic acids.

Electron donating or releasing substituents (EDG): Generally, electron-donating groups decrease the stability of the carboxylate ion by intensifying the negative charge relative to the formate ion. Alkyl is an electron releasing group. If the H atom of formic acid is replaced by \({{\rm{C}}{{\rm{H}}_3}}\) group to form acetic acid \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{COOH}}} \right),\) the alkyl group will increase the electron density on the oxygen atom of the \({\rm{O – H}}\) bond. Hence, the release of \({{{\rm{H}}^ + }}\) ions in acetic acid is more difficult as compared to formic acid. On the other hand, the methyl group will also destabilize the acetate ion by intensifying the negative charge relative to the formate ion.

In general, the greater the \({\rm{ + I}}\) effect (Inductive effect) of the alkyl group attached to the carboxyl group, the lesser will be the acidic strength of the carboxylic acid. The \({\rm{ + I}}\) effect of the alkyl groups increases in the order:

\({\rm{C}}{{\rm{H}}_3} < {{\rm{C}}_2}{{\rm{H}}_5} < {\left( {{\rm{C}}{{\rm{H}}_3}} \right)_2}{\rm{CH}} < {\left( {{\rm{C}}{{\rm{H}}_3}} \right)_3}{\rm{C}}\)

The decreasing acidic strength of few acids are as follows:

\({\rm{HCOOH}}({\rm{Formic}}\,{\rm{acid}}) > {\rm{C}}{{\rm{H}}_3}{\rm{COOH}}({\rm{Acetic}}{\mkern 1mu} \,{\rm{acid}}) > {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{COOH}}({\rm{Propionic}}\,{\rm{acid}})\)

\( > {\left( {{\rm{C}}{{\rm{H}}_3}} \right)_2}{\rm{CHCOOH}}({\rm{Isobutyric}}\,{\mkern 1mu} {\rm{acid}}) > {\left( {{\rm{C}}{{\rm{H}}_3}} \right)_3}{\rm{CCOOH}}(2,2 – {\rm{Dimethylpropanoic}}{\mkern 1mu} \,{\rm{acid}})\)

Electron withdrawing substituents (EWG): In general, electron withdrawing substituents increase the stability of the carboxylate ion by decreasing the negative charge density and hence increase the acidity of the carboxylic acid.  

The electron withdrawing groups such as the halogen atom withdraw the electrons from the carbon to which it is attached, and this effect is transmitted throughout the chain. As a result, the electrons are withdrawn more strongly towards oxygen of \({\rm{O – H}}\) bond and promotes the release of the proton, and results in an increase in acidic strength. Therefore, chloroacetic acid is more acidic than acetic acid.

Because of electron withdrawing tendency of \({\rm{Cl}}\) atom (\({\rm{ – I}}\) effect), it stabilizes the carboxylate anion by dispersing the negative charge and, therefore, strengthening the acid.

Effect of the number of halogen atoms: The inductive effect increases with an increase in the number of chlorine atoms, and therefore, acidic strength also increases.

Thus, the acidic strength decreases in the order:

\({\rm{CC}}{{\rm{l}}_3}{\rm{COOH}} > {\rm{CHC}}{{\rm{l}}_2}{\rm{COOH}} > {\rm{C}}{{\rm{H}}_2}{\rm{ClCOOH}} > {\rm{C}}{{\rm{H}}_3}{\rm{COOH}}\)

Position of the halogen atom: As the distance between the electron withdrawing group like halogen and the \({\rm{ – COOH}}\) group increases, the electron withdrawing influence decreases.  Beyond a few(generally three) methylene groups, the effect becomes negligible.

Acidity of Aromatic Carboxylic Acid: Benzoic Acid

Benzoic acid is more acidic than acetic acid because in benzoic acid, \({\rm{ – COOH}}\) is attached to \({\rm{s}}{{\rm{p}}^2}\) hybridized \({\rm{C}}\) atom of the phenyl ring while in the acetic acid -\({\rm{COOH}}\) is attached to \({\rm{s}}{{\rm{p}}^3}\) hybridized \({\rm{C}}\) atom of the methyl group. 

However, formic acid does not contain any alkyl group, and therefore, it is a stronger acid than benzoic acid. Thus, the relative acid strength of these three acids is:

Formic acid\(>\)Benzoic acid\(>\)Acetic acid
The electron is releasing groups like \( – {\rm{C}}{{\rm{H}}_3}, – {\rm{OH}}, – {\rm{OC}}{{\rm{H}}_3}\) and \( – {\rm{N}}{{\rm{H}}_2}\) decreases the acidity of benzoic acid while electron withdrawing groups like \( – {\rm{Cl}}, – {\rm{N}}{{\rm{O}}_2},\) etc., make benzoic acid stronger.

The ortho isomer of every substituted benzoic acid is the strongest acid among ortho, para and meta isomers, no matter whether the substituent is electron withdrawing or electron donating.

Among \({\rm{p – }}\) and \({\rm{m – }}\) isomers, \({\rm{p – }}\) isomer has a more acidic character than m-isomer for electron withdrawing groups by \({\rm{ – M}}\) effect. For electron releasing groups like \({\rm{ – OH}}\) group, \({\rm{m – }}\) isomer is a stronger acid than benzoic acid while \({\rm{p – }}\) isomer is weaker than benzoic acid.

Comparison of Acidity of Carboxylic Acid and Alcohol

Monocarboxylic acids are stronger acids than monohydric alcohols. This is due to the fact that both the carboxylic acids and the carboxylate ions are resonance stabilized, and the carboxylate is more resonance stabilized in comparison to carboxylic acid. Therefore, carboxylic acids have a tendency to give \({{\rm{H}}^{\rm{ + }}}\) forming stable carboxylate ions. However, in alcohol, no resonance is possible for the alcohol as well as alkoxide ions. Therefore, alcohols are very weak acids.

Comparison of Acidity of Carboxylic Acid and Phenol

Phenols are acidic because the phenoxide ion is stabilized by the delocalization of the negative charge into the benzene ring. However, phenols are less acidic (\({\rm{p}}{{\rm{K}}_{\rm{a}}}\) is about \(16\)) than carboxylic acids.

In resonance structure, the electron charge in the carboxylate ion is more dispersed in comparison to the phenoxide ion since there are two electronegative oxygen atoms in the carboxylate ion as compared to only one oxygen atom in the phenoxide ion. The carboxylate ion is relatively more stable as compared to the phenoxide ion. Thus, the release of \({{\rm{H}}^{\rm{ + }}}\) ion from carboxylic acid is comparatively easier and hence carboxylic acid is a stronger acid than phenol.

Summary

This article taught you the definition of carboxylic acid. Carboxylic acid acidity, as well as the effects of electron-withdrawing and electron-releasing groups on acidity. It is possible to compare the acidity of aromatic and aliphatic carboxylic acids. Using the information on this page, you may compare the acidity of carboxylic acid, alcohol, and phenol.

FAQs on Acidity of Carboxylic Acids

Q.1. Out of Formic acid, acetic acid and benzoic acid, Which carboxylic acid is most acidic?
Ans: Formic acid (methanoic acid, \({\rm{HCOOH}}\)) is the most acidic.

Q.2. How do you determine the acidity of carboxylic acids?
Ans: The acidity of the carboxylic acid is measured by the dissociation constant (acidity constant) of an acid. It is expressed in terms of \({\rm{p}}{{\rm{K}}_{{\rm{a,}}}}\) which is defined as
\({\rm{p}}{{\rm{K}}_{\rm{a}}}{\rm{ = \, – log}}{{\rm{K}}_{\rm{a}}}\)
A stronger acid will have a higher \({{\rm{K}}_{\rm{a}}}\) value but a smaller \({\rm{p}}{{\rm{K}}_{{\rm{a}}}}\) value, while a weaker acid will have a smaller \({{\rm{K}}_{\rm{a}}}\) value but a higher \({\rm{p}}{{\rm{K}}_{{\rm{a,}}}}\) value. In simple words, the larger the value of \({\rm{p}}{{\rm{K}}_{{\rm{a,}}}}\) the weaker is the acid and vice versa.

Q.3. Why are carboxylic acids so acidic?
Ans: In aqueous solutions, carboxylic acids undergo ionization to form carboxylate anion \(({\rm{RCO}}{{\rm{O}}^ – })\) and hydronium ion \(\left( {{{\rm{H}}_3}{{\rm{O}}^ + }} \right)\)

The carboxylate ion is resonance stabilized. Hence carboxylic acids are so acidic.

Q.4. Which is the most acidic? \({\rm{C}}{{\rm{H}}_3}{\rm{COOH}},{\rm{CHC}}{{\rm{l}}_2}{\rm{COOH}},{\rm{CC}}{{\rm{l}}_3}{\rm{COOH}}.\)
Ans: As the number of halogens (electron withdrawing group) increases acidity increases. Hence \({\rm{CC}}{{\rm{l}}_3}{\rm{COOH}}\) is more acidic.

Q.5. Which is more acidic among alcohols and carboxylic acid?
Ans: Carboxylic acids and the carboxylate ions are resonance stabilized. Therefore, carboxylic acids have a tendency to give \({{\rm{H}}^{\rm{ + }}}\) forming stable carboxylate ions. However, in alcohols, no resonance is possible for the alcohols as well as alkoxide ions. Therefore, carboxylic acids are stronger acids than alcohol.

Q.6. Which is more acidic, alcohol or phenol?
Ans: Phenol is more acidic than alcohol since phenoxide ion is more stabilized than alkoxide ion due to the resonance effect.

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