Why Carboxylic Acids Are Acidic Resonance Inductive Effect and pKa
Carboxylic acid can be described as an organic acid containing a carboxyl group (C(=O)OH) attached to an R-group. The carboxylic acid's general formula can be given as R–COOH, where R refers to the alkyl group.
Also, carboxylic acids occur widely. A few of the important examples are fatty acids and amino acids. The deprotonation of a carboxylic acid produces a carboxylate anion.
Industrially, several carboxylic acids are produced on a large scale.
They are found frequently in nature. Polyamides of amino carboxylic acids are the primary components of proteins, and the esters of fatty acids are the primary components of lipids.
Carboxylic Acids - Acidity
The carboxylic acid can be described as an organic compound that contains a carboxyl group (COOH), which is attached either to an aryl or alkyl group. They react with alkalis and metals to generate carboxylate ions. These carboxylic acid reactions indicate their acid nature.
Contrarily, the acidity of carboxylic acids is higher when compared to the simple phenols because they react with weak bases such as bicarbonates and carbonates to liberate CO2 gas. The naming of Carboxylic Acid takes place when a substance donates a proton; in general, hydrogen to other things. Carboxylic acids are said to be acidic in nature due to the reason hydrogen belongs to the -COOH group.
2R - COOH + 2Na → 2R-COŌNa⁺ + H2
R - COOH + NaOH → 2R-COŌNa⁺ + H2O
R - COOH + NaHCO₃ → 2R-COŌNa⁺ +H2O + CO2
Acidity of Carboxylic Acids and Its Derivatives
Carboxylic acids can dissociate in water to produce hydronium and carboxylate ions. The carboxylate ion, which is formed, will stabilise through the resonance by an effective delocalisation of the negative charge.
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Carboxylic acids are weaker compared to mineral acids, but they are strongest among the organic compounds. And, the acidity of a carboxylic acid is higher compared to alcohols and even phenols. Also, carboxylic acid is more acidic than phenols.
As discussed, carboxylate ion, which is the conjugate base of a carboxylic acid, is stabilised with the help of two equivalent resonance structures. At the same time, the negative charge is delocalised effectively between the two more electronegative oxygen atoms.
Besides, in the case of phenols, a negative charge is delocalized with less electronegative carbon atoms in phenoxide ion and less effective over one oxygen atom. Thus, the carboxylate ion exhibits higher stability than that of phenoxide ion.
Thus, carboxylic acids are more acidic than phenols. When carboxylic acids react with metals and the alkalis, they produce carboxylate ions, which only stabilise because of the resonance. A simple way to understand carboxyl groups is by understanding that electrons withdrawal leads to the carboxyl group's increased acidity, whereas an electron donation leads to the decrease of the carboxyl group's acidity.
The carboxylic acid's acidity further depends on the substituent aryl or alkyl group's nature, which is attached to the carboxyl group. An electron-withdrawing group ensures the effective negative charge delocalization via inductive or resonance effect. Therefore, the electron-withdrawing groups increase the stability of the conjugate base that is formed.
Contrarily, the electron-donating groups destabilise the conjugate base that is formed and thereby decrease the acidity of the carboxylic acid. The general trend of acidic strength of carboxylic acid or the order of acidity of carboxylic acids can be represented as follows.
CF3COOH > CCl3COOH > CHCl3COOH >NO2COOH > NC- CH2COOH
We can also call it the order of acidic strength of carboxylic acids. Moreover, because of the resonance effect, vinyl or phenyl groups increase the carboxylic acids’ acidity rather than decreasing it. This is because of the inductive effect.
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The Acidity of Carboxylic Acid
Some of the carboxylic acids with their pKa value have been mentioned in the table below. Thus, when we compare them to that of alcohols like ethanol (pKa = 16) and 2-methyl-2-propanol (pKa = 19), it becomes clear that the acidity of the carboxylic acid is stronger than acid by ten powers of ten. Furthermore, the electronegative substitutes also contribute to increasing the acidity when substituted near the carboxyl group.
It is important to know why the presence of the carbonyl group adjacent to a hydroxyl group has such a profound effect on the acidity of the hydroxyl proton. For this, we must take a look into the nature of the acid-base equilibrium and the definition of pKa that is illustrated by the general equation that has been described below.
H-A + H2O ⇄ H3O+ + A-
Therefore, Keq = \[\frac {[H_{3}O^+][A^-]}{[HA][H_2O]}\]
Keq = \[\frac {[H_{3}O^+][A^-]}{[HA]}\] pKa = - logKa = \[log(\frac{1}{k_a})\]
As the equilibrium always prefers the thermodynamically stable side and the magnitude of the equilibrium shows the difference between energies of the components on each side of an equation. Equilibrium is always seen to be favouring the side of the weaker acid and the base in an acid-base reaction. Water is constantly used for the measurement as a standard base used for pKa measurements, anything that stabilises the conjugate base (A-) of an acid. This in turn will make the acid stronger and the equilibrium will necessarily shift to the right. The resonance thus stabilises the carboxyl group and the carboxylate anion. But the stabilisation of anion becomes much greater than that of the neutral function as shown in the equation below. The two contributing structures have equal weight in the hybrid, and the C–O bonds are of equal length in a carboxylate anion that is between the single bond and the double bond. This results in a marked increase in the acidity.
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Nomenclature and Examples
Commonly, carboxylic acids are identified using their trivial names. They often contain the suffix -ic acid. There also exist the recommended IUPAC names; in this system, carboxylic acids contain an -oic acid suffix.
For example, the IUPAC name of the butyric acid (C3H7CO2H) is butanoic acid, according to the IUPAC guidelines. For the nomenclature of complex molecules that contain carboxylic acid, carboxyl is considered as one of the parent chains even if there exist other substituents, like 3-chloro propanoic acid. In an alternate way, it is named either as a "carboxylic acid" or "carboxy" substituent on another parent structure, like 2-carboxy furan.
Usually, the carboxylate anion (RCO2 or R–COO−) of a carboxylic acid is named with a suffix -ate, with the normal pattern of -ic acid and -ate for the conjugate acid and the conjugate base of it, respectively. As an example, the conjugate base of acetic acid is given as acetate.
Carbonic acid, which takes place in bicarbonate buffer systems in nature, is not classed as one of the carboxylic acids generally, despite that it contains a moiety that is like a COOH group.
Applications of Carboxylic Acids
Carboxylic acids can be used in the production of pharmaceuticals, polymers, food additives, and solvents.
The important industrial carboxylic acids are given as follows.
Acrylic & methacrylic acids (precursors to adhesives, polymers),
Acetic acid (a component of vinegar, a precursor to coatings and solvents),
Citric acid (a preservative and flavour in food & beverages),
Adipic acid (polymers),
Maleic acid (polymers),
Fatty acids (coatings),
Terephthalic acid (polymers)
Propionic acid (food preservatives).
FAQs on Acidity of Carboxylic Acids Explained for Exams
1. What makes carboxylic acids acidic?
Carboxylic acids are acidic because they can donate a proton (H+) from the –COOH (carboxyl) group to form a resonance-stabilized carboxylate ion (R–COO−).
- The O–H bond in the carboxyl group is polar, making H+ easy to release.
- After deprotonation, the negative charge is delocalized over two oxygen atoms by resonance.
- This stabilization increases the acidity of carboxylic acids compared to alcohols.
2. Why are carboxylic acids more acidic than alcohols?
Carboxylic acids are more acidic than alcohols because their conjugate base is stabilized by resonance, while alkoxide ions are not.
- In carboxylate ions, the negative charge is shared between two oxygen atoms.
- In alcohols (ROH), the alkoxide ion (RO−) holds the charge on only one oxygen atom.
- This greater stability lowers the pKa of carboxylic acids (around 4–5) compared to alcohols (around 16–18).
3. What is the pKa value of carboxylic acids?
Most simple carboxylic acids have a pKa value between 4 and 5, indicating they are weak acids.
- Example: Acetic acid (CH3COOH) has pKa ≈ 4.76.
- Lower pKa means stronger acid.
- Electron-withdrawing groups can decrease the pKa further.
4. How do electron-withdrawing groups affect the acidity of carboxylic acids?
Electron-withdrawing groups increase the acidity of carboxylic acids by stabilizing the negative charge on the carboxylate ion through the −I (inductive) effect.
- They pull electron density away from the –COO− group.
- This stabilizes the conjugate base and lowers the pKa.
- Example: Trichloroacetic acid (CCl3COOH) is stronger than acetic acid due to three chlorine atoms.
5. How do electron-donating groups affect the acidity of carboxylic acids?
Electron-donating groups decrease the acidity of carboxylic acids by destabilizing the carboxylate ion.
- They push electron density toward the –COO− group.
- This increases electron–electron repulsion in the conjugate base.
- As a result, the pKa increases and acidity decreases.
6. What is the order of acidity of substituted carboxylic acids?
The acidity of substituted carboxylic acids increases with stronger and more numerous electron-withdrawing groups, especially when they are closer to the –COOH group.
- Greater electronegativity → stronger −I effect → stronger acid.
- Substituents at the α-carbon have the greatest effect.
7. How does resonance stabilize the carboxylate ion?
Resonance stabilizes the carboxylate ion by delocalizing the negative charge equally over two oxygen atoms.
- The ion has two equivalent resonance structures.
- Both C–O bonds become equal in length.
- This delocalization lowers the energy of the conjugate base.
8. Are carboxylic acids strong or weak acids?
Carboxylic acids are classified as weak acids because they partially ionize in aqueous solution.
- They establish an equilibrium in water.
- Example: CH3COOH(aq) + H2O(l) ⇌ CH3COO−(aq) + H3O+(aq).
- The equilibrium constant is expressed as Ka.
9. How does the position of a substituent affect the acidity of carboxylic acids?
The closer an electron-withdrawing substituent is to the –COOH group, the greater its effect on acidity.
- Substituents at the α-carbon show the strongest inductive effect.
- The effect decreases with distance along the carbon chain.
- This is due to weakening of the inductive effect over distance.
10. Can you give an example of a reaction that shows the acidity of carboxylic acids?
Carboxylic acids react with bases to form a salt and water, demonstrating their acidic nature.
- Example neutralization reaction:
- They also react with carbonates to release CO2 gas:
