Why Are Carboxylic Acids More Acidic? Factors & Applications
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 Carboxylic Acids Acidity Explained: Key Trends & Examples
1. What makes carboxylic acids acidic in nature?
Carboxylic acids are acidic because the hydrogen atom in the carboxyl group (–COOH) can be released as a proton (H⁺). This is due to the high polarity of the O-H bond and the resonance stabilisation of the resulting carboxylate anion (RCOO⁻). The negative charge is delocalised over both oxygen atoms, making the conjugate base very stable and favouring the donation of the proton.
2. Why are carboxylic acids generally stronger acids than phenols?
Carboxylic acids are stronger acids than phenols because their conjugate base, the carboxylate ion (RCOO⁻), is more stable than the phenoxide ion. In the carboxylate ion, the negative charge is delocalised over two highly electronegative oxygen atoms. In the phenoxide ion, the negative charge is delocalised over one oxygen atom and the less electronegative carbon atoms of the benzene ring. This greater and more effective delocalisation in the carboxylate ion makes it a more stable conjugate base, thus making the parent carboxylic acid a stronger acid.
3. How do different substituents attached to the alkyl chain affect the acidity of carboxylic acids?
Substituents significantly influence the acidity of carboxylic acids by altering the stability of the carboxylate anion. The main effects are:
- Electron-withdrawing groups (EWGs): Groups like -NO₂, -CN, and halogens (-F, -Cl, -Br) pull electron density away from the carboxyl group. This disperses the negative charge on the carboxylate ion, increasing its stability and thereby increasing the acidity of the acid. For example, trichloroacetic acid is much stronger than acetic acid.
- Electron-donating groups (EDGs): Groups like alkyl groups (-CH₃, -C₂H₅) push electron density towards the carboxyl group. This intensifies the negative charge on the carboxylate ion, decreasing its stability and thereby decreasing the acidity of the acid.
4. Why is formic acid (HCOOH) a stronger acid than acetic acid (CH₃COOH)?
Formic acid is stronger than acetic acid due to the electronic effect of the substituent attached to the carboxyl group. In acetic acid, the methyl group (-CH₃) is an electron-donating group (+I effect). It pushes electron density towards the carboxylate ion, destabilising it and making the acid weaker. In formic acid, the hydrogen atom has no significant electronic effect. Since there is no destabilisation from an electron-donating group, formic acid is more willing to donate its proton and is therefore a stronger acid.
5. What is the general trend for the solubility of carboxylic acids in water?
Carboxylic acids with up to four carbon atoms are generally miscible with water. This is because the polar carboxyl group can form strong hydrogen bonds with water molecules. However, as the length of the nonpolar alkyl chain increases, the hydrophobic character of the molecule dominates. This disrupts hydrogen bonding, causing a sharp decrease in solubility for larger carboxylic acids.
6. Why are carboxylic acids considered weak acids despite being more acidic than alcohols?
Carboxylic acids are classified as weak acids because they only partially dissociate in an aqueous solution. While they are significantly more acidic than alcohols (due to the resonance-stabilised carboxylate ion), they are much less acidic than mineral acids like HCl or H₂SO₄, which dissociate completely. For example, in a 1 M solution, only a small fraction of acetic acid molecules will donate their proton at any given time.
7. How does the sodium bicarbonate test demonstrate the acidity of carboxylic acids?
The sodium bicarbonate test is a characteristic test for the acidity of carboxylic acids. When a carboxylic acid (RCOOH) is added to a sodium bicarbonate (NaHCO₃) solution, it acts as an acid and donates a proton. This reaction forms a sodium carboxylate salt, water, and carbonic acid (H₂CO₃). Carbonic acid is unstable and immediately decomposes into water and carbon dioxide gas (CO₂), which is observed as brisk effervescence. This reaction is strong enough to occur with carboxylic acids but not with less acidic compounds like phenols or alcohols, making it a reliable identification test.
