Why are Carboxylic Acids Stronger Acid than Phenol, Alcohol, and Water, but Weaker than Mineral Acids?

Is carboxylic acid a strong acid? In the field of chemistry, the research on acidic substances has never stopped. In chemistry, acids are compounds that can release hydrogen ions, they play important roles in both scientific research and daily life. The strength of acid is a fundamental but crucial factor, that relates to substances’ chemical properties and applications.

The strength of acidity refers to the degree of dissociation in aqueous solution. Strong acids can completely dissociate in water, and they can release all hydrogen ions. While weak acids can only release some hydrogen ions. When we talk about carboxylic acids, we usually refer to organic compounds with one or more carboxyl groups (-COOH). Is carboxylic acid a strong acid? This article will explore this issue from some aspects.

Carboxylic acid

Carboxylic acids are kinds of organic compounds, that contain at least one carboxyl group (-COOH) in their molecules. Carboxylic acid’s general formula is R-COOH, the R refers to hydrocarbon or hydrogen atom. Carboxylic acid is a kind of organic compound with carbon-oxygen double bonds and carbon hydroxyl groups. Carboxyl groups are composed of one carbon atom, two oxygen atoms, and one hydrogen atom. They contain carbon-oxygen double bonds (C=O) and hydroxyl groups (-OH).

Carboxylic acids’ acidity is from the hydroxyl group (-OH) in the carboxyl group, they can release protons and transform into carboxyl ions (-COO;). Biomolecules such as amino acids and fatty acids belong to carboxylic acids, they are highly common. Carboxylic acids can release protons in water, and then form carboxylate ions (RCOO⁻) and hydrogen ions (H⁺). That is their acidic reaction. Because of its unique chemical properties, carboxylic acids have a wide range of applications in medicine and daily life.

Acidity strength

In chemistry, acids are defined as substances that can release hydrogen ions (H⁺). The strength of acidity is mainly dependent on their dissociation ability to produce hydrogen ions in water. Generally speaking, strong acids can fully ionize in water, they can release a large amount of hydrogen ions (H⁺). However, weak acids only partially ionize in water, they can only release fewer hydrogen ions. Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid are common strong acids. Weak acids such as acetic acid and phenol, have an ionization degree of less than 100% in water.

Carboxylic acid is a stronger acid than phenol because it has a similar resonance structure and maximum charge dispersion, which makes it stronger than phenol. The acidity of phenol is also acceptable because when it loses an H⁺ ion, it forms a stable phenoxy ion (resonance stable). They are all acids, but compared to inorganic acids, they are weak acids. Compared to inorganic acids, all organic acids are weak.

Acidity of carboxylic acids

Is carboxylic acid a strong acid? Carboxylic acids are usually considered to be weak acids. This is associated with carboxylic acids’ molecular structure. The hydrogen bond formed between the oxygen atom and the hydrogen atom in the carboxyl group is relatively strong, they can not easily be broken. Therefore, that allows carboxylic acids to only partially release hydrogen ions. A relatively stable internal hydrogen bond is formed between the carbonyl group (C=O) and hydroxyl group (-OH) within the carboxylic acid molecule. Because of that, carboxylic acids’ dissociation ability is further reduced. Carboxylic acid ions’ stability means that the reaction tends to remain in an undissociated carboxylic acid form. thereby exhibiting weaker acidity.

The oxygen atom’s lone pair of electrons resonate, and the oxygen atom connected to hydrogen generates a positive charge. The oxygen atom with a positive charge attracts the bond pair to itself, which makes protons’ removal easier. We know that acids are stronger when stabilized by conjugated bases. In benzene oxide ions, the negative charge is only located on one oxygen atom. In carboxylate ions, the negative charge is dispersed on two oxygen atoms, which are more stable through resonance. Therefore, carboxylic acids are stronger than phenol. Phenol is more acidic than alcohol because it forms phenoxy ions that are stabilized through resonance. Whether water is acidic, alkaline, or neutral mainly depends on the solution it exists in.

Conclusion

Is carboxylic acid a strong acid? The answer is "no", carboxylic acids are weak acids. Their molecular structure determines that they are partially ionized in water. A deeper understanding of carboxylic acids can help us better apply these types of compounds.