What Is Phenolic Acid Definition Types Reactions and Applications
Phenolic acids, which are also called s phenol carboxylic acids, are the aromatic acids that hold a carboxyl functional group and a phenolic ring.
Phenolic acids and Flavonoids constitute one of the most widespread groups of plant phenolics. Due to their significance in human health and plants, a deeper understanding of the flavonoid concentration and biological activities can suggest their potential as therapeutic agents, including predicting and monitoring the medicinal herbs' quality as well, would be useful.
Thus, these compounds contain at least one hydroxy group, one carboxyl group, and one aromatic ring. Salicylic acid is also given as one of the simplest phenolic acids.
Structure of Salicylic Acid
The structure of salicylic acid, which is given as a common phenolic acid, can be illustrated as follows.
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Some other important naturally occurring phenolic acids can be given as hydroxybenzoic acids and hydroxycinnamic acids. Hydroxybenzoic acids are derived from the benzoic acid molecules, which are non-phenolic, whereas the Hydroxycinnamic acids are derived from the acid molecules, which are non-phenolic cinnamic.
Occurrences of Phenolic Acid
Phenolic acids are found in several plant species. Their content present in the dried fruits can be high.
Natural phenols in the horse grams (which is Macrotyloma uniflorum) are mostly given as phenolic acids, which are namely p-hydroxybenzoic, 3,4-dihydroxy benzoic, vanillic, p-coumaric, caffeic, ferulic, sinapinic acids, and syringic.
Phenolic acids are also found in the species of mushroom Basidiomycetes. It is a part of the humic substances, which are given as the major organic constituents of soil humus.
Several phenolic acids are also found in human urine.
Uses of Phenolic Acid
Phenolic compounds are the ones among the phytochemicals that are present in cereals that promote health. Despite their antioxidant properties, phenolic compounds receive more attention. The most common types of phenolic compounds found in whole grains are given as flavonoids and phenolic acids.
Phenolic acids are the most abundant compounds found in cereals, and the flavonoids also exist in grains. The total flavonoid and phenolic content of 20 genotypes of very small grain cereals, including durum wheat, bread wheat, rye, hulless oat, and hull-less barley, were analyzed. These phytochemicals consisted of the highest content in hull-less barley, followed by the rye, durum wheat, bread wheat, and hulless oat.
The role of gut microbiota in bio-transformation of the phytochemicals, along with the phenolic acid, is widely accepted, causing the circulation of food-derived metabolites excreted in the urine.
Medicinal Uses of Phenol
Phenol was also used for skin cleaning and itching reducing. Phenol is also used in drugs such as Chloraseptic to treat pharyngitis as an oral analgesic or anaesthetic. Phenol and its related compounds are also used in the diagnosis of a surgical ingrown toenail, a procedure called phenolization.
Deciding the Acidic Nature of Phenols
Acidic strength is always directly proportional to the conjugate base's stability, which means the more stable is the conjugate base and more acidic is the compound. And, in phenols, once H+ leaves Phenol, a negative charge is acquired by the Oxygen atom. Hence, there is no problem with that as the Oxygen is much more electronegative, containing a value of 3.5. Therefore, it won't raise any issues if it contains a negative charge on it.
Secondly, the most important thing is given as resonance, as the negative charge on the Oxygen atom and π bonds of the Carbon present in the ring are in conjugation, which is given as the basis of resonance. And, resonance lessens oxygen's negative charge by pulling its negative charge towards itself as in a benzene ring. Also, carbon atoms can effectively hold a negative charge where the negative charge is circulated throughout the ring and on oxygen as well, which lessens the compound's energy adding to its stability.
Natural Occurrences of Phenolic Acids
Let us understand a few of the natural occurrences of the phenolic acids as listed below:
Many of the dried fruits hold phenolic acids.
Phenolic acids are also found in the horse grams.
The Basidiomycetes species of the mushroom is much known to hold these organic compounds.
These aromatic acids are also found in the soil humus.
Finally, human urine also comprises some phenolic acids.
Why is Acetic Acid Said to Be Stronger Than Phenol?
Acetic acid (with the chemical form, CH3-CO-OH) is defined as a typical carboxylic acid. In carboxylic acids, the OH hydrogen release as a proton is much facilitated by the electron-withdrawing carbonyl group. There is no such type of feature available in phenol, and it majorly depends on the stability of resultant phenoxide ions to release OH proton.
Therefore, acetic acid is described as a much stronger acid than the other.
Examples of Phenolic Acids
A few of the examples of phenolic acids can be listed as follows:
p-hydroxybenzoic acid (PHBA)
Protocatechuic acid (PCA)
Caffeic acid
Vanillic acid
Ferulic acid
p-Coumaric acid
Sinapinic acid
Syringic acid
FAQs on Phenolic Acid Structure Classification and Chemical Properties
1. What is a phenolic acid in chemistry?
A phenolic acid is an organic compound that contains both a phenol group (–OH attached to an aromatic ring) and a carboxylic acid group (–COOH) in the same molecule. These compounds are classified as aromatic acids because:
- They have a benzene ring (aromatic structure).
- They contain at least one hydroxyl (–OH) group directly bonded to the ring.
- They also contain a carboxyl functional group (–COOH).
2. What is the general structure of a phenolic acid?
The general structure of a phenolic acid consists of an aromatic ring bearing both a –OH (phenolic) group and a –COOH (carboxylic acid) group. Structurally, it can be represented as:
- Ar–OH (phenol part)
- Ar–COOH (aromatic acid part)
3. Why are phenolic acids acidic?
Phenolic acids are acidic because they contain a carboxylic acid group (–COOH) that can donate a proton (H+) in aqueous solution. The acidity arises due to:
- Release of H+ from the –COOH group.
- Stabilization of the resulting carboxylate ion (–COO–) by resonance.
- Additional resonance stabilization from the aromatic ring.
4. What is the difference between phenol and phenolic acid?
The key difference is that phenol contains only a hydroxyl group (–OH) on a benzene ring, while a phenolic acid contains both –OH and –COOH groups on the ring.
- Phenol: Formula C6H5OH; weakly acidic due to phenolic –OH.
- Phenolic acid: Contains –OH and –COOH; stronger acidity due to the carboxylic acid group.
5. What are the types of phenolic acids?
Phenolic acids are mainly classified into hydroxybenzoic acids and hydroxycinnamic acids.
- Hydroxybenzoic acids: Derived from benzoic acid; example: salicylic acid.
- Hydroxycinnamic acids: Contain a three-carbon side chain attached to the benzene ring; example: cinnamic acid derivatives.
6. Can you give an example of a phenolic acid?
A common example of a phenolic acid is salicylic acid (C7H6O3). Its structure includes:
- A benzene ring.
- One hydroxyl group (–OH).
- One carboxylic acid group (–COOH).
7. How do phenolic acids react with bases?
Phenolic acids react with bases to form a salt and water through a neutralization reaction. For example, salicylic acid reacts with sodium hydroxide as follows:
- C7H6O3(aq) + NaOH(aq) → C7H5O3Na(aq) + H2O(l)
8. Are phenolic acids stronger than carboxylic acids?
Phenolic acids are generally comparable in strength to simple aromatic carboxylic acids, but their acidity mainly depends on the –COOH group rather than the phenolic –OH.
- The –COOH group provides primary acidity.
- Electron-withdrawing substituents increase acidity.
- The phenolic –OH contributes only weak acidity.
9. What are the uses of phenolic acids?
Phenolic acids are used in pharmaceuticals, food chemistry, and antioxidants applications. Their main uses include:
- Manufacture of drugs such as aspirin (from salicylic acid).
- Natural antioxidants in plants and foods.
- Preservatives and antimicrobial agents.
10. How are phenolic acids formed?
Phenolic acids are typically formed by introducing a carboxyl group (–COOH) onto a phenolic aromatic ring through chemical reactions such as carboxylation. One general laboratory method involves:
- Reaction of a phenoxide ion with carbon dioxide (Kolbe–Schmitt reaction).
- Acidification to produce the phenolic acid.
