What Are Aromatic Carboxylic Acids Definition Structure Types and Examples
Aromatic acids are compounds that have a COOH group that is bonded to an aromatic ring. One of the simplest aromatic acids is given as benzoic acid.
About Aromatic Compounds
Aromatic carboxylic acids exhibit not only the acidity and the other reactions expected of carboxylic acids (as an acid, benzoic acid is slightly stronger compared to the acetic acid) but, also, the same as the other aromatic compounds, undergo electrophilic substitution reactions.
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The COOH group is the deactivating, which means that electrophilic substitutions occur less readily than with benzene itself (Friedel-Crafts reactions do not take place), and meta-directing, which means that the incoming entity will enter at a meta-position to the COOH group, rather than either at an ortho or para position.
Example of Aromatic Acids
Nitration of Benzoic Acid
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Benzoic acid, at a solid at room temperature (with a melting point 122 °C [252 °F]), was first described in 1560, which has been prepared by the distilling gum benzoin - a resin that is obtained from certain Asian trees. It takes place in different plants, both in ester form and free acid form. Also, it is a constituent of the urine of certain animals, specifically the horses, as an amide of glycine, known as hippuric acid, C6H5CONHCH2COOH. The sodium benzoate, sodium salt, is used as a preservative in several foods.
Important Aromatic Acids
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Salicylic acid is both phenol and carboxylic acid, so it may be esterified in two ways, with both giving rise to familiar products. In methyl salicylate (the wintergreen oil), the COOH group of the salicylic acid can be esterified with methanol (CH3OH). Whereas, in the acetylsalicylic acid (otherwise aspirin), the ester’s acid component is acetic acid, and the salicylic acid contributes phenolic - OH group.
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Gallic Acid
Gallic acid may be found in tea and other plants as well, and it also takes place as the part of a larger molecule, known as tannin, that is present in galls (like the tissue’s swellings of oak trees caused by the wasps’ attack). Tannins can be used in making leather, and gallic acid is employed in ink production.
Hydroxy and Keto Acids
All the hydroxycarboxylic of type 2-, 3-, 4-, and 5- acids lose water upon heating, although these products are not similar. The 2-hydroxy acids produce cyclic dimeric esters (which are formed by two molecules’ esterification of the acid) known as lactides, whereas the 3- and 4-hydroxy acids undergo intramolecular esterification to produce cyclic esters known as lactones.
These specific reactions occur so readily, even without heating, that in the majority of the cases, the only way to keep these hydroxy acid kinds from forming cyclic esters is to convert them to their salts of either potassium or sodium salts. 2-Hydroxy acids lose the water upon heating to yield the unsaturated acids of both α and β.
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The simplest hydroxy acids, lactic and glycolic, occur in nature.
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Lactic acid can be formed when milk turns sour (thus the name, from Latin lactis, “milk”) and was first isolated in 1780 from sour milk by a Swedish chemist named Carl Wilhelm Scheele. It takes place in plants also.
Amino Acids
Compounds having both an amino group and a carboxyl group are known as amino acids. Twenty of these compounds are found in proteins, where all are α-amino acids with the below formula:
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Glutamic acid is one of the amino acids that is found in proteins, as well as its sodium salt, monosodium glutamate (MSG), which is often used as a food additive. However, it imparts no flavour of its own, whereas it enhances the flavours of fish, vegetables, and meats. A few people experience an allergic reaction to MSG; commonly, the allergy is called “Chinese restaurant syndrome” because MSG has been a major widely used ingredient in the cuisine of several Chinese restaurants.
Synthesis of Carboxylic Acids
Many methods for the synthesis of carboxylic acids may be put into one of the two categories:
Hydrolysis of acid derivatives and
Oxidation of various compounds.
Hydrolysis of Acid Derivatives
All the acid derivatives may be hydrolyzed (which is cleaved by water) to yield carboxylic acid aromatic acids; the conditions needed range from mild to severe, based on the compound involved.
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The easiest acid derivatives to hydrolyze are called acyl chlorides that need only the addition of water. Carboxylic acid salts can be instantaneously converted to the corresponding acids at room temperature just on the treatment with water and a strong acid like hydrochloric acid (which is H+ in the equations given above).
Oxidation
The oxidation of major alcohols is a general method for the carboxylic acid synthesis: RCH2OH → RCOOH. It needs a strong oxidizing agent, which is the most common being potassium permanganate (KMnO4), nitric acid (HNO3), and chromic acid (H2CrO4). Aldehydes can be oxidized to carboxylic acids very easily (by several oxidizing agents), but often, this is not useful because, usually, the aldehydes are less available than that of corresponding acids.
FAQs on Aromatic Carboxylic Acids Structure Properties and Reactions
1. What are aromatic carboxylic acids?
Aromatic carboxylic acids are organic compounds that contain a carboxyl group (–COOH) directly attached to an aromatic ring, usually benzene.
- General structure: Ar–COOH, where Ar represents an aromatic ring.
- Example: benzoic acid (C6H5COOH).
- The –COOH group shows typical acid behavior, such as donating H+ in aqueous solution.
- They combine properties of aromatic compounds and carboxylic acids.
2. What is the formula and structure of benzoic acid?
The formula of benzoic acid is C6H5COOH, consisting of a benzene ring attached to a carboxyl group.
- Molecular formula: C7H6O2.
- Structural form: a benzene ring (C6H5–) bonded to –COOH.
- The carboxyl carbon is sp2 hybridized and conjugated with the aromatic ring.
- It is the simplest and most common aromatic carboxylic acid.
3. Why are aromatic carboxylic acids acidic?
Aromatic carboxylic acids are acidic because the carboxyl group (–COOH) can donate a proton (H+) and form a resonance-stabilized carboxylate ion (–COO−).
- Ionization example: C6H5COOH(aq) ⇌ C6H5COO−(aq) + H+(aq).
- The negative charge in the carboxylate ion is delocalized over two oxygen atoms.
- The aromatic ring can further stabilize the conjugate base by electron-withdrawing effects (if substituents are present).
4. How are aromatic carboxylic acids prepared in the laboratory?
Aromatic carboxylic acids are commonly prepared by oxidation of alkylbenzene side chains using strong oxidizing agents like alkaline KMnO4.
- Example reaction: C6H5CH3(aq) + 2KMnO4(aq) → C6H5COOK(aq) + 2MnO2(s) + KOH(aq) + H2O(l) (on heating).
- Acidification gives benzoic acid: C6H5COOK(aq) + HCl(aq) → C6H5COOH(s) + KCl(aq).
- Any alkyl group attached to the benzene ring with at least one benzylic hydrogen can be oxidized to –COOH.
5. What is the difference between aromatic and aliphatic carboxylic acids?
The main difference is that aromatic carboxylic acids have the –COOH group attached to an aromatic ring, while aliphatic carboxylic acids have it attached to an alkyl chain.
- Example (aromatic): C6H5COOH (benzoic acid).
- Example (aliphatic): CH3COOH (ethanoic acid).
- Aromatic acids are often slightly stronger due to resonance and inductive effects.
- They undergo electrophilic substitution on the aromatic ring in addition to typical acid reactions.
6. How do aromatic carboxylic acids react with bases?
Aromatic carboxylic acids react with bases to form carboxylate salts and water in a neutralization reaction.
- Example: C6H5COOH(aq) + NaOH(aq) → C6H5COONa(aq) + H2O(l).
- With sodium bicarbonate: C6H5COOH(aq) + NaHCO3(aq) → C6H5COONa(aq) + H2O(l) + CO2(g).
- Effervescence of CO2 confirms the presence of a carboxylic acid group.
7. What happens when aromatic carboxylic acids are heated with soda lime?
When heated with soda lime (NaOH + CaO), aromatic carboxylic acids undergo decarboxylation to form the corresponding aromatic hydrocarbon.
- Example: C6H5COONa(s) + NaOH(s) → C6H6(g) + Na2CO3(s).
- The –COO− group is removed as carbonate.
- Benzoic acid thus gives benzene on decarboxylation.
8. What are substituted aromatic carboxylic acids?
Substituted aromatic carboxylic acids are compounds where additional groups (like –NO2, –OH, –CH3) are attached to the aromatic ring along with the –COOH group.
- Example: o-nitrobenzoic acid (NO2 group on benzene ring).
- Substituents can be electron-withdrawing or electron-donating.
- Electron-withdrawing groups (e.g., –NO2) increase acidity.
- Electron-donating groups (e.g., –CH3) decrease acidity.
9. What are the uses of aromatic carboxylic acids?
Aromatic carboxylic acids are widely used in food preservation, pharmaceuticals, dyes, and polymers.
- Benzoic acid is used as a food preservative.
- Salicylic acid is used in medicines and skincare products.
- Terephthalic acid is used to manufacture PET plastics.
- They serve as intermediates in organic synthesis and industrial chemistry.
10. How does the –COOH group affect electrophilic substitution in aromatic carboxylic acids?
The –COOH group is a strong electron-withdrawing and deactivating group that directs electrophilic substitution to the meta position on the aromatic ring.
- It reduces electron density on the ring via –I and –M effects.
- Nitration of benzoic acid mainly gives m-nitrobenzoic acid.
- The reaction is slower compared to benzene due to deactivation.
