Aromatic Carboxylic Acids

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.