The carboxylic acid is a term ascribed to any organic compound containing a carboxyl group. They can also be referred to as organic acids, and are present in many natural sources. For example, from ants and other insects such as the galerite beetle, formic acid, a carboxylic acid, is distilled.
That is, an anthill is a rich source of formic acid. Also, acetic acid is extracted from vinegar, the smell of rancid butter is due to butyric acid, valerian herbs contain valeric acid and capers are obtained capric acid, all these carboxylic acids.
Lactic acid gives sour milk a bad taste, and fatty acids are present in some fats and oils. Examples of natural sources of carboxylic acids are innumerable, but all their assigned names are derived from Latin words. Thus, in Latin the word Formica means “ant.”
As these acids were extracted in different chapters of history, these names became common, consolidating in popular culture.
The general formula of carboxylic acid is R – COOH, or in more detail: R– (C = O) –OH. The carbon atom is linked to two oxygen atoms, which causes a decrease in its electron density and, consequently, a positive partial charge.
This charge reflects the oxidation state of carbon in an organic compound. In no other is carbon as oxidized as in the case of carboxylic acids, this oxidation being proportional to the degree of reactivity of the compound.
For this reason the –COOH group has predominance over other organic groups, and defines the nature and the main carbon chain of the compound.
Hence, there are no acid derivatives of amines (R-NH 2 ), but amines derived from carboxylic acids (amino acids).
The common names derived from Latin for carboxylic acids do not clarify the structure of the compound, nor its arrangement or the arrangement of the groups of its atoms.
Given the need for these clarifications, the IUPAC systematic nomenclature arises to name carboxylic acids.
This nomenclature is governed by several rules, and some of these are:
To mention a carboxylic acid, the name of its alkane must be modified by adding the suffix “ico”. Thus, for ethane (CH 3 –CH 3 ) its corresponding carboxylic acid is ethanoic acid (CH 3 –COOH, acetic acid, the same as vinegar).
Another example: for CH 3 CH 2 CH 2 –COOH the alkane becomes butane (CH 3 CH 2 CH 2 CH 3 ) and, therefore, butanoic acid is named (butyric acid, the same as rancid butter).
The group –COOH defines the main chain, and the number corresponding to each carbon is counted from the carbonyl.
For example, CH 3 CH 2 CH 2 CH 2 -COOH is pentanoic acid, counting from one to five carbons up to methyl (CH 3 ). If another methyl group is attached to the third carbon, it would be CH 3 CH 2 CH (CH 3 ) CH 2 -COOH, the resulting nomenclature now being: 3-methylpentanoic acid.
Substituents are preceded by the number of the carbon to which they are attached. Also, these substituents can be double or triple bonds, and add the suffix “ico” equally to alkenes and alkynes. For example, CH 3 CH 2 CH 2 CH = CHCH 2 -COOH is referred to as (cis or trans) 3-heptenoic acid.
When the chain R consists of a ring (φ). The acid is mentioned starting with the ring name and ending with the suffix “carboxylic.” For example, φ – COOH, is named as benzenecarboxylic acid.
In the upper image the general structure of the carboxylic acid is represented. The R side chain can be of any length or have all kinds of substituents.
The carbon atom is sp 2 hybridized , allowing it to accept a double bond and generate bond angles of approximately 120º.
Therefore, this group can be assimilated as a flat triangle. The upper oxygen is rich in electrons, while the lower hydrogen is poor in electrons, turning into acidic hydrogen (electron acceptor). This is observable in double bond resonance structures.
Hydrogen is transferred to a base, and for this reason this structure corresponds to an acid compound.
Carboxylic acids are very polar compounds, with intense odors and with the facility to interact effectively with each other through hydrogen bonds, as illustrated in the image above.
When two carboxylic acids interact in this way, dimers are formed, some stable enough to exist in the gas phase.
Hydrogen bonds and dimers cause carboxylic acids to have higher boiling points than water. This is because the energy provided in the form of heat must evaporate not only a molecule, but also a dimer, also linked by these hydrogen bonds.
Small carboxylic acids have a strong affinity for water and polar solvents. However, when the number of carbon atoms is greater than four, the hydrophobic character of the R chains predominates and they become immiscible with water.
In the solid or liquid phase, the length of the R chain and its substituents play an important role. Thus, when the chains are very long, they interact with each other through London dispersion forces, as in the case of fatty acids.
When the carboxylic acid donates a proton, it is converted to the carboxylate anion, represented in the image above. In this anion, the negative charge is delocalized between the two carbon atoms, stabilizing it and, therefore, favoring the reaction to occur.
How does this acidity vary from one carboxylic acid to another? It all depends on the acidity of the proton in the OH group: the poorer it is in electrons, the more acidic it is.
This acidity can be increased if one of the R chain substituents is an electronegative species (which attracts or removes electronic density from its surroundings).
For example, if in CH 3 –COOH an H of the methyl group is replaced by a fluorine atom (CFH 2 –COOH), the acidity increases considerably because the F removes the electronic density of carbonyl, oxygen, and then hydrogen. If all the H are replaced by F (CF 3 –COOH) the acidity reaches its maximum value.
What variable determines the degree of acidity? The pK a . The lower the pK a and the closer to 1, the greater the ability of the acid to dissociate in water and, in turn, the more dangerous and harmful. From the previous example, CF 3 –COOH has the smallest value of pK a .
Due to the immense variety of carboxylic acids, each of these has a potential application in the industry, be it polymer, pharmaceutical or food.
– In the preservation of food, non-ionized carboxylic acids penetrate the cell membrane of bacteria, lowering the internal pH and stopping their growth.
– Citric and oxalic acids are used to remove rust from metal surfaces, without properly altering the metal.
– Tons of polystyrene and nylon fibers are produced in the polymer industry.
– Fatty acid esters find use in the manufacture of perfumes.
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