Carboxylic acids are used today in many industries. They make various types of plastic and fibers, medicines and detergents, they are used as preservatives in the food and cosmetic industries. This list can be continued for a very long time. However, in nature, organic acids are found everywhere: in plants, fungi, insects, animals, as well as in the products of their vital functions. That is why it is important to understand what you are dealing with when you come across the name of one of them.
Carboxylic acids are organic compounds that contain at least one group of atoms — COOH, called carboxylic. The nomenclature of carboxylic acids does not seem so complicated if you have an idea of the variety of substances of this class and understand the principles of compiling the names of any organic substances.
Functional group
A feature of the carboxyl group is that its properties are almost independent of the structural features of the carbon skeleton of the acid molecule, as well as the presence or absence of substituents in the group.
It can be considered as a complex group consisting of two simple ones: carbonyl> C = O and hydroxyl-OH, which affect each other. In this case, the electron density shifts toward carbonyl oxygen and causes additional polarization of the OH bond in the hydroxo group. As a result, an excess negative is formed on the carbonyl oxygen, and positive charges on the hydroxyl hydrogen. This leads to the appearance of hydrogen bonds between neighboring acid molecules, as well as the appearance of dimers and associates. This explains their high boiling points.
Classification of Organic Acids
Before you begin to understand the features of the nomenclature of compounds of this class, you need to form an idea of what they are.
By type of hydrocarbon skeleton:
- limit - C atoms in the acid molecule are connected by simple single bonds;
- unsaturated - in the hydrocarbon chain there are C = C or C≡C;
- aromatic - as part of the carboxylic acid molecule there is a benzene ring.
By the number of UNCS groups:
- monobasic - contain one group;
- dibasic - two;
- polybasic - three or more groups - COOH.
Isomerism
The isomerism and nomenclature of carboxylic acids are closely related. Indeed, in compounds with the same composition, but having a different structure, the names will differ. For most carboxylic acids, structural isomerism is typical. So, the composition of C 5 Η 10 O 2 corresponds to the acid:
- pentane Η 3 ―Η 2 - Η 2 ―Η 2 ―Η;
- 2-methylbutane Η 3 ―Η 2 ―Η (Η 3 ) ―Η.
If the molecules have C = C or C≡C, then positional isomerism is possible for them. For example, 3-butene and 2-butenoic acids: CΗ 2 = CΗ ― CΗ 2 ―COO― and CΗ 3 ―CΗ = CΗ ― COOΗ.
Interclass isomerism is also possible. Unsaturated dihydric alcohols, hydroxycarbonyl compounds and esters are isomeric to carboxylic acids. For example, pentanoic acid With 3 H 9 -COOH corresponds to several esters:
- C 2 Η 5 ―CO ― O ― C 2 Η 5 ;
- Η 3 ― ― ― 3 Η 7 .
In addition, spatial isomerism is characteristic of organic acid molecules.
Trivial nomenclature
The very first nomenclature of carboxylic acids was precisely trivial. The names given to the compounds did not in any way reflect the structural features of the molecule, but gave an idea of where the substances are contained or used:
- ant (UNSF) is found in ants and causes a burning sensation when bitten;
- vinegar (Η 3 Η) is formed in the process of turning wine into vinegar;
- butter (C 3 Η 7 COOΗ) appears in butter when it becomes rancid and causes its specific smell;
- kapron (C 5 Η 11 COOH) is found in goat milk (from Latin caper - goat);
- apple (C 4 H 6 O 5 ) was named in accordance with the fruit in which it was found;
- lemon (C 6 H 8 O 7 ) is found in large quantities in lemons and other citrus fruits;
- wine (C 4 H 6 O 6 ) is formed in the sour juice of most fruits;
- salicylic ( 6 Η 4 (Η) Η) was first obtained from willow bark (from the Latin salix - willow).
And today the names most often used are those that correspond specifically to the trivial nomenclature of carboxylic acids. For molecules with a more complex structure, the name of the substituents is added to the trivial name.
If at the end of the chain there is a branching in the form of a methyl group —CH 3 , then the iso: isovalerianic, isobutyric, etc.
If the branches are in another part of the carbon chain, then the location of the attached group is indicated by the corresponding capital letters of the Greek alphabet:
θ ― η ― ζ ― ε ― δ ― γ ― β ― α COOH
Example. Name the compound Η 3 ―Η 2 ―Η ( 2 Η 5 ) ―Η 2 ―Η according to the trivial nomenclature.
Decision:
- The C atoms in the main chain are denoted sequentially: δ CΗ 3 - γ CΗ 2 - β CΗ (C 2 Η 5 ) - α CH 2 COOH.
- The names of the substituents are recorded and their position is indicated in letters: β-ethyl.
- The trivial name of the organic acid is written corresponding to the main chain, in this case valerianic: β-ethylvalerianic acid.
IUPAC Nomenclature
According to the systematic nomenclature, carboxylic acids are called using the following rules:
- A hydrocarbon backbone is selected that necessarily includes a carboxyl group. As a rule, it is the longest and includes the maximum possible number of substituents, functional groups, C = C and C≡C, if any.
- Atoms C in the main chain are assigned sequentially numbers starting from carbon in the -COOH group: 4 C- 3 C- 2 C- 1 COOH.
- The name begins to make up by listing the substituents in alphabetical order. For this, the position of the substituent (to which C atom from the main chain it is connected) is marked with a number and its name is written through a hyphen. If several substituents of the same type are contained, their location is indicated by numbers separated by commas, and the multiplication prefix di, three, tetra, etc. are assigned to the name. The multiplying prefix does not affect the alphabetical order.
- The chain length is reflected by the name of the corresponding hydrocarbon (butane, hexane, octane).
- The presence of a carboxyl functional group of —COOH is indicated by the addition of -ovic acid at the end of the recording.
5 Η 3 - 4 Η 2 - 3 Η ( 2 Η 5 ) - 2 Η 2 - 1 Η 3-ethyl pentanoic acid
4 CH 3 - 3 CH (CH 3 ) - 2 C (CH 3 ) 2 - 1 COOH 2,2,3-trimethylbutanoic acid
Example: Name the systematic nomenclature for carboxylic acid CΗ 3 -CΗ (CΗ 3 ) ―CH (C 2 Η 5 ) ―CH 2 ―COOH.
Decision:
- The C atoms in the main chain are numbered: 5 CΗ 3 - 4 CΗ (CΗ 3 ) - 3 CΗ (C 2 Η 5 ) - 2 CΗ 2 - 1 COOH.
- The names of the substituents are recorded, and their positions are indicated in numbers: 4-methyl-3-ethyl.
- The name of the corresponding hydrocarbon backbone is recorded. Five carbon atoms - pentane: 4-methyl-3-ethylpentane.
- Indicating the presence of –COOH, –– acid is recorded: 4-methyl-3-ethylpentanoic acid.
Rational nomenclature
According to the rules of rational nomenclature, the names are formed from a radical, or rather, a corresponding hydrocarbon associated with a carboxyl group and the phrase "carboxylic acid":
- CΗ 3 -COOΗ methanecarboxylic acid;
- C 3 Η 7 COOΗ propanecarboxylic acid;
- C 5 Η 11 COOΗ pentanecarboxylic acid.
Sometimes acetic acid is taken as the basis of the name and the radicals attached to it indicate:
- CΗ 3 ―CH 2 ―COOH methylacetic acid;
- Η 3 ―Η 2 ―Η (Η 3 ) ―Η methylethylacetic acid.
However, calling compounds with complex branched molecules can be problematic. Therefore, the rational nomenclature of carboxylic acids is rarely used.
Unsaturated and aromatic carboxylic acids
The basis for the nomenclature of unsaturated carboxylic acids is the corresponding alkene, alkine or diene, and the terminal acid is added. The carbon numbering in the main chain begins with the carboxyl group, and the position of the multiple bond is written with the corresponding number:
- CΗ 2 = CΗ ― CΗ 2 ―COOΗ 3-butenoic acid;
- CH 3 ―C≡C ― COOH 2-butic acid.
Aromatic acids are most often called derivatives of benzoic acid. For example, m-methylbenzoic acid, p-bromobenzoic acid. According to the systematic nomenclature, the presence of a benzene ring is reflected by the prefix "phenyl-".
Polybasic carboxylic acids
From the nomenclature of monobasic carboxylic acids, the compilation of the names of substances with two or more COOH groups differs insignificantly. The name of the dibasic acids is also based on the corresponding hydrocarbon, but before the end of the -ova acid, the multiplying prefix di. For instance,
- COOH ― CH 2 ―CH 2 ― COOH butanedioic acid;
- COOH ― CH (CH 3 ) COOH 2-methylpropanedioic acid.
If a molecule contains more than two carboxyl groups, then they are already indicated by the prefix "carboxy-":
HOOC-CH (COOH) -COOH 2-carboxypropanedioic acid.
Acid Residues and Esters
There are a lot of esters of various carboxylic acids in nature. These are fats, and waxes, and aromatic substances of fruits and flowers. All of them contain an ester functional group —COO—. There are two approaches to the nomenclature of carboxylic acid esters.
In the first case, first write the name of the radical, which took the place of hydrogen in the carboxyl group, and then the name of the acid residue:
- CH 3 ―COO ― C 2 Η 5 ethyl acetate;
- NSOO ― CH 3 methyl formate.
In the second case, add-ester to the name of the radical and write the trivial name of the acid in the genitive case:
- C 3 H 5 ―COO ― CΗ 3 methyl ester of butyric acid;
- Η 3 ― ― 2 Η 5 ethyl ester of acetic acid.
Names of carboxylic acids and their residues
In order to name any organic acid according to the rules of IUPAC, you need to know only a few rules. They have already been discussed above. However, the trivial names of acids and the names of their residues have to memorize or use a cheat sheet.
Summing up, it can be noted that the structure, isomerism, and nomenclature of carboxylic acids are considered above in rather detail. And after a careful study of the material there should be no questions about how to properly name one or another organic acid. Conversely, having heard or read the scientific name of the substance on the label, you can compare it with the trivial and make a conclusion for yourself about its harmfulness or safety.