Chemical Nomenclature

We explain what chemical nomenclature is and its different types. Also, nomenclatures in organic and inorganic chemistry.

What is chemical nomenclature?

In chemistry, the set of rules and formulas that determine the way of naming and representing the various chemical compounds known to human beings, depending on the elements that compose them and the proportion in each element.

The importance of chemical nomenclature lies in the possibility of naming, organizing and classifying the various types of chemical compounds, in such a way that only With their identifying term you can have an idea of ​​what type of elements they are made up of and, therefore, what type of reactions can be expected from these compounds.

There are three chemical nomenclature systems:

• Stoichiometric or systematic system (recommended by IUPAC). Name the compounds based on the number of atoms of each element that make them up. For example: The compound Ni₂EITHER₃ It is called dinickel trioxide.
• Functional, classic or traditional system. It uses various suffixes and prefixes (such as -oso, -ico, hypo-, per-) depending on the atomic valence of the elements of the compound. This nomenclature system is largely out of use. For example: The compound Ni₂EITHER₃ It is called nickel oxide.
• STOCK system. In this system the name of the compound includes in Roman numerals (and sometimes as a subscript) the valence of the atoms present in the compound molecule. For example: The compound Ni₂EITHER₃ It is called nickel(III) oxide.

On the other hand, the chemical nomenclature varies depending on whether it is organic or inorganic compounds.

See also: Avogadro's number

Nomenclature in organic chemistry

Before talking about the nomenclature of the different types of organic compounds, it is necessary to define the term “locator”. The locator is the number used to indicate the position of an atom in a hydrocarbon chain or cycle. For example, in the case of pentane (C₅h₁₂) and cyclopentane (C₅h₁₀), each carbon atom is numbered as shown in the following figure:

On the other hand, it is worth mentioning the carbon tetravalence which means that this element has 4 valences, therefore, it can form only 4 bonds with a wide combination of them. This explains the reason why in each organic compound we will almost never see or put a carbon atom with more than 4 bonds.

In organic chemistry there are mainly two nomenclature systems:

Substitute nomenclature One hydrogen of the hydrocarbon structure is replaced by the corresponding functional group. Depending on whether the functional group acts as a substituent or as a primary function, it will be named as a prefix or suffix to the name of the hydrocarbon. For example:

• Main function A hydrogen on carbon 3 of pentane is replaced by the -OH (-ol) group. It is named: 3-pentanol.
  
• Substituent. A hydrogen on carbon 1 of pentane is replaced by the -Cl (chloro-) group, named 1-chloropentane. If a hydrogen is substituted on carbon 2, it is named 2-chloropentane.
  

Clarification: The hydrogens in the above structures are implied for simplicity. Each union between two lines means that there is a carbon atom with its corresponding hydrogens, always respecting tetravalence.

Radical-function nomenclature The name of the radical corresponding to the hydrocarbon is put as a suffix or prefix to the name of the functional group. If it is a functional group of the main function type it would be, for example, pentylamine or 2-pentylamine. If it were a substituent type functional group it would be, for example, pentyl chloride (you can see that it is the same structure as 1-chloropentane but using another nomenclature to name it).

Table 1: Very common substituent names.

Table 2: Names of very common organic radicals.

Hydrocarbon nomenclature

Hydrocarbons are compounds made up of carbon (C) and hydrogen (H) atoms. They are classified into:

Aliphatic hydrocarbons

They are non-aromatic compounds. If their structure closes and forms a cycle, they are called alicyclic compounds. For example:

Alkanes They are compounds of acyclic nature (that do not form cycles) and saturated (all their carbon atoms are linked to each other with simple covalent bonds). They respond to the general formula C_nh_2n+2where n represents the number of carbon atoms. In all cases the suffix -ano is used to name them. They can be:

• Linear alkanes They have a linear chain. To name them, the suffix -ane will be combined with the prefix that denotes the number of carbon atoms present. For example, hexane has 6 carbon atoms (hex-) (C₆h₁₄). In the Table 3 Some examples are shown.
  

Table 3: Alkane names according to the number of carbon atoms contained in their structure.

• Branched alkanes. If they are not linear but branched, the longest hydrocarbon chain that has the most branches (the main chain) must be looked for, its carbon atoms are counted from the end closest to the branch and the branches are named indicating their position in the chain. main (as we saw with the locator), replacing the suffix -ano with -il (see Table 2 ) and adding the corresponding numerical prefixes in case there are two or more identical strings. The main chain is chosen so that it has the smallest possible combination of locators. Finally the main chain is named normally. For example, 5-ethyl-2-methylheptane has a heptane backbone (hep-, 7 carbon atoms) with a methyl radical (CH₃-) on the second carbon atom and one ethyl (C₂h₅-) in the fifth. This is the smallest possible combination of branch positions for this compound.
• Alkane radicals (produced by losing a hydrogen atom attached to one of its carbons). They are named by replacing the suffix -ane with -yl and indicating it with a hyphen in the chemical bond. For example, from methane (CH₄) the methyl radical (CH₃-). (See Table 2 ). It is worth clarifying that, for nomenclature, the ending -yl can also be used for radicals when they act as substituents. For example:
  
• Cycloalkanes They are alicyclic compounds that respond to the general formula C_nh_2n. They are named like linear alkanes but adding the prefix cyclo- to the name, for example, cyclobutane, cyclopropane, 3-isopropyl-1-methyl-cyclopentane. In these cases, the smallest possible combination of the numbers of the atoms that have substituents must also be chosen. For example:
  

Alkenes and alkynes They are unsaturated hydrocarbons, as they have a double (alkenes) or triple (alkynes) carbon-carbon bond. They respond, respectively, to the general formulas C_nh_2n and C._nh_2n-2. They are named similarly to alkanes, but different rules apply to them based on the location of their multiple bonds:

• When there is a carbon-carbon double bond, the suffix -ene is used (instead of -ane as in alkanes) and the respective number prefixes are added if the compound has more than one double bond, for example, -diene, -triene, -tetraene.
• When there is a carbon-carbon triple bond, the suffix -yne is used and the respective number prefixes are added if the compound has more than one triple bond, for example, -diyne, -triyne, -tetraino.
• When there are carbon-carbon double and triple bonds, the suffix -enyne is used and the respective number prefixes are added if there are several of these multiple bonds, for example, -dienyne, -trienyne, -tetraenyne.
• The location of the multiple bond is indicated by the number of the first carbon of that bond.
• If there are branches, the longest chain with the greatest number of double or triple bonds is chosen as the main chain. The chain is chosen so that the double or triple bond locator is as small as possible.
• The organic radicals that come from alkenes are named by replacing the suffix -ene with -enyl (if it acts as a substituent, -enyl) and the radicals coming from alkynes are replaced -yne with -ynyl (if it acts as a substituent, -ynyl).

Table 4: Names of substituent radicals of alkenes and alkynes.

Aromatic hydrocarbons

They are known as arenas. They are conjugated cyclic compounds (that alternate a single bond and a multiple bond in their structure). They have rings with flat and very stable structures due to conjugation. Many include benzene (C₆h₆) and its derivatives, although there are numerous other varieties of aromatic compounds. They can be classified into:

• Monocyclic They are named from derivations of the name benzene (or some other aromatic compound), listing their substituents with numerator prefixes (locators). If the aromatic ring has several substituents, they are named in alphabetical order, always looking for the smallest possible combination of locators. If any substituent involves a ring, it is placed in position one on the aromatic ring, and continues to be named according to the alphabetical order of the rest of the substituents. On the other hand, the benzene ring radical is called phenyl (if it acts as a substituent, -phenyl). For example:
  
  Another way to define the position of substituents in aromatic hydrocarbons is using the ortho, meta and para nomenclature. This consists of locating the position of other substituents based on the position of an initial substituent, for example:
  
• Polycyclic. They are mostly named by their generic name, since they are very specific compounds. But the suffix -ene or -enyl can also be used for them. These polycycles can be formed by several fused aromatic rings, or connected by CC bonds. In these compounds, the locators are usually given with numbers for the main structure (the one with the most cycles) and with numbers with “primes” for the secondary structure. For example:
  

Alcohols

Alcohols are organic compounds that contain a hydroxyl group (-OH). Their structure is formed by replacing an H with the -OH group in a hydrocarbon, therefore, they are defined by the general formula R-OH, where R is any hydrocarbon chain. They are named using the suffix -ol instead of the ending -o of the corresponding hydrocarbon. If the -OH group acts as a substituent, then it is named hydroxy-. If a compound has several hydroxyl groups, it is called a polyalcohol or polyol, and is named using numerator prefixes.

Phenols

Phenols are similar to alcohols, but have the hydroxyl group attached to an aromatic benzene ring, rather than a linear hydrocarbon. They respond to the formula Ar-OH. To name them, the suffix -ol attached to that of the aromatic hydrocarbon is also used.

Some examples of alcohols and phenols are:

Ethers

Ethers are governed by the general formula RO-R', where the radicals at the ends (R- and R'-) can be identical or different groups, from the alkyl or aryl group. Ethers are named with the term of each alkyl or aryl group in alphabetical order, followed by the word “ether.” For example:

Amines

They are organic compounds derived from ammonia by replacing one or some of its hydrogens with alkyl or aryl radical groups, obtaining aliphatic amines and aromatic amines respectively. In both cases they are named using the suffix -amine or the general name is retained. For example:

Carboxylic acids

They are organic compounds that have a carboxyl group (-COOH) as part of their structure. This functional group is composed of a hydroxyl group (-OH) and a carbonyl group (-C=O). To name them, the main chain is considered to be the chain with the highest number of carbons that contains the carboxyl group. Then it is used as the ending -ico or -oico to name them. For example:

Aldehydes and ketones

They are organic compounds that have a carbonyl functional group. If the carbonyl is at one end of the hydrocarbon chain we will speak of an aldehyde, and it will in turn be linked to a hydrogen and an alkyl or aryl group. We will talk about ketones when the carbonyl is within the hydrocarbon chain and linked through the carbon atom to alkyl or aryl groups on both sides. To name aldehydes, the suffix -al is used at the end of the name of the compound, following the same numbering rules according to the number of atoms. They can also be named using the general name of the carboxylic acid from which they come, and changing the suffix -ico to -aldehyde. For example:

To name ketones, the suffix -one is used at the end of the name of the compound following the same numbering rules according to the number of atoms. You can also name the two radicals attached to the carbonyl group followed by the word ketone. For example:

Esters

They should not be confused with ethers, since they are acids whose hydrogen is replaced by an alkyl or aryl radical. They are named by changing the suffix -ico of the acid to -ate, followed by the name of the radical that replaces hydrogen, without the word “acid”. For example:

Amides

They should not be confused with amines. They are organic compounds that are produced by replacing the -OH group of a reference acid with the -NH group.₂. They are named by replacing the -ic ending of the referring acid with -amide. For example:

acid halides

They are organic compounds derived from a carboxylic acid in which the -OH group is replaced by an atom of a halogen element. They are named by replacing the suffix -ico with -yl and the word “acid” with the name of the halide. For example:

acid anhydrides

They are organic compounds derived from carboxylic acids. They can be symmetrical or asymmetrical. If they are symmetrical, they are named by replacing the word acid with “anhydride”. For example: acetic anhydride (from acetic acid). If they are not, both acids are combined and preceded by the word “anhydride”. For example:

Nitriles

They are organic compounds that have the -CN functional group. In this case, the -ic ending of the referring acid is replaced by -nitrile. For example:

Nomenclature in inorganic chemistry

• Oxides They are compounds that are formed with oxygen and some other metallic or non-metallic element. They are named using prefixes according to the number of atoms each oxide molecule has. For example: digallium trioxide (Ga₂EITHER₃), carbon monoxide (CO). When the oxidized element is metallic, they are called basic oxides; When it is non-metallic, they are called acid anhydrides or oxides. Generally, the oxygen in oxides has an oxidation state of -2.
• Peroxides They are compounds formed by the combination of the peroxo group (-OO-) O₂^-2 and another chemical element. Oxygen generally has oxidation state -1 in the peroxo group. They are named the same as oxides but with the word “peroxide”. For example: calcium peroxide (CaO₂), dihydrogen peroxide (H₂EITHER₂).
• Superoxides They are also known as hyperoxides. Oxygen has a -½ oxidation state in these compounds. They are regularly named like oxides, but using the word “hyperoxide” or “superoxide”. For example: potassium superoxide or hyperoxide (KO₂).
• Hydrides They are compounds formed by hydrogen and another element. When the other element is metallic, they are called metal hydrides and when it is not metallic, they are called non-metal hydrides. Its nomenclature depends on the metallic or non-metallic nature of the other element, although in some cases common names are used, such as ammonia (or nitrogen trihydride).
  • Metal hydrides To name them, the numerical prefix is ​​used according to the number of hydrogen atoms followed by the term “hydride”. For example: potassium monohydride (KH), lead tetrahydride (PbH₄).
  • Non-metallic hydrides. The ending -ide is added to the non-metallic element and then the phrase “of hydrogen” is added. They are usually found in a gaseous state. For example: hydrogen fluoride (HF_(g)), dihydrogen selenide (H₂HE_(g)).
• Oxacids They are compounds that are also called oxoacids or oxyacids (and popularly “acids”). They are acids that contain oxygen. Its nomenclature requires using the prefix corresponding to the number of oxygen atoms, followed by the word “oxo” attached to the name of the non-metal ending in “-ate”. At the end the phrase “hydrogen” is added. For example: hydrogen tetraoxosulfate or sulfuric acid (H₂SW₄), hydrogen dioxosulfate or hyposulfurous acid (H₂SW₂).
• Hydracids They are compounds formed by hydrogen and a non-metal. When dissolved in water they give acidic solutions. They are named using the prefix “acid” followed by the name of the nonmetal, but with the ending “hydric.” For example: hydrofluoric acid (HF_(ac)), hydrochloric acid (HCl_(ac)), hydrogen sulfide (H₂Yes_(ac)), hydroselide acid (H₂HE_(ac)). Whenever the formula of a hydracid is represented, it must be clarified that it is in aqueous solution (aq) (otherwise it can be confused with a non-metallic hydride).
• Hydroxides or bases. They are compounds formed by the union of a basic oxide and water. They are recognized by the -OH functional group. They are generically named as hydroxide, attached to the respective prefixes depending on the amount of hydroxyl groups present. For example: lead dihydroxide or lead (II) hydroxide (Pb(OH)₂ ), lithium hydroxide (LiOH).
• Salts Salts are the product of the union of acidic and basic substances. They are named according to their classification: neutral, acidic, basic and mixed.
  • Neutral salts. They are formed by the reaction between an acid and a base or hydroxide, releasing water in the process. They can be binary and ternary depending on whether the acid is a hydracid or an oxacid.
    • If the acid is a hydracid, they are called haloid salts. They are named using the suffix -ide on the non-metallic element, and the prefix corresponding to the amount of this element. For example: sodium chloride (NaCl), iron trichloride (FeCl₃).
    • If the acid is an oxacid, they are also called oxysalts or ternary salts. They are named using the numerical prefix according to the number of “oxo” groups (number of O oxygens^2-), and the suffix -ate in the nonmetal, followed by the oxidation state of the nonmetal written in Roman numerals and in parentheses. They can also be named using the name of the anion followed by the name of the metal. For example: calcium tetraoxosulfate (VI) (Ca²⁺S⁶⁺EITHER^2- ) or calcium sulfate (Ca²⁺(SO₄)^2-) (Case₄), sodium tetraoxophosphate (V) (Na¹⁺P⁵⁺EITHER^2-) or sodium phosphate (Na¹⁺(P.O.₄)^3-) (Na₃P.O.₄).
  • acid salts They are formed by replacing hydrogen in an acid with metal atoms. Its nomenclature is the same as that of ternary neutral salts, but adding the word “hydrogen”. For example: sodium hydrogen sulfate (VI) (NaHSO₄), a hydrogen is changed from sulfuric acid (H₂SW₄) for a sodium atom, potassium hydrogen carbonate (KHCO₃), a hydrogen is exchanged from the carbonic acid (H₂CO₃) by a potassium atom.
  • Basic salts They are formed by replacing the hydroxyl groups of a base with the anions of an acid. Its nomenclature depends on an oxyacid.
    • If the acid is a hydracid, the name of the nonmetal is used with the suffix -ide and the numeral prefix of the number of -OH groups is preceded, followed by the term “hydroxy”. At the end the oxidation state of the metal is set if necessary. For example: FeCl(OH)₂ it would be iron (III) dihydroxychloride.
    • If the acid is an oxacid, the term “hydroxy” is used with its corresponding numeral prefix. Then the suffix corresponding to the number of “oxo” groups is added and the ending -ate is added to the nonmetal, followed by its oxidation state written in Roman numerals and in parentheses. Finally, the name of the metal is given followed by its oxidation state written in Roman numerals and in parentheses. For example: Neither₂(OH)₄SW₃ it would be nickel (III) tetrahydroxytrioxosulfate (IV).
  • Mixed salts They are produced by replacing the hydrogens of an acid with metal atoms of different hydroxides. Its nomenclature is identical to that of acid salts, but including both elements. For example: sodium potassium tetraoxosulfate (NaKSO₄).

IUPAC nomenclature

The IUPAC (acronym for International Union of Pure and Applied Chemistry, that is, International Union of Pure and Applied Chemistry ) is the international organization dedicated to establishing universal rules of chemical nomenclature.

Their system, proposed as a simple and unifying system, is known as IUPAC nomenclature and differs from traditional nomenclature in that it is more specific when naming the compounds, since it not only names them but also clarifies the amount of each chemical element in the compound.

Continue with: Chemical formula

References

• “Chemical nomenclature” https://es.wikipedia.org/
• “Nomenclature and chemical formulas” (video) in Your Virtual Teacher. https://www.youtube.com/
• “Chemical nomenclature” https://www.profesorenlinea.cl/
• “Organic nomenclature: alkanes, alkenes and alkynes” (video) in Quimiayudas. https://www.youtube.com/