Classification and Nomenclature

In chemistry, especially organic chemistry, the number of compounds runs into millions. To avoid confusion, chemists use classification and nomenclature systems to systematically group and name these compounds.

• Classification means arranging compounds into categories based on structure, functional groups, or properties.
• Nomenclature means assigning standard names to these compounds following internationally accepted rules, primarily set by IUPAC (International Union of Pure and Applied Chemistry).

1.0Classification of Organic Compounds

Organic compounds can be broadly classified into two main categories: acyclic and cyclic. Let's break down each class.

1. Acyclic (Open-Chain) Compounds

Also known as aliphatic compounds, these compounds have a straight or branched chain of carbon atoms. They do not contain any rings.

• Saturated Hydrocarbons: These contain only single bonds between carbon atoms. Examples include alkanes like methane (CH₄​), ethane (C₂​H₆​), and propane (C₃​H₈​).
• Unsaturated Hydrocarbons: These compounds contain at least one double or triple bond between carbon atoms.
• • Alkenes: Contain at least one carbon-carbon double bond (C=C). Examples include ethene (C₂​H₄​) and propene (C₃​H_6​).
  • Alkynes: Contain at least one carbon-carbon triple bond (C≡C). Examples include ethyne (C₂​H₂​) and propyne (C₃​H₄​).

2. Cyclic (Closed-Chain) Compounds

These compounds contain a ring structure of carbon atoms. They are further divided into:

• Homocyclic (Carbocyclic) Compounds: The ring is made up entirely of carbon atoms.
• • Alicyclic Compounds: These are saturated or unsaturated cyclic compounds that behave like aliphatic compounds. Examples include cyclopropane and cyclohexane. They don't have the characteristic aromatic properties of benzene.
  • Aromatic Compounds: These compounds contain a benzene ring or a related structure with a special type of stability due to electron delocalization. The most famous example is benzene (C6​H6​). Other examples include toluene and naphthalene.
• Heterocyclic Compounds: The ring contains at least one atom other than carbon, such as oxygen, nitrogen, or sulfur. These are known as heteroatoms.
• • Heteroalicyclic Compounds: Example is tetrahydrofuran (THF).
  • Heteroaromatic Compounds: Examples include pyridine, furan, and thiophene.

2.0IUPAC Nomenclature Principles

IUPAC (International Union of Pure and Applied Chemistry) sets standardized rules for naming compounds. Key concepts:

• Identify the longest chain (organic) or central atom (inorganic).
• Determine functional groups or ligands and assign priority.
• Use proper prefixes, infixes, and suffixes (e.g., -ane, -ene, -ol, -ic acid).
• Number the chain or coordination sphere to give lowest possible locants.

Naming Inorganic Compounds

Binary Compounds

For ionic compounds: metal name + nonmetal with “‑ide” suffix (e.g., sodium chloride, MgO = magnesium oxide).

For covalent compounds, use Greek prefixes (mono‑, di‑, tri‑) plus element names (e.g., CO₂ = carbon dioxide, PCl₅ = phosphorus pentachloride).

Oxoacids and Oxyanions

Oxoacids (like H₂SO₄, HNO₃): use “‑ic” and “‑ous” endings (sulfuric acid, nitric acid).

Oxyanions: “‑ate” (higher oxygen) vs. “‑ite” (lower oxygen) (e.g., SO₄²⁻ = sulfate, SO₃²⁻ = sulfite).

Coordination Compounds

Name ligands alphabetically before the central metal. Use prefixes (di‑, tri‑) for multiple ligands.

Anionic ligands end in “o” (e.g., chloro, aqua, ammine).

Indicate oxidation state of metal in parentheses (e.g., K₄[Fe(CN)₆] = potassium hexacyanoferrate(II)).

3.0Naming Organic Compounds

Alkanes, Alkenes, Alkynes

Alkanes: single bonds, suffix “‑ane” (methane, ethane, propane).

Alkenes: one or more double bonds, suffix “‑ene” (ethylene, propene).

Alkynes: triple bonds, suffix “‑yne” (ethyne, propyne).

Functional Groups

Alcohols: “‑ol” (e.g., ethanol, 2‑propanol).

Aldehydes: “‑al” (e.g., ethanal, benzaldehyde).

Ketones: “‑one” (e.g., propanone, butanone).

Carboxylic acids: “‑oic acid” (e.g., ethanoic acid, propanoic acid).

Others: amines (“‑amine”), amides (“‑amide”), esters (“‑oate”), nitriles (“‑nitrile”), etc.

4.0Common Functional Groups and Their Suffixes

A functional group is an atom or a group of atoms that is responsible for the characteristic chemical reactions of an organic compound. Here's a quick reference for some of the most important functional groups and their suffixes/prefixes:

5.0Examples 

Inorganic Nomenclature Examples

FeCl₂: iron(II) chloride (transition metal with oxidation state).

N₂O₅: dinitrogen pentoxide (covalent binary).

H₂SO₃: sulfurous acid (less oxygen than sulfuric acid).

[Cu(NH₃)₄]SO₄: tetraamminecopper(II) sulfate (coordination complex).

Practice problem: Name K₃[Fe(C₂O₄)₃]·3H₂O

Solution: potassium tris(oxalato)ferrate(III) trihydrate

Organic Nomenclature Examples

CH₃–CH₂–OH: ethanol (alkanol).

CH₂=CH–CH₃: propene (alkene).

CH₃–CO–CH₃: propanone (ketone).

CH₃–COOH: ethanoic acid (carboxylic acid).

CH₃–CH₂–COOCH₃: methyl propanoate (ester).

6.0Nomenclature Rules in Practice

Let's apply the IUPAC rules step by step.

Rule 1: Finding the Longest Carbon Chain

Identify the longest continuous chain of carbon atoms. This chain is the parent chain.

The name of the parent chain is derived from the number of carbons in it using the word root.

• 1 C: Meth-
• 2 C: Eth-
• 3 C: Prop-
• 4 C: But-
• 5 C: Pent-
• 6 C: Hex-
• ... and so on.

Rule 2: Naming Substituents

• Any group attached to the parent chain that is not part of the primary functional group is called a substituent or side chain.
• Substituents are named by changing the -ane suffix of the corresponding alkane to -yl. For example, CH3​− is a methyl group.
• The position of the substituent is indicated by a number.

Rule 3: Numbering the Parent Chain

• The carbon atoms in the parent chain are numbered to give the lowest possible numbers to the substituents. This is known as the Lowest Sum Rule or Lowest Locant Rule.
• If a functional group is present, the numbering must give the principal functional group the lowest possible number. The priority order for functional groups must be memorized for the JEE exam.

$\mathrm{COOH}>\mathrm{S}{\mathrm{O}}_3\mathrm{H}>\mathrm{COOR}>\mathrm{COCl}>\mathrm{CON}{\mathrm{H}}_2>\mathrm{CN}>\mathrm{CHO}>\mathrm{C}=\mathrm{O}>\mathrm{OH}>\mathrm{N}{\mathrm{H}}_2>\mathrm{C}=\mathrm{C}>\mathrm{C}\equiv \mathrm{C}>\mathrm{Halogen}>\mathrm{Alkyl}$

Rule 4: Handling Multiple Identical Substituents

If there are two or more identical substituents, use a prefix like di-, tri-, tetra-, etc., before the name of the substituent.

The positions of all identical substituents must be listed, separated by commas.

Rule 5: Alphabetical Order

When multiple different substituents are present, they are listed in alphabetical order. The prefixes di-, tri-, etc., are ignored when alphabetizing.