Classification and IUPAC Nomenclature

An accurate understanding of classification and IUPAC nomenclature is foundational for mastering organic chemistry, particularly at the JEE level. Classification enables students to organize organic compounds by structure and reactivity, while the IUPAC system ensures uniform, systematic naming—essential for clear communication, problem-solving, and exam success.

1.0Classification of Organic Compounds

Based on Functional Groups

• Alkanes: Single bonds only (C–C), saturated hydrocarbons
• Alkenes: Double bonds (C=C), unsaturated hydrocarbons
• Alkynes: Triple bonds (C≡C), unsaturated hydrocarbons
• Aromatic compounds: Contain benzene rings
• Alcohols: Contain –OH group
• Phenols: –OH directly bonded to the benzene ring
• Ethers: R–O–R’
• Aldehydes: –CHO group
• Ketones: C=O group bonded to two carbon atoms
• Carboxylic acids: –COOH
• Amines: –NH₂ group
• Amides: –CONH₂

Based on Chain Structure

Organic compounds are also distinguished by their carbon skeleton:

• Straight-chain (normal) vs. branched
• Cyclic: alicyclic (e.g., cycloalkanes) vs. aromatic (benzene derivatives)
• Heterocyclic: rings containing elements other than carbon (e.g., pyridine, furan)

Understanding chain structure aids in identifying parent names and substituents.

Based on Saturation

• Saturated hydrocarbons: only single C–C bonds (alkanes)
• Unsaturated hydrocarbons: contain double (alkenes) or triple (alkynes) C–C bonds
• Aromatic compounds: delocalized π‑electron systems, e.g., benzene

Saturation influences reactivity, nomenclature, and classification.

2.0IUPAC Nomenclature System

General Principles

IUPAC nomenclature provides a systematic, unambiguous method to name organic compounds. Key principles include:

• Longest chain rule: identify the longest continuous carbon chain containing the principal functional group
• Functional group priority: certain groups take precedence in suffix naming
• Numbering: assign locants (numbers) to minimize sum of positions and give lowest possible set of numbers
• Alphabetical order: list substituents in alphabetical order (ignoring prefixes like di-, tri‑)

Steps in Naming Organic Compounds

1. Identify the parent chain.
2. Identify and name substituents.
3. Number the chain to assign the lowest possible locants to substituents and functional groups.
4. Select appropriate suffix based on the highest-priority functional group.
5. Assemble the name: locant‑substituent(s)‑parent‑suffix.

Nomenclature of Alkanes, Alkenes, Alkynes

Alkanes (C_nH_2n+2)

• Parent names: methane, ethane, propane, butane, pentane, etc.
• Example: CH₃–CH₂–CH₂–CH₃ → butane

Alkenes (C_nH_2n)

• Use suffix ‑ene.
• Number the chain to give the double bond the lowest possible locant.
• Example: CH₂=CH–CH₂–CH₃ → 1‑butene
• If double bond starts at second carbon: CH₃–CH=CH–CH₃ → 2‑butene (cis‑ and trans‑ or E/Z isomers if applicable).

Alkynes (C_nH_2n‑2)

• Use suffix ‑yne.
• Example: CH≡C–CH₂–CH₃ → 1‑butyne; CH₃–C≡C–CH₃ → 2‑butyne.

Nomenclature of Alcohols, Ethers, Aldehydes, Ketones

Alcohols (–OH)

• Suffix ‑ol.
• Number the chain to give the –OH the lowest possible locant.
• Example: CH₃–CH₂–CH₂–OH → propan‑1‑ol; CH₃–CHOH–CH₃ → propan‑2‑ol.

Ethers (R–O–R′)

• Named as alkoxy‑alkane.
• Example: CH₃–O–CH₂–CH₃ → methoxyethane.

Aldehydes (–CHO)

• Suffix ‑al.
• The –CHO group is always at carbon‑1, no need for a locant.
• Example: CH₃–CHO → ethanal; CH₃–CH₂–CHO → propanal.

Ketones (>C=O)

• Suffix ‑one.
• Number the chain so the carbonyl carbon gets the lowest possible number.
• Example: CH₃–CO–CH₃ → propan‑2‑one (acetone); CH₃–CH₂–CO–CH₃ → butan‑2‑one.

Nomenclature of Carboxylic Acids and Derivatives

Carboxylic Acids (–COOH)

• Suffix ‑oic acid.
• Example: CH₃–COOH → ethanoic acid; CH₃–CH₂–COOH → propanoic acid.

Esters (–COOR)

• Named as alkyl alkanoate.
• Example: CH₃–COO–CH₂–CH₃ → ethyl ethanoate.

Acid Chlorides, Anhydrides

• Acid chloride: suffix ‑oyl chloride. Example: CH₃–COCl → ethanoyl chloride.
• Anhydride: named as alkanoic anhydride. Example: (CH₃CO)₂O → ethanoic anhydride.

Nomenclature of Amines, Amides, and Nitriles

Amines (–NH₂, –NHR, –NR₂)

• Suffix ‑amine or prefix amino‑ (depending on priority).
• Example: CH₃–NH₂ → methanamine; CH₃–CH₂–NH–CH₃ → N‑methylethanamine.

Amides (–CONH₂)

• Suffix ‑amide.
• Example: CH₃–CONH₂ → ethanamide.

Nitriles (–C≡N)

• Suffix ‑nitrile.
• Example: CH₃–C≡N → ethanenitrile (also ethyl cyanide).

Nomenclature of Aromatic Compounds

Simple Benzene Derivatives

• Parent: benzene.
• Substituents named as prefixes: e.g., chlorobenzene, nitrobenzene, methylbenzene (toluene), etc.

Disubstituted Benzenes

• Use ortho‑, meta‑, para‑ or numeric locants (1,2‑; 1,3‑; 1,4‑).
• Example: o‑dichlorobenzene (1,2‑dichlorobenzene), m‑xylene (1,3‑dimethylbenzene).

Polysubstituted Aromatics

• Number to give substituents lowest set of locants; use alphabetical order for listing.

Polyfunctional and Complex Molecules

When multiple functional groups are present:

• Determine the principal functional group (highest priority) and use its suffix.
• Other groups become prefixes (e.g., hydroxy‑, oxo‑, amino‑).
• Number the chain so the principal group gets the lowest possible locant.
• Example: HO–CH₂–CH₂–COOH → 2‑hydroxyethanoic acid (glycolic acid).
• Example: CH₃–CO–CH₂–CH₂–OH → 4‑hydroxybutan‑2‑one.