Nomenclature is the system of naming chemical compounds in a standardized way. Organic compounds can be named using two main systems: the common (or trivial) system and the International Union of Pure and Applied Chemistry (IUPAC) system.
Common Nomenclature System
The common system of nomenclature often uses historical or traditional names that are not systematically derived. This system is widely used for simpler molecules, especially those with well-known common names.
1. Alkanes (Paraffins):
– Methane (CH4)
– Ethane (C2H6)
– Propane (C3H8)
– Butane (C4H10)
– Pentane (C5H12)
– Hexane (C6H14)
– Heptane (C7H16)
– Octane (C8H18)
– Nonane (C9H20)
– Decane (C10H22)
2. Alkenes (Olefins):
– Ethylene (C2H4)
– Propylene (C3H6)
– Butylene (C4H8)
3. Alkynes:
– Acetylene (C2H2)
4. Alcohols:
– Methyl alcohol (Methanol, CH3OH)
– Ethyl alcohol (Ethanol, C2H5OH)
– Isopropyl alcohol (Isopropanol, C3H7OH)
5. Carboxylic Acids:
– Formic acid (Methanoic acid, HCOOH)
– Acetic acid (Ethanoic acid, CH3COOH)
– Propionic acid (Propanoic acid, C2H5COOH)
– Butyric acid (Butanoic acid, C3H7COOH)
6. Aromatic Compounds:
– Benzene (C6H6)
– Toluene (Methylbenzene, C6H5CH3)
– Xylene (Dimethylbenzene, C6H4(CH3)2)
IUPAC Nomenclature System
The IUPAC system is systematic and follows a set of rules to name organic compounds, ensuring each compound has a unique name.
 IUPAC Rules for Alkanes, Alkenes, and Alkynes:
1. Alkanes (CnH2n+2):
– Name is derived from the number of carbon atoms in the longest continuous chain.
– Suffix: -ane.
– Prefixes indicate the number of carbons: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10).
Examples:
– Methane (CH4)
– Ethane (C2H6)
– Propane (C3H8)
– Butane (C4H10)
2. Alkenes (CnH2n):
– Name is based on the longest chain containing the double bond.
– Suffix: -ene.
– Numbering starts from the end nearest the double bond.
Examples:
– Ethene (C2H4)
– Propene (C3H6)
– But-1-ene (C4H8)
– But-2-ene (C4H8)
3. Alkynes (CnH2n-2):
– Name is based on the longest chain containing the triple bond.
– Suffix: -yne.
– Numbering starts from the end nearest the triple bond.
Examples:
– Ethyne (C2H2)
– Propyne (C3H4)
– But-1-yne (C4H6)
– But-2-yne (C4H6)
IUPAC Rules for Alcohols, Aldehydes, Ketones, Carboxylic Acids:
1. Alcohols (R-OH):
– Name is based on the longest chain containing the hydroxyl group.
– Suffix: -ol.
– Numbering starts from the end nearest the hydroxyl group.
Examples:
– Methanol (CH3OH)
– Ethanol (C2H5OH)
– Propan-1-ol (C3H7OH)
– Propan-2-ol (C3H7OH)
2. Aldehydes (R-CHO):
– Name is based on the longest chain containing the carbonyl group.
– Suffix: -al.
– Carbonyl group is always at position 1, so no need for a number.
Examples:
– Methanal (Formaldehyde, HCHO)
– Ethanal (Acetaldehyde, CH3CHO)
– Propanal (C2H5CHO)
3. Ketones (R-CO-R’):
– Name is based on the longest chain containing the carbonyl group.
– Suffix: -one.
– Numbering starts from the end nearest the carbonyl group.
Examples:
– Propan-2-one (Acetone, C3H6O)
– Butan-2-one (C4H8O)
4. Carboxylic Acids (R-COOH):
– Name is based on the longest chain containing the carboxyl group.
– Suffix: -oic acid.
– Carboxyl group is always at position 1, so no need for a number.
Examples:
– Methanoic acid (Formic acid, HCOOH)
– Ethanoic acid (Acetic acid, CH3COOH)
– Propanoic acid (C2H5COOH)
– Butanoic acid (C3H7COOH)
IUPAC Rules for Aromatic Compounds:
1. Simple Aromatic Compounds:
– Named as derivatives of benzene.
– Substituents are prefixed to the word benzene.
– Numbering starts from the substituent that comes first alphabetically.
Examples:
– Methylbenzene (Toluene, C6H5CH3)
– 1,2-Dimethylbenzene (o-Xylene, C6H4(CH3)2)
– 1,3-Dimethylbenzene (m-Xylene, C6H4(CH3)2)
– 1,4-Dimethylbenzene (p-Xylene, C6H4(CH3)2)
 Carbocyclic Compounds
1. Cycloalkanes:
– Named similarly to alkanes but with the prefix cyclo-.
– Numbering starts from a substituent if present.
Examples:
– Cyclopropane (C3H6)
– Cyclobutane (C4H8)
– Cyclopentane (C5H10)
2. Cycloalkenes:
– Named similarly to alkenes but with the prefix cyclo-.
– Numbering starts from the double bond.
Examples:
– Cyclopropene (C3H4)
– Cyclobutene (C4H6)
– Cyclopentene (C5H8)
Summary
The common system of nomenclature uses traditional names for compounds, which are widely recognized but not always systematic. The IUPAC system provides a systematic and standardized approach to naming organic compounds, based on specific rules that ensure each compound has a unique name reflecting its structure. This system is essential for clear communication and understanding in the scientific community.
