Alkane Naming Rules

Overview

This lecture covers the systematic IUPAC rules for naming alkanes, focusing on identifying parent chains, naming and numbering substituents, handling multiple or branched substituents, and resolving naming ambiguities.

Steps for Systematic Naming of Alkanes

• Identify the longest continuous carbon chain (parent chain), which determines the base name (e.g., pentane, hexane, octane).
• Number the parent chain to give the first substituent the lowest possible number.
• Name substituents based on the number of carbons: methyl (1), ethyl (2), propyl (3), butyl (4), etc.
• Place substituent names in front of the parent chain with their position numbers, separated from letters by dashes (e.g., 2-methylpentane).
• When multiple substituents are present, number the chain to produce the lowest set of numbers for all substituents.
• List substituents in alphabetical order; ignore numerical prefixes (di-, tri-, etc.) for alphabetizing except with iso.
• For multiple identical substituents, use prefixes (di-, tri-, tetra-) and separate their numbers with commas (e.g., 2,4-dimethylhexane).
• If two chains are the same length, choose the one with more substituents as the parent chain.

Special Cases in Naming

• If numbering gives the same lowest number to a substituent in both directions, assign the next substituent the next lowest number.
• If both numbering directions are equivalent, the substituent that comes first alphabetically gets the lower number.
• Halogen substituents are named by dropping the "-ine" and adding "-o" (e.g., chloro, bromo).

Naming Branched Substituents

• Number the branched substituent starting from the attachment point to the parent chain.
• The carbon attached directly to the parent is always carbon 1 in the substituent.
• Name the substituent in parentheses (e.g., 1-methylethyl), or use the common name if available (e.g., isopropyl).
• Alphabetize parenthetical names, including the "di", "tri" prefix for branched cases.

Key Terms & Definitions

• Parent Chain — the longest continuous chain of carbon atoms in the molecule.
• Substituent — a group of atoms attached to the parent chain, replacing a hydrogen.
• Alkyl Group — a substituent derived from an alkane by removing a hydrogen (e.g., methyl, ethyl).
• Branched Substituent — a substituent itself containing branches or side chains.
• IUPAC Naming — systematic method for naming chemical compounds.

Action Items / Next Steps

• Practice identifying parent chains and naming substituents on provided structures.
• Complete homework problems on naming alkanes, especially those with branched substituents.
• Use online tools for skeletal structure naming practice as recommended.