4.1 IUPAC Nomenclature of Alkanes and Cycloalkanes

Overview

This lecture covers the IUPAC rules for naming alkanes and cycloalkanes, including identifying the parent chain, naming substituents, and correctly numbering and ordering substituents.

Basics of Alkanes and Nomenclature

• Alkanes are molecules with only carbon and hydrogen, containing single C–C bonds.
• The parent chain is the longest continuous chain of carbon atoms in a molecule.
• Substituents are groups not part of the parent chain, often alkyl groups.
• The parent chain uses the "-ane" suffix; substituents use the "-yl" suffix.

Prefixes for Chain Length

• 1: meth-, 2: eth-, 3: prop-, 4: but-, 5: pent-, 6: hex-, 7: hept-, 8: oct-, 9: non-, 10: dec-
• 11: undec-, 12: dodec- (usually required only up to 10 or 12 carbons)
• Example: 6-carbon chain is hexane; a 2-carbon substituent is ethyl.

Rules for Naming Alkanes

• Name substituents first, then the parent chain.
• Assign the lowest possible number (chain locator) to substituents.
• When multiple substituents are present, use alphabetical order to name them, not by chain locator.
• Use hyphens between numbers and letters; use commas between numbers.
• Multiple identical substituents use prefixes: di-, tri-, tetra-, etc., and list all locations (e.g., 2,3,6-trimethyl).

Choosing and Numbering the Parent Chain

• If there’s a tie for the longest chain, choose the path with more substituents.
• When a tie exists in numbering, use the next substituent’s position or alphabetical order as a tiebreaker.

Cycloalkane Nomenclature

• Ring structures are named with a "cyclo" prefix (e.g., cyclohexane).
• A ring is usually the parent chain unless a longer linear chain is present.
• Single substituent: automatically at position 1; omit the number in the name (e.g., chlorocyclohexane).
• Multiple substituents: assign numbers to give the lowest set of locators; use alphabetical order for numbering ties.
• Halogen substituents: end in -o (fluoro-, chloro-, bromo-, iodo-).

Special Cases

• If the ring is a substituent, use the "-yl" suffix (e.g., cyclobutyl).
• Do not mix ring and chain when picking the parent chain; it's one or the other.

Key Terms & Definitions

• Parent chain — the longest continuous carbon chain in a molecule.
• Substituent — a group attached to the parent chain, not included within it.
• Chain locator — the number assigned to a carbon atom to indicate substituent position.
• Cycloalkane — a ring-shaped alkane; use “cyclo” prefix.
• Alkyl group — substituent formed by removing one hydrogen from an alkane (e.g., methyl, ethyl).

Action Items / Next Steps

• Practice naming alkanes and cycloalkanes using provided rules.
• Review and memorize numerical prefixes and alkyl group names.
• Complete any assigned homework or practice problems on alkane nomenclature.