Overview
This lecture covers how to systematically and commonly name complex substituents in organic chemistry, extending basic alkane nomenclature to branched and larger substituent groups.
Review of Simple Substituents
- Simple substituents are groups like methyl, ethyl, propyl, and butyl, attached by a single carbon chain.
- When naming alkanes, identify and number the longest continuous carbon chain as the parent chain.
Introduction to Complex Substituents
- Complex substituents are substituents that have their own branches (i.e., not straight-chain groups).
- Two naming methods exist: common names (for some small substituents) and systematic IUPAC naming (always applicable).
Systematic Naming of Complex Substituents
- Number the substituent’s longest chain starting at the carbon attached to the parent chain (always carbon 1).
- Name the substituent as an alkyl group using "-yl" and indicate branches as substituents on that group.
- List numbers and names of substituents on the main substituent, e.g., (1,2-dimethylpropyl).
- Enclose systematic complex substituent names in parentheses in the full compound name.
- Alphabetize complex substituents by the first letter inside the parentheses, including prefixes if present.
Common Names for Branched Substituents
- Three-carbon substituents: propyl (straight) and isopropyl (branched).
- Four-carbon substituents: butyl (straight), sec-butyl (attached via a secondary carbon), isobutyl (branched with a Y shape), tert-butyl (attached via a tertiary carbon).
- Sec- and tert-butyl are hyphenated and alphabetized by "B," while iso groups are not hyphenated and are alphabetized by "I."
- For five or more carbons, use only the systematic method.
Alphabetization and Parentheses Rules
- Systematic complex substituents in parentheses are alphabetized by the first letter inside, including prefixes such as di-, tri-.
- Common names are not placed in parentheses and are alphabetized as usual.
Examples and Practice
- When naming entire molecules, always choose the longest parent chain and assign the lowest numbers to substituents.
- When multiple naming options exist (e.g., common vs systematic), either may be used if IUPAC allows.
- Order of substituent names can change based on which naming system is used, affecting alphabetical listing.
Key Terms & Definitions
- Substituent — An atom or group attached to the main carbon chain.
- Complex substituent — A substituent with its own branching.
- Systematic name — IUPAC-approved approach for naming compounds using standardized rules.
- Common name — Traditional, often shorter or widely used names for certain groups (e.g., isopropyl, tert-butyl).
- Parent chain — The longest continuous carbon chain in a molecule.
- Isopropyl — A branched three-carbon group attached by the middle carbon.
- Sec-butyl — Four-carbon group attached by a secondary carbon.
- Isobutyl — Four-carbon group with a Y-shaped branch at the end.
- Tert-butyl — Four-carbon group attached by a tertiary carbon.
Action Items / Next Steps
- Practice identifying and naming complex substituents both systematically and by common names.
- Memorize common names for three- and four-carbon branched substituents.
- Review how to apply alphabetical order and parentheses in naming compounds.