Overview
This lecture introduces the rules for naming branched alkanes in organic chemistry by identifying the longest carbon chain and correctly numbering branches for systematic IUPAC naming.
Identifying the Longest Chain
- Find the longest continuous chain of carbon atoms; this determines the root name of the molecule.
- The root name is based on the number of carbons in the longest chain (e.g., nonane for nine carbons).
Naming Alkyl Branches
- Branches attached to the main chain are called alkyl groups and are named using prefixes (e.g., methyl for one carbon, ethyl for two, propyl for three, butyl for four).
- Replace the "-ane" ending with "-yl" for branches (e.g., methane → methyl).
Numbering the Chain
- Number the main chain so that the branch or substituent gets the lowest possible number.
- Indicate the position of the branch before its name using numbers and hyphens (e.g., 3-methylnonane means a methyl group on the third carbon of nonane).
Constructing and Deconstructing Names
- For names like "2-propylheptane," the main chain is heptane (seven carbons), and a propyl group (three carbons) is attached to the second carbon.
- For "6-butyltetradecane," the main chain is tetradecane (fourteen carbons), and a butyl group (four carbons) is on the sixth carbon.
Correct Numbering and Avoiding Redundancy
- Always number from the end nearest a branch to minimize the numbers used (e.g., don't use 9-butyltetradecane if 6-butyltetradecane is possible).
- Lowest set of numbers rule ensures clarity and avoids ambiguity in naming.
Key Terms & Definitions
- Alkane — Hydrocarbon with only single bonds between carbons.
- Alkyl group — A side chain formed by removing a hydrogen from an alkane (named with "-yl" ending).
- Longest chain — The primary sequence of carbons used for base naming.
- Prefix — Indicates branch identity/size (meth-, eth-, prop-, but-).
- Numbering — Assigning numbers to chain carbons to locate substituents.
Action Items / Next Steps
- Memorize alkane and alkyl prefixes up to at least ten carbons.
- Practice naming and drawing branched alkanes using these rules.
- Prepare for more complex naming in upcoming lectures.