Overview
This lecture covers the rules for naming alkanes, focusing on identifying the parent chain, numbering substituents, and following alphabetical and numerical order conventions in naming.
Naming the Parent Chain
- Identify the longest continuous chain of carbon atoms to determine the parent alkane.
- Alkanes are named according to the number of carbons: methane (1), ethane (2), propane (3), butane (4), pentane (5), hexane (6), heptane (7), octane (8), nonane (9), decane (10).
Numbering and Positioning Substituents
- Number the chain to give substituents the lowest possible numbers.
- Substituents are side groups like methyl (-CH₃), ethyl (-C₂H₅), etc.
Naming with Single and Multiple Substituents
- Name as [number]-[substituent][parent chain], e.g., 2-methylpentane.
- For multiple identical groups, use prefixes (di-, tri-, tetra-) and list their positions separated by commas (e.g., 2,3-dimethylhexane).
- Write each position number separately if multiple identical groups are on the same carbon (e.g., 3,3,3-trimethyl hexane).
Alphabetizing Substituents
- List different substituents in alphabetical order, ignoring prefixes like di-, tri-, etc.
- Alphabetical order has priority over numerical order when listing substituents (e.g., 4-ethyl-3-methylheptane).
Numbering When Order is Ambiguous
- When numbering creates the same set of numbers both ways, choose the numbering that gives the substituent listed first in alphabetical order the lowest number.
Special Cases and Common Mistakes
- Do not alphabetize using prefixes (di-, tri-); use the actual group names for order.
- Always use hyphens between numbers and words, commas between numbers.
Key Terms & Definitions
- Alkane — Saturated hydrocarbon with single bonds only.
- Parent Chain — The longest continuous carbon chain in a molecule.
- Substituent — An atom or group of atoms replacing hydrogen on the parent chain.
- Methyl — CH₃ group.
- Ethyl — C₂H₅ group.
- Prefix — Di- (2), Tri- (3), Tetra- (4) indicate multiple identical substituents.
Action Items / Next Steps
- Practice naming alkanes with various substituent arrangements.
- Review the alkane parent chain names and rules for alphabetical order in naming.