What NEET Asks
- Direct IUPAC naming of complex organic molecules with multiple functional groups.
- Identification of the principal functional group (PFG) in a given structure.
- Questions often involve selecting the correct parent chain and assigning locants based on priority.
Key Points
- Principal Functional Group (PFG): The functional group with the highest priority in a polyfunctional compound dictates the suffix of the IUPAC name. All other functional groups are treated as substituents and named with prefixes.
- Longest Chain: The parent chain must include the PFG and the maximum number of carbons, ideally including other functional groups or multiple bonds.
- Numbering: Start numbering the parent chain from the end that gives the PFG the lowest possible locant. If PFG is at the chain end (e.g., -COOH, -CHO, -CN), its carbon is always C-1.
- Prefixes & Suffixes: Each functional group has a specific prefix (when it's a substituent) and a suffix (when it's the PFG). E.g., -OH is 'hydroxy' as prefix, 'ol' as suffix.
- Alphabetical Order: When multiple substituents are present, they are listed alphabetically in the name, ignoring numerical prefixes like di-, tri-.
Must-Know Formula / Reaction
Functional Group Priority Order: -COOH > -SO₃H > -COOR > -COCl > -CONH₂ > -CN > -CHO > -CO- > -OH > -NH₂ > -C≡C- > -C=C- > -X (Halo) > -NO₂ > -OR (Alkoxy) > -R (Alkyl)
- Explanation: This hierarchy determines which functional group is the principal one, taking the primary suffix. Others are prefixes.
Common Mistakes
- Students often forget to correctly identify the principal functional group, leading to incorrect parent chain selection and numbering.
- Don't confuse the prefix and suffix forms of functional groups (e.g., 'hydroxy' for -OH as prefix vs. 'ol' as suffix).
- Incorrectly applying the lowest locant rule, especially when multiple functional groups or multiple bonds are present alongside the PFG.
Rapid Revision
Always identify the highest priority functional group first to determine the suffix. Select the longest carbon chain containing this PFG and number it to give the PFG the lowest possible locant. List other functional groups as prefixes alphabetically.