What NEET Asks
- Questions on IUPAC nomenclature of unsaturated hydrocarbons (alkenes, alkynes) are frequently asked.
- Expect direct naming of given structures or drawing structures from names. Often combined with isomerism concepts.
- Typically carries 4 marks for a correct answer, with negative marking for incorrect ones.
Key Points
- Longest Carbon Chain: Select the longest continuous carbon chain that includes the maximum number of double/triple bond(s), even if a longer chain without them exists.
- Numbering Rule: Start numbering from the end closest to the multiple bond. This gives the lowest possible locant to the multiple bond.
- Multiple Bonds Priority: If equidistant from both ends, the double bond (alkene) gets preference over the triple bond (alkyne) for lower numbering.
- Naming Order: Name substituents alphabetically, followed by the parent chain prefix, then locant(s) for multiple bonds, and finally the suffix (-ene for double, -yne for triple).
- Both Double & Triple Bonds: Use the suffix '-en-yne'. The double bond position is stated first, then the triple bond position.
Must-Know Rule Summary
- Rule: Identify the longest carbon chain containing the maximum number of multiple bonds. Number from the end that gives the lowest locant to the first multiple bond. If equidistant, a double bond receives lower numbering than a triple bond.
Common Mistakes
- Students often fail to include all multiple bonds in the principal chain, or choose the absolutely longest chain without including them.
- Don't confuse numbering priority: multiple bonds > substituents. Double bond > triple bond only if equidistant in the numbering decision.
- Forgetting the '-en-yne' suffix order (always double bond first, then triple bond) and incorrect numbering when both are present.
Rapid Revision
Prioritize multiple bonds for parent chain selection and numbering. Always give the lowest possible locant to the first multiple bond encountered. If a tie in numbering from both ends, the double bond gets the lower number. Remember the '-en-yne' suffix order.