What is the IUPAC name of the following compound?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 1-Bromo-2-methylbut-2-ene
The compound is \( \ce{CH3-CH=C(CH3)-CH2Br} \). To name an alkene, choose the longest chain that contains the double bond, number it so that the double bond gets the lowest locant, and, if that is the same from both ends, so that the substituents get the lowest locants. Prefixes are written in alphabetical order.
The longest chain through the double bond has four carbons (but-ene), running from the \( \ce{CH2Br} \) carbon to the \( \ce{CH3} \) at the other end. The double bond lies between C2 and C3 whichever end is chosen, so the substituents decide:
From the \( \ce{CH2Br} \) end: bromo at C1, methyl at C2, locants {1, 2}.
From the other end: methyl at C3, bromo at C4, locants {3, 4}.
The set {1, 2} is lower, so the name is 1-bromo-2-methylbut-2-ene.