What is the correct IUPAC name of the following compound?

Step 1: Identify the parent chain and principal characteristic group

The molecule is a cyclopentene (5-membered ring with one double bond) with an $\ce{-OH}$ group and an ethyl ($\ce{-C2H5}$) substituent. The $\ce{-OH}$ group is the principal characteristic group, so the suffix is -ol. The parent name is cyclopent-2-en-1-ol (the ring).

Step 2: Number the ring to give the -OH the lowest locant

In IUPAC 2013 rules for cyclic alcohols:

• The carbon bearing $\ce{-OH}$ gets locant 1 (since -ol suffix takes priority).
• The double bond runs C2–C3 (giving the lowest locant to the double bond).
• The ethyl group is at C4.

Step 3: Common wrong approaches

Students often number to give the double bond locant 1, which is incorrect — the principal characteristic group ($\ce{-OH}$) always gets the lowest possible locant before the double bond in IUPAC nomenclature.

Another mistake is using "hydroxy" as a prefix (option a style) — when $\ce{-OH}$ is the principal group, it must appear as the suffix "-ol", not as "hydroxy-".

Step 4: Verify

• Parent: cyclopentane → cyclopentene (double bond) → cyclopent-2-en-1-ol (OH at C1, double bond at C2)
• Substituent: ethyl at C4
• Full name: 4-Ethylcyclopent-2-en-1-ol ✓

Key Points to Remember:

• In IUPAC nomenclature, the principal characteristic group (-OH for alcohols) gets the lowest locant.
• The suffix "-ol" is used, not "hydroxy-" as prefix, when -OH is the principal group.
• Numbering direction is chosen to give lowest locant set to substituents after fixing the principal group position.