Select the correct IUPAC name for the branched alcohol.

The IUPAC name of the alcohol is:

2-ethyl-4-methyl-5-propyl-1-hexanol
2-ethyl-4,5-dimethyl-1-octanol
4,5-dimethyl-7-propyl-8-octanol
3-hydroxymethyl-5,6-dimethylnonane

The Correct Answer and Explanation is:

The correct IUPAC name is:
2-ethyl-4,5-dimethyl-1-octanol

Explanation:

To determine the correct IUPAC name for a branched alcohol, we follow the IUPAC nomenclature rules:

Step 1: Identify the Longest Carbon Chain Containing the OH Group

In IUPAC nomenclature for alcohols, the longest continuous carbon chain must include the hydroxyl (-OH) group. In this case, the correct name “2-ethyl-4,5-dimethyl-1-octanol” suggests the parent chain is octane (8 carbon atoms) and the OH group is on carbon 1.

Step 2: Number the Carbon Chain

Number the carbon atoms in such a way that the hydroxyl group gets the lowest possible number. That’s why the OH is at position 1 in “1-octanol”.

So the main chain is:

CH₃–CH(Et)–CH₂–CH(Me)–CH(Me)–CH₂–CH₂–CH₂OH

Which gives us the following numbering:

–OH on C1
Ethyl on C2
Methyl on C4 and C5

This matches “2-ethyl-4,5-dimethyl-1-octanol”

Step 3: Name the Substituents and Their Positions

Ethyl group at carbon 2 → “2-ethyl”
Methyl groups at carbons 4 and 5 → “4,5-dimethyl”
OH group at carbon 1 → “1-octanol”

Why the Other Options Are Incorrect:

2-ethyl-4-methyl-5-propyl-1-hexanol
→ Parent chain is only 6 carbons (hexanol), even though a longer chain exists. Violates Rule 1.
4,5-dimethyl-7-propyl-8-octanol
→ The OH group is on carbon 8, which is not the lowest possible number. Should be carbon 1.
3-hydroxymethyl-5,6-dimethylnonane
→ The –OH group is not on the main chain; it’s in a substituent (hydroxymethyl). Less preferred.

Conclusion:

“2-ethyl-4,5-dimethyl-1-octanol” correctly follows all IUPAC rules:
✔ Longest chain includes OH
✔ OH has the lowest possible number
✔ Substituents are correctly named and located

Hence, it is the correct IUPAC name.