Q.Write IUPAC name of the following compound:
$CH_3-\underset{OH}{\underset{|}{CH}}-\underset{CH_3}{\underset{|}{CH}}-CH_2-OH$

Step 1. Identify the longest chain and count OH groups. The structure CH3-CH(OH)-CH(CH3)-CH2-OH has a 4-carbon main chain bearing two -OH groups (a diol) and one methyl branch on the third chain carbon (counting from the CH3 end as printed).

Step 2. Number from each end and compare locant sets. Numbering from the printed CH3 end: C1(CH3)-C2(OH)-C3(CH3 branch)-C4(CH2OH) -- OH locants {2,4}. Numbering instead from the CH2OH end: C1(CH2OH)-C2(CH3 branch)-C3(OH)-C4(CH3) -- OH locants {1,3}.

Step 3. Apply the lowest-locant rule for the principal group. Since -OH is the principal characteristic group here (expressed as the '-diol' suffix), the numbering direction must give the LOWEST locant SET to the -OH groups first, ahead of the methyl substituent's locant. Comparing {1,3} (from the CH2OH end) against {2,4} (from the CH3 end): {1,3} is lower at the very first point of difference (1 < 2), so numbering from the CH2OH end is correct.

Step 4. Assemble the name. With this numbering, the -OH groups sit at C1 and C3, and the methyl branch sits at C2. Name: 2-methylbutane-1,3-diol.

Comparing both possible numbering directions of the 4-carbon diol chain and selecting the one giving the lowest locant SET to the principal -OH groups (per the IUPAC rule in section 11.3.1), then citing the methyl substituent's locant under that same numbering

• Numbering from the wrong end (the end nearer the printed CH3, rather than the end that actually gives the OH groups the lower locant set), which would incorrectly give '3-methylbutane-1,3-diol' or a similarly wrong name.
• Giving the substituent (methyl) priority for the lowest locant instead of the principal -OH groups, which must always be numbered lowest first when present as the compound's suffix.