Counting C4H10O isomers with branched skeleton and secondary alcohol

Steps:

• List structural isomers of C4H10O alcohols: butan-1-ol (primary, linear), butan-2-ol (secondary, linear), 2-methylpropan-1-ol (primary, branched), 2-methylpropan-2-ol (tertiary, branched).
• Select secondary alcohols: only butan-2-ol, where OH attaches to a carbon bonded to two alkyl groups (methyl and ethyl).
• Assess branched skeleton: butan-2-ol qualifies as the functional carbon branches into distinct alkyl chains.
• Confirm no others match both criteria simultaneously.

Why A is correct:

• The structural formula requires exactly one isomer where the secondary carbon creates the branch point in the skeleton, defining butan-2-ol uniquely.

Why the others are wrong:

• B. No second isomer exists; branched options are primary or tertiary.
• C. Overcounts nonexistent additional matches.
• D. Equals total alcohol isomers, ignoring specific constraints.

Final answer: A