Secondary alcohol C5H12O dehydrates to C5H10 alkene (M+=70)

Steps:

• Secondary alcohol has OH on carbon bonded to two alkyl groups.
• Dehydration yields alkene by losing H2O (mass loss of 18).
• Alkene M+=70 corresponds to C5H10 (5×12 + 10×1 = 70), so alcohol is C5H12O (M+=88).
• Evaluate choices for secondary C5H12O structure.

Why B is correct:

• B is 3-pentanol (CH3CH2CH(OH)CH2CH3), a secondary alcohol (C5H12O) that dehydrates to pentene (C5H10, M+=70) per Zaitsev's rule.

Why the others are wrong:

• A: 1-Pentanol is primary (OH on terminal carbon).
• C: 2-Methyl-2-butanol is tertiary (OH on carbon with three alkyl groups).
• D: Cyclohexanol is C6H12O (M+=100), not C5H12O.

Final answer: B