(i) Draw the structures of all isomeric alcohols of molecular formula C₅H₁₂O and give their IUPAC names. (ii) Classify the isomers of alcohols in question 7.3 (i) as primary, secondary and tertiary alcohols.
Structures and IUPAC names of isomeric alcohols (C₅H₁₂O):
Primary Alcohols (4 isomers):
- Pentan-1-ol: CH₃-CH₂-CH₂-CH₂-CH₂OH
- 2-Methylbutan-1-ol: CH₂OH-CH(CH₃)-CH₂-CH₃
- 3-Methylbutan-1-ol: (CH₃)₂CH-CH₂-CH₂OH
- 2,2-Dimethylpropan-1-ol: (CH₃)₃C-CH₂OH
Secondary Alcohols (3 isomers): 5. Pentan-2-ol: CH₃-CH₂-CH₂-CH(OH)-CH₃ 6. Pentan-3-ol: CH₃-CH₂-CH(OH)-CH₂-CH₃ 7. 3-Methylbutan-2-ol: CH₃-CH(OH)-CH(CH₃)₂
Tertiary Alcohol (1 isomer): 8. 2-Methylbutan-2-ol: (CH₃)₂C(OH)-CH₂-CH₃
Classification of Isomers:
Primary Alcohols (1°): The —OH group is attached to a primary carbon atom (carbon bonded to only one other carbon).
- Pentan-1-ol
- 2-Methylbutan-1-ol
- 3-Methylbutan-1-ol
- 2,2-Dimethylpropan-1-ol
Secondary Alcohols (2°): The —OH group is attached to a secondary carbon atom (carbon bonded to two other carbons).
- Pentan-2-ol
- Pentan-3-ol
- 3-Methylbutan-2-ol
Tertiary Alcohol (3°): The —OH group is attached to a tertiary carbon atom (carbon bonded to three other carbons).
- 2-Methylbutan-2-ol
Explanation
This question tests understanding of structural isomerism in alcohols and classification based on the degree of the carbon bearing the —OH group. The molecular formula C₅H₁₂O corresponds to saturated monohydric alcohols. Students must systematically derive all possible carbon skeletons (n-pentane, isopentane/2-methylbutane, and neopentane/2,2-dimethylpropane) and place the —OH group at all unique positions. Primary alcohols have —OH on a carbon bonded to one other carbon (—CH₂OH), secondary on a carbon bonded to two others (—CHOH—), and tertiary on a carbon bonded to three others (—C(OH)—).