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Difficulty: MediumStructural Isomerism and Stereoisomerism

An organic compound with the molecular formula C4H10OC_4H_{10}O forms four isomeric alkanols. Which of these alkanol isomers contains an asymmetric carbon atom and exhibits optical isomerism?

  1. A
    Butan-1-ol
  2. Butan-2-olAnswer
  3. C
    2-Methylpropan-1-ol
  4. D
    2-Methylpropan-2-ol

Answer

Butan-2-ol is the only isomeric alkanol of formula C4H10OC_4H_{10}O containing a chiral carbon atom.
Butan-2-ol has a chiral carbon at position 2 (CH3−CH(OH)−CH2−CH3CH_3-CH(OH)-CH_2-CH_3), which is bonded to four different substituent groups: −H-H, −OH-OH, −CH3-CH_3, and −CH2CH3-CH_2CH_3. The presence of this asymmetric center causes optical activity.

Step-by-Step Solution

1
Identify the requirement for optical isomerism.
A molecule exhibits optical isomerism if it contains at least one chiral (asymmetric) carbon atom—a carbon atom bonded to four different groups or atoms.
Chirality causes non-superimposable mirror images (enantiomers) capable of rotating plane-polarized light.
2
Analyze the structural formulas of the four isomeric alkanols of C4H10OC_4H_{10}O.
1) Butan-1-ol: CH3−CH2−CH2−CH2OHCH_3-CH_2-CH_2-CH_2OH
2) Butan-2-ol: CH3−CH(OH)−CH2−CH3CH_3-CH(OH)-CH_2-CH_3
3) 2-Methylpropan-1-ol: (CH3)2CH−CH2OH(CH_3)_2CH-CH_2OH
4) 2-Methylpropan-2-ol: (CH3)3C−OH(CH_3)_3C-OH
Writing structural formulas reveals the substituent groups attached to each carbon atom.
3
Examine carbon-2 in butan-2-ol.
Carbon-2 is attached to −H-H, −OH-OH, −CH3-CH_3, and −CH2CH3-CH_2CH_3. All four substituents are distinct.
Since carbon-2 has four different attached groups, it is an asymmetric (chiral) carbon atom.

Key Concept

Optical isomerism and chirality in alkanols
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Topics

Structural Isomerism and Stereoisomerism

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