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Basics of Organic Chemistry question

2006 · Shift 0 · Q9
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Basics of Organic Chemistry question

2006 · Shift 0 · Q9

JEE MainChemistryBasics of Organic ChemistryMCQ+4 / −1
Increasing order of stability among the three main conformations (i.e. Eclipse, Anti, Gauche) of 2-fluoroethanol is
  1. A
    Eclipse, Gauche, Anti
  2. B
    Gauche, Eclipse, Anti
  3. C
    Eclipse, Anti, Gauche
  4. D
    Anti, Gauche, Eclipse
View written solutionFree

Correct answer: C

  1. Identify the molecule and the relevant conformations

    2-fluoroethanol is HO−CH2−CH2−F\mathrm{HO{-}CH_2{-}CH_2{-}F}HO−CH2​−CH2​−F.
    Rotation about the C−C\mathrm{C{-}C}C−C bond gives three main conformations:

    • Eclipsed
    • Gauche: OH\mathrm{OH}OH and F\mathrm{F}F are staggered with dihedral angle 60∘60^\circ60∘
    • Anti: OH\mathrm{OH}OH and F\mathrm{F}F are opposite, dihedral angle 180∘180^\circ180∘
  2. General stability of eclipsed vs staggered

    Eclipsed conformations are usually least stable because of:

    • torsional strain
    • unfavorable eclipsing interactions

    Therefore, Eclipsed is least stable\text{Eclipsed is least stable}Eclipsed is least stable

  3. Compare Gauche and Anti in 2-fluoroethanol

    Normally, for simple substituted ethanes, anti is more stable than gauche due to less steric repulsion.
    However, in HO−CH2−CH2−F\mathrm{HO{-}CH_2{-}CH_2{-}F}HO−CH2​−CH2​−F, the gauche conformation is especially stabilized.

  4. Reason for gauche stabilization

    In the gauche form, OH\mathrm{OH}OH and F\mathrm{F}F come close enough for favorable intramolecular interaction. This is commonly attributed to the gauche effect and can also be viewed as a weak intramolecular hydrogen-bonding/electrostatic stabilization between O−H\mathrm{O{-}H}O−H and F\mathrm{F}F.

    Thus, Gauche is more stable than Anti\text{Gauche is more stable than Anti}Gauche is more stable than Anti

  5. Overall increasing order of stability

    Combining the above: Eclipsed<Anti<Gauche\text{Eclipsed} < \text{Anti} < \text{Gauche}Eclipsed<Anti<Gauche

  6. Match with options

    • A: Eclipse, Gauche, Anti →\to→ wrong
    • B: Gauche, Eclipse, Anti →\to→ wrong
    • C: Eclipse, Anti, Gauche →\to→ correct
    • D: Anti, Gauche, Eclipse →\to→ wrong

Therefore, the correct answer is: C: Eclipse, Anti, Gauche\boxed{\text{C: Eclipse, Anti, Gauche}}C: Eclipse, Anti, Gauche​

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