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Which of the following conformers for ethylene glycol is most stable?
A
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B
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C
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D
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Detailed Solution
Ethylene glycol is $HOCH_2-CH_2OH$; its conformations arise by rotation about the C–C bond.
Eclipsed conformations have torsional strain and are less stable than staggered ones.
Normally the anti (staggered) conformation, with the two bulky groups $180^\circ$ apart, would be the most stable.
In ethylene glycol, however, the gauche (staggered) conformation, in which the two –OH groups are $60^\circ$ apart, allows an intramolecular hydrogen bond between the –OH groups.
This hydrogen bond more than compensates for the steric repulsion, so the gauche form is more stable than the anti form.
Hence the most stable conformer is the staggered Newman projection in which the two –OH groups are on adjacent positions (gauche).
Eclipsed conformations have torsional strain and are less stable than staggered ones.
Normally the anti (staggered) conformation, with the two bulky groups $180^\circ$ apart, would be the most stable.
In ethylene glycol, however, the gauche (staggered) conformation, in which the two –OH groups are $60^\circ$ apart, allows an intramolecular hydrogen bond between the –OH groups.
This hydrogen bond more than compensates for the steric repulsion, so the gauche form is more stable than the anti form.
Hence the most stable conformer is the staggered Newman projection in which the two –OH groups are on adjacent positions (gauche).