Why does a peptide bond possess partial double bond character and restricted rotation?
Answer and explanation
Correct answer: A. Resonance delocalization of the nitrogen lone pair into the carbonyl pi system
The peptide bond (-CO-NH-) has ~40% double bond character due to resonance between O=C-NH- and O(-)=C=NH(+)-. This results in a planar geometry, shorter C-N distance (1.32 A vs 1.47 A for single bond), and prevents free rotation around the C-N axis.
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