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Question #44253

Difficulty: 8/10 | 🟢 Free

The Question

Consider the peptide linkage in $N$ -methylacetamide (\ce{CH3CONHCH3}) at $298\text{ K}$ . Due to restricted rotation, the \ce{O-C-N} atomic framework is coplanar and lies entirely in the $xy$ -plane. The delocalized $\pi$ -system is formed by the overlap of the unhybridized $p_z$ orbitals of Oxygen, Carbon, and Nitrogen. According to Molecular Orbital Theory, which of the following accurately describes the nodal planes of the Highest Occupied Molecular Orbital (HOMO) of this 3-center, 4-electron $\pi$ -system?

Options

A.
Two planar nodes: the $xy$ -plane and a plane perpendicular to the $xy$ -plane passing through the carbonyl carbon
B.
One planar node: the $xy$ -plane, representing the localized $p_z$ orbital of the nitrogen atom
C.
One planar node: the $xy$ -plane, representing the fully delocalized bonding $\pi$ -molecular orbital
D.
Zero planar nodes, as the nitrogen lone pair resides in an $sp^3$ hybridized orbital

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Angular Nodes and Planar Nodes|Hybridization and Geometry|Peptide Bond Planarity

Why This Question Is Tricky

Common Mistakes

The question tests the transition from localized Valence Bond Theory to Molecular Orbital Theory in the context of peptide bond planarity.

The 'hidden variable' is the nodal structure of the 3-center 4-electron $\pi$ -system.

Students often memorize that $sp^2$ hybridization implies planarity but fail to recognize that resonance creates a new MO (the non-bonding HOMO) which introduces an additional vertical nodal plane at the central carbon atom.

Option B is the primary trap, catching students who assume the nitrogen lone pair remains in an isolated $p_z$ orbital with only one nodal plane.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Structure of Atom - Class 11 Part 1
Concept	Angular Nodes and Planar Nodes
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free