Organic Chemistry: Resonance and Mesomerism

Questions and Answers

What is resonance or mesomerism in organic chemistry?

A method to draw a single structure for a molecule

A representation of a molecule as a weighted average of two or more hypothetical structures (correct)

A condition where a molecule has multiple structures with different atomic positions

A process of changing atomic positions in a molecule

What are resonance structures in a molecule?

Hypothetical structures with different arrangement of electrons but identical positions for atoms (correct)

Actual structures of a molecule with different atomic positions

Delocalized electrons in a molecule

Energy levels of a molecule

What is the resonance hybrid?

The hypothetical structure with the highest energy

The structure with the lowest energy

The weighted average of contributing resonance structures (correct)

The actual structure of the molecule with changed atomic positions

Why is the energy of the resonance hybrid less than the energy of any contributing resonance structure?

Due to the delocalization of electrons Signup and view all the answers

What is delocalization of electrons in resonance?

A hypothetical process shown using curved arrows Signup and view all the answers

What do curved arrows represent in resonance?

Delocalization of electrons Signup and view all the answers

What is true about the resonance structures in a molecule?

They are imaginary and the molecule resonates between them Signup and view all the answers

What happens to the positions of atoms in resonance structures?

They remain the same Signup and view all the answers

What is the expected behavior of urea based on its initial structure?

It should be a diacidic base with a normal C-N bond length and no dipole moment Signup and view all the answers

What is the reason for the shorter than expected bond length of C-N bond in urea?

Due to the resonance hybrid of the actual structure Signup and view all the answers

What is the condition for valid resonance structures?

They must be valid Lewis structures obeying octet rule and have same number of electrons Signup and view all the answers

What is the condition for the atoms in resonance structures?

The positions of atoms should be same in all the resonance structures Signup and view all the answers

What is the reason for the planar arrangement of atoms in resonance structures?

To get maximum overlap between the orbitals Signup and view all the answers

What is the term used to describe the molecules that are in dynamic equilibrium with each other?

Tautomers Signup and view all the answers

Why is the nitrogen atom in structure (II) not allowed?

Because it has 10 electrons around it Signup and view all the answers

What is the symbol used to link resonance structures?

Double headed arrow Signup and view all the answers

Study Notes

Resonance or Mesomerism in Organic Chemistry

Sometimes, a molecule or ion cannot be represented by a single structure, and multiple structures must be proposed to explain its properties.
The representation of a molecule's structure as a weighted average of two or more hypothetical structures is referred to as resonance or mesomerism or delocalization.

Salient Features of Resonance

Resonance structures or canonical forms or contributing structures have different arrangements of electrons but identical positions for atoms.
Resonance structures are imaginary, and the actual structure of the molecule is considered a hybrid of all valid resonance structures, known as the resonance hybrid.
The energy of the resonance hybrid is always less than the energy of any contributing resonance structure.
Resonance structures are formed due to delocalization of electrons, not by changing atom positions.
Delocalization of electrons is shown using curved arrows.
The actual molecule is simply the hybrid of all imaginary resonance structures.

Illustration of Resonance

The structure of urea (I) is expected to have certain properties, but observed properties differ, such as being a monoacidic base, having a shorter C-N bond length, and showing a dipole moment.
To account for these observations, the actual structure of urea is represented as a resonance hybrid of multiple resonance structures.

Rules for Writing Valid Resonance Structures

Resonance structures must satisfy the following rules:
- They must be valid Lewis structures obeying the octet rule.
- They should possess the same number of electrons and equal net charge.
- The number of unpaired electrons in them must be the same.
- The positions of atoms should be the same in all resonance structures.
- The bond order of two connecting atoms may vary between two different resonance structures.
- Resonance structures may or may not be equivalent.
- Atoms that are part of the delocalized system must be arranged in one plane or nearly so.

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Description

Learn about resonance or mesomerism in organic chemistry, where multiple structures are proposed to explain observed properties of a molecule or ion.