Advices For Studying Organic Chemistry

13.5 SUMMARY OF RULES FOR RESONANCE

13.5A RULES FOR WRITING RESONANCE STRUCTURES

1. Resonance structures exist only on paper.
   
   
   1. Resonance structures are useful because they allow us to describe molecules,
      radicals, and ions for which a single Lewis structure is inadequate.
   
   
   2. Resonance structures or resonance contributors are connected by double-headed
      arrows (↔) ⇒ the real molecule, radical, or ion is a hybrid of all of them.
   
   
   
   


2. In writing resonance structures, only electrons are allowed to be moved.
   
   
   1. The positions of the nuclei of the atoms must remain the same in all of the
      structures.
   
   
   
   

CH 3 CH+ CH CH 2 CH 3 CH CH CH+ 2 CH+ 2 CH 2 CH CH 2

123

These are resonance structures for

the allylic cation formed when

1,3-butadiene accepts a prooton.

This is not a proper resonance

structure for the allylic cation

because a hydrogen atom has

been moved.

2) Only the electrons of π bonds and those of lone pairs are moved.

3. All of the structures must be proper Lewis structures.

This not a proper resonance structure for methanol

because carbon has five bonds.

Elements of the first major row of the periodic table

cannot have more than eight electrons in their valence.

C

H

H

H − O+ H

4. All structures must have the same number of unpaired electrons.

H 2 C CH 2

CH

H 2 C CH 2

CH

This is not a proper resonance structure for the allyl radical because it does not

contain the same number of unpaired electrons as CH 2 =CHCH 2 •.