H H H Resonance Draw the Lewis structure for nitromethane, CH 3 NO - - PowerPoint PPT Presentation

Resonance Draw the Lewis structure of [CH2=CHCH2]+

C C H H H C H H C C H H H C H H C C H H H C H H C C H

H

C

H

H

H

C C H

H

C

H

H

H

C C H

H

C

H

H

H

+ + + . . + + +

Resonance Draw the Lewis structure for nitromethane, CH3NO2

N C O O H H H N C O O H H H

+

• .

. . . .. .. . . +

• .

. .. . . . . . .

Csp

3

Nsp

2

Osp

2

Osp

2

H1s H1s H1s Csp

3

Nsp

2

Osp

2

Osp

2

H1s H1s H1s

C N O O H H H + .. : : .. .. C N O O H H H + .. : : . .. .

Common structural classes where resonance is observed Compounds with allylic lone pairs

O O : : .. -

• ..

.. ..

Common structural classes where resonance is observed Compounds with allylic positive charge

+ + +

Common structural classes where resonance is observed Compounds with lone pairs next to positive charge

Br Br .. .. : + + . .. .

Nitromethane, CH3NO2, combines two structural classes where resonance is observed It is a compound with lone pairs next to positive charge and allylic lone pairs

N C O O H H H N C O O H H H

+

• .

. . . .. .. . . +

• .

. .. . . . . . .

Common structural classes where resonance is observed Compounds with pi bonds between atoms of different electronegativities

O O . . .. : : .. - +

Common structural classes where resonance is observed Compounds with aromatic rings

H H

H H

H H

The concepts of resonance

• 1. Resonance is not something that is happening to a molecule. It is a way to describe the electronic

distribution of a molecule.

• 2. Individual resonance isomers are imaginary, not real.
• a. They are representations of a hybrid structure
• b. The resonance hybrid is more stable than any individual resonance isomer contributing to it
• 3. Resonance isomers must be valid Lewis structures.
• 4. Resonance isomers differ only by the relative placement of their pi and nonbonding electrons in p
• rbitals.
• 5. Resonance isomers must have identical constitutions. Therefore, the hybridization states of the

constituent atoms cannot change.

• 6. Resonance isomers must have the same number valence electrons and the same number of

paired and unpaired electrons.

• 7. Different resonance isomers don’t have to be equivalent. Resonance isomers that describe the

electronic configuration of a molecule most realistically have

• a. all octets full
• b. minimal formal charge
• c. minimal formal charge separation
• d. formal charge assignment according to relative electronegativities

N H H H H

H

Heterocyclic aromatic compounds: pyridine

N H H H H H N H H H H H N H H H H H .. .. . .- + X

Consider resonance isomers of spinacine

.. . .. : O N N N O H H H . . . . . +

• ..