Drawing Resonance Structures: Recognize the Patterns

Common resonance patterns:

  1. πœ‹ Bond Between Atoms of Differing Electronegativity:
    A πœ‹ bond is formed between two atoms with different electronegativities (e.g., C=O, C=NR).
    The more electronegative atom pulls the πœ‹ electrons towards itself, forming a lone pair and a negative charge.
    The other atom, now lacking the electrons, develops a positive charge.

  2. Lone Pair Next to a Positive Charge:
    A lone pair on an atom is adjacent to a positively charged carbon atom.
    The lone pair shifts to form a new πœ‹ bond with the positively charged carbon, neutralizing the charge on the carbon.
    This results in a new resonance structure with a positive charge on the atom that donated the lone pair.

  3. Allylic Carbocation:
    A positively charged carbon atom is next to a carbon-carbon double bond (πœ‹ bond).
    The πœ‹ electrons shift to form a new πœ‹ bond with the positively charged carbon, neutralizing the charge on the carbon.
    This results in a new resonance structure with a positive charge on the adjacent carbon.

  4. Lone Pair Next to a πœ‹ Bond (Allylic Lone Pair): A lone pair on an atom is located next to a carbon-carbon double bond (πœ‹ bond).
    The lone pair can shift to form a new πœ‹ bond, pushing the existing πœ‹ bond electrons onto the adjacent carbon atom.
    This results in a new resonance structure with the lone pair on the adjacent carbon.
    Example: An allylic alcohol (R-CH=CH-CH2-OH).

  5. Conjugated πœ‹ Bonds in a Ring:
    Alternating double and single bonds within a ring structure (conjugated system).
    πœ‹ electrons can shift around the ring, creating multiple equivalent resonance structures.
    Example: Benzene (C6H6).

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