Resonance Between Two Atoms

Resonance: Electron Redistribution Between Two Atoms

This section explores a common and essential resonance pattern: a 𝜋 bond between two atoms of differing electronegativity. In organic chemistry, you'll most frequently encounter this with carbonyls (C=O) or imines (C=N).

This resonance pattern occurs when a 𝜋 bond exists between two atoms, where one is significantly more electronegative than the other (e.g., carbon double-bonded to oxygen or nitrogen). These systems have the following general structure, where 'Z' represents the more electronegative atom:

General structure for two-atom resonance: a carbon double-bonded to a more electronegative atom (Z), which typically has lone pairs.

You'll encounter this pattern many times in organic chemistry. This resonance typically indicates that the carbon atom connected to the more electronegative atom (Z) becomes electron-deficient. The resonance structures are generated by depicting the shifting of a pair of π electrons from the double bond onto the more electronegative atom (Z) as a lone pair, resulting in a formal positive charge on the carbon and a formal negative charge on Z:

Example of electron redistribution: the 𝜋 bond's electrons are depicted as moving to the more electronegative atom, creating a positive charge on carbon and a negative charge on Z.

You should be able to draw the resonance hybrid for these structures, which will show partial double bond character between C and Z, and partial charges on both atoms.

Key Observations for Resonance Between Two Atoms:

When drawing these resonance structures, observe the following crucial points:

• Charge Change on Z: The formal charge on the more electronegative atom (Z, e.g., O or N) i
  ncreases by -1 (becomes more negative) when the π electrons are depicted as shifting onto it.
• Charge Change on Carbon: The formal charge on the carbon atom directly involved in the π bond decreases by +1 (becomes more positive) as it is depicted as losing π electron density.
• Electron Conservation: The total number of valence electrons remains the same in all valid resonance structures.
• Charge Conservation: The overall net charge of the molecule or ion is conserved across all resonance structures.
• No Atom Movement: Atoms never change their positions; only electrons (specifically π electrons and lone pairs) are delocalized.

𝜋 Bond Between Two Atoms

Molecules that have a π bond between two different atoms will exhibit resonance, provide the atoms are different, having different electronegativities.  They have the following general structure.

You will encounter this pattern many times in organic chemistry.  This resonance typically indicates that the carbon connected to Z is deficient in electrons.  You should be able to draw the resonance hybrid for these.

Your Turn!