2.6 Drawing Resonance Forms

Resonance structures reveal how electrons are shared across molecules, impacting their stability and reactivity. This concept is crucial for understanding bonding patterns, charge distribution, and overall molecular behavior in organic compounds.

Resonance forms show different ways to arrange electrons without changing atom positions. By exploring various resonance structures, we can better predict a molecule's properties and how it might interact in chemical reactions.

Resonance Structures

Resonance in three-atom groupings

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• Allyl cation
  • Two equivalent resonance forms where the positive charge is spread out (delocalized) over the two terminal carbons at the ends of the molecule
  • In each resonance form, the central carbon forms a single bond to each terminal carbon and one terminal carbon has the positive charge (carbocation)
  • Real structure is a hybrid of these two forms, with partial positive charges on both terminal carbons (delocalized carbocation)
• Allyl anion
  • Two equivalent resonance forms where the negative charge is delocalized over the two terminal carbons
  • In each resonance form, the central carbon forms a double bond to one terminal carbon and a single bond to the other, with the negative charge on the single-bonded terminal carbon (carbanion)
  • Real structure is a hybrid, with partial negative charges on both terminal carbons and partial double bond character between the central carbon and each terminal carbon
• Azide anion
  • Two equivalent resonance forms where the negative charge is delocalized over the two terminal nitrogens at the ends of the molecule
  • In each resonance form, the central nitrogen forms a double bond to one terminal nitrogen and a single bond to the other, with the negative charge on the single-bonded terminal nitrogen
  • Real structure is a hybrid, with partial negative charges on both terminal nitrogens and partial double bond character between the central nitrogen and each terminal nitrogen

Multiple resonance forms of molecules

• 2,4-pentanedione anion has three major resonance forms due to delocalization of the negative charge
  • Negative charge can be placed on either terminal oxygen or the central carbon
  • When the negative charge is on a terminal oxygen:
    • That oxygen forms a single bond to the adjacent carbon
    • The adjacent carbon forms a double bond to the other terminal oxygen to maintain octet (enolate form)
  • When the negative charge is on the central carbon:
    • Both terminal oxygens form double bonds to their adjacent carbons
    • The central carbon forms single bonds to both adjacent carbons (carbanion form)
  • Real structure is a hybrid of these forms, with the negative charge delocalized over all three positions

Resonance in ions and molecules

• Phosphate ion ($PO_4^{3-}$)
  • Four equivalent resonance forms due to delocalization of the negative charge over all four oxygens
  • In each resonance form, one oxygen forms a double bond to phosphorus and the other three oxygens form single bonds, with a negative charge on each single-bonded oxygen
  • Real structure is a hybrid with partial negative charges on all oxygens and partial double bond character between phosphorus and each oxygen
• Nitrate ion ($NO_3^-$)
  • Three equivalent resonance forms due to delocalization of the negative charge over all three oxygens
  • In each resonance form, one oxygen forms a double bond to nitrogen and the other two oxygens form single bonds, with a negative charge on each single-bonded oxygen
  • Real structure is a hybrid with partial negative charges on all oxygens and partial double bond character between nitrogen and each oxygen
• Carboxylate ion ($RCOO^-$)
  • Two equivalent resonance forms due to delocalization of the negative charge over the two oxygens
  • In each resonance form, one oxygen forms a double bond to carbon and the other oxygen forms a single bond, with the negative charge on the single-bonded oxygen
  • Real structure is a hybrid with a partial negative charge on each oxygen and partial double bond character between carbon and each oxygen
• Amide group ($RCONH_2$)
  • Two major resonance forms due to delocalization of the nitrogen lone pair
  • In one form, carbon forms a double bond to oxygen (carbonyl) and a single bond to nitrogen
  • In the other form, carbon forms a single bond to oxygen and a double bond to nitrogen (iminium), with a positive charge on nitrogen
  • Real structure is a hybrid with partial double bond character between carbon and both oxygen and nitrogen
• Benzene ($C_6H_6$)
  • Two equivalent resonance forms due to delocalization of the pi electrons around the ring
  • In each resonance form, there are three alternating double bonds and three alternating single bonds around the ring
  • The double bonds can be drawn pointing in either direction around the ring
  • Real structure is a hybrid with equal bond lengths between all carbons (delocalized pi system)

Drawing and Evaluating Resonance Forms

• Use curved arrows to show electron movement when drawing resonance forms
• Calculate formal charge for each atom to determine the most stable resonance forms
• Consider conjugation (alternating single and double bonds) when evaluating stability
• The resonance hybrid represents the actual structure, with bond order between single and double for resonating bonds