Resonance & Tautomerism

• What are localised electrons?

  Electrons belonging to a single atom or bond
• What color indicates electron-rich areas in an electrostatic potential map?

  Red
• What is the molecular formula of benzene?

  C6H6
• What is the degree of unsaturation for benzene?

  Four
• What was the Kekule hypothesis regarding benzene?

  It proposed alternating single and double bonds

  
• What is the bond length of benzene?

  1.39 Å
• How does the bond length of benzene compare to C-C and C=C bonds?

  It lies between C-C and C=C bond lengths

  
• What are resonance structures?

  Different placements of π and nonbonding electrons

  
• What does the curly arrow represent in resonance structures?

  Movement of a pair of electrons

  
• How does the number of resonance forms affect stability?

  More resonance forms increase compound stability
• What is a conjugated system?

  A series of overlapping p orbitals
• What is tautomerism?

  Rapid interconversion between isomers

  
• What are tautomers?

  Isomers with differently arranged atoms
• What factors can affect keto-enol equilibrium?

  Conjugation and hydrogen bonding
• What is the role of base in enol formation?

  It catalyzes the formation of enols
• What happens to a covalent bond when one electron is removed?

  The bond is destroyed
• Why are certain carbocations more stable?

  Positive charge is shared among more carbons
• Why is 2-aminopyridine preferentially protonated on the ring N?

  More good resonance structures increase stability
• What is the effect of delocalised electrons on pKa?

  They can affect the acidity of compounds
• What are the rules for resonance forms?

  1. Only electrons move; atoms never move.
  2. Only π electrons and lone-pair electrons can move; σ electrons never move.
  3. Total number of electrons does not change.
  4. Resonance forms must be valid Lewis structures.
  5. The octet rule still applies.
• What is the difference between tautomers and resonance forms?

  • Tautomers: different compounds with atom shifts.
  • Resonance forms: different representations of a single compound with electron shifts.
• What is resonance stabilization energy?

  • Energy gained from delocalised electrons.
  • Increases stability of the compound.
• What is the significance of resonance in organic chemistry?

  • Helps explain stability of compounds.
  • Provides insight into electron distribution.