Structure, Bonding and Reactivity of Heterocyclics

  • Created by: E.H13
  • Created on: 23-05-20 20:44
View mindmap
  • Structure, Bonding and Reactivity of Heterocyclics
    • Introduction
      • Heteroaromatics; compounds which are aromatic and contain at least one heteroatom as part of their ring system
        • Aromatic; cyclic, fully conjugated, planar and have 4n+2 pi electrons
          • 4n + 2 pi electrons > Huckel's rule
          • First 3 rules set up a continuous pi system, the final one dictates whether the system is stabilised or destabilised
      • 4 important types of hetero aromatic structure
        • Pyridine
        • Quinoline/ Isoquinoline
        • Pyrrole/ Furan / Thiophene
        • Indole /Benzofuran / Benzothiophene
      • Grouped by structural similarities
        • Results in chemical similarities
    • Structure and Bonding of 6 membered rings
      • Heteroaromatics often compared to benzene
        • Planar
        • p orbitals orthoganal to the ring
        • Symmetric
        • Each bond of equal length
      • Aromatic systems undergo electrophilic substitution fairly readily and nucleophilic substitution slowly.
      • Pyridine
        • Atomic numbering starting at the N
        • N is sp2 hybridized, and contributes 1e to the ring system
        • Basic, and acts as a nucleophile through the N
        • Electron deficient, as N is more eletcronegative, so undergoes EArS slowly
        • Has a permanent dipole
        • Inductive effects alone cannot explain dipole, so need to consider resonance structures
          • Consider charged resonance forms as they contribute more in pyridine as the -ve charge can be placed on the N
    • Structure and Bonding of 5 membered heteroaromatics
      • Aromatic system must obey Huckel's rule.
        • Heteroaromatic must contribute 2 pi e
        • and by sp2 hybridised
      • Dipole on pyrrole and pyrrolidine are almost exactly reversed
        • Inductive effects must operate in the same direction
        • Must be due to the resonance effects
      • 6e over 5 'carbons', so electron rich
        • Undergoes EArS readily - opposite of pyridine
      • Degree of aromaticity is influencd by the extent of delocalisation
        • Delocalisation results in electron loss from heteroatom
          • So degree of aromaticity varies with electronegativity of heteroatom
        • Pyrrole> Thiophene> Furan
          • Thiophene has poor overlap
            • Large atomic radius decreases angular strain and stabilises aromaticity
          • O is much more electronegative
    • Quinoline and Isopuinoline
      • Structure and bonding essentially identical to pyridine
      • Numbering the same for both - 1 at lowest position on right ring
      • 7 resonance structures, but final 3 disrupt aromaticity of both rings
        • So basically useless
    • Indoles
      • Only one significant resonance form
        • Others disrupt aromaticity
      • Can be compared to enamines
    • Heterocyclics with more than one heteroatom
      • Must be able to form 3 bonds to form membered ring
        • Why we don't find neutral sulphur or oxygen
      • Azoles
        • 1xN and 1x(other) in a 5 membered ring
          • Imine N - nitrogen i pyridine
          • 2nd heteroatom = N-H in pyrrole
    • A hellscape, visualised


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Snaith Handout 1 resources »