Structure, Bonding and Reactivity of Heterocyclics
- Created by: E.H13
- Created on: 23-05-20 20:44
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- 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
- Aromatic; cyclic, fully conjugated, planar and have 4n+2 pi electrons
- 4 important types of hetero aromatic structure
- Pyridine
- Quinoline/ Isoquinoline
- Pyrrole/ Furan / Thiophene
- Indole /Benzofuran / Benzothiophene
- Grouped by structural similarities
- Results in chemical similarities
- Heteroaromatics; compounds which are aromatic and contain at least one heteroatom as part of their ring system
- 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
- Heteroaromatics often compared to benzene
- 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
- Thiophene has poor overlap
- Delocalisation results in electron loss from heteroatom
- Aromatic system must obey Huckel's rule.
- 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
- Only one significant resonance form
- 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
- 1xN and 1x(other) in a 5 membered ring
- Must be able to form 3 bonds to form membered ring
- A hellscape, visualised
- Introduction
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