JW 1-2. Intro & Electrophilic aromatic substitution

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What are the three rules of aromaticity?

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  1. 4n + 2pi electrons

  2. Planar molecule for the overlap of p orbitals

  3. Fully conjugated pi-system

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What is a Frost-Muslin Diagram? How are they constructed?

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<ul><li><p>shows the bonding and anti-bonding and non-bonding molecular orbitals in cyclic structures </p></li><li><p>along the dotted line draw the molecules ring structure with the point facing down, there will always be one lowest energy MO on this bottom point </p></li><li><p>an MO space will sit on every corner of the structure </p></li><li><p>How to fill them: e.g. benzene has six pi-bonds</p></li></ul><p></p>
  • shows the bonding and anti-bonding and non-bonding molecular orbitals in cyclic structures

  • along the dotted line draw the molecules ring structure with the point facing down, there will always be one lowest energy MO on this bottom point

  • an MO space will sit on every corner of the structure

  • How to fill them: e.g. benzene has six pi-bonds

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1
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What are the three rules of aromaticity?

  1. 4n + 2pi electrons

  2. Planar molecule for the overlap of p orbitals

  3. Fully conjugated pi-system

2
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What is a Frost-Muslin Diagram? How are they constructed?

  • shows the bonding and anti-bonding and non-bonding molecular orbitals in cyclic structures

  • along the dotted line draw the molecules ring structure with the point facing down, there will always be one lowest energy MO on this bottom point

  • an MO space will sit on every corner of the structure

  • How to fill them: e.g. benzene has six pi-bonds

<ul><li><p>shows the bonding and anti-bonding and non-bonding molecular orbitals in cyclic structures </p></li><li><p>along the dotted line draw the molecules ring structure with the point facing down, there will always be one lowest energy MO on this bottom point </p></li><li><p>an MO space will sit on every corner of the structure </p></li><li><p>How to fill them: e.g. benzene has six pi-bonds</p></li></ul><p></p>
3
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What are the requirements for an anti-aromatic system?

  1. cyclic and conjugated molecule

  2. With only 4n electrons

4
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When is something planar?

When there is sp2 hybridisation

5
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What can cause a structure (like cyclooctaterene) to not be aromatic even though it looks like it would follow all the rules of aromaticity?

  • although its fully conjugated with 4n +2 pi electrons

  • It is not planar as it is more stable in its boat conformation

  • When constructing the Frost-Muslin diagram for it, there are 8 electrons to be placed on, however two of these land on non-bonding lines and are not paired, this destabilises the system forcing it to be more stable in boat conformation.

<ul><li><p>although its fully conjugated with 4n +2 pi electrons</p></li><li><p>It is not planar as it is more stable in its boat conformation </p></li><li><p>When constructing the Frost-Muslin diagram for it, there are 8 electrons to be placed on, however two of these land on non-bonding lines and are not paired, this destabilises the system forcing it to be more stable in boat conformation. </p></li></ul><p></p>
6
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How can we force some cyclic structures into aromaticity?

  • by forming aromatic anions with negative charges

  • the addition of 2 electrons often causes structures to be able to be aromatic as it adds the possibilities of resonance and conjugation to the ring

  • Even cycloocaterene can have two electrons donated into the ring to become cycloocatetrene which is now planar and fully conjugated.

<ul><li><p>by forming aromatic anions with negative charges </p></li><li><p>the addition of 2 electrons often causes structures to be able to be aromatic as it adds the possibilities of resonance and conjugation to the ring </p></li><li><p>Even cycloocaterene can have two electrons donated into the ring to become cycloocatetrene which is now planar and fully conjugated. </p></li></ul><p></p>
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What is the general mechanism with Wheland intermediates for electrophilic aromatic substitution?

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