Aromatic Compounds - Mindmap

Aromatic Compounds - Mindmap

1. August Kekule

   • For
     • Benzene can be hydrogenated with 3 hydrogens to form cyclohexene. This would suggest there are 3 double bonds.
   • Against
     • Enthalpy of hydrogenation would be expected to be -320 kJ/mol. However benzene has an enthalpy of hydrogenation of -208 kJ/mol. This means benzene is more stable than the Kekule Model.
     • Kekule suggested that benzene has alternating double and single bonds meaning the bonds have different lengths. However all the bonds in benzene have been found to have a length of 0.139 nm; this length is between a single and double bond.
     • Benzene doesn't decolourise bromine water. Since Kekule's structure included double bonds you would expect this.
     • Doesn't undergo electrophilic addition.

2. Delocalised Pi System

   • This is the proposed structure of benzene. It is a cloud in the middle of a benzene molecule. It forms due to the sideways overlap of the pi bonds. This creates a region of high electron density due to there being delocalised electrons inside. This is split into two regions, one above and below the carbon ring.

3. Nitration

   • Requires: Concentrated sulphuric acid, nitric acid and a temperature of 50 degrees celsius.
   • Electrophilic substitution.
   • HNO3 is not the electrophile. This is created from the reaction between concentrated sulphuric acid and nitric acid. A nitronium ion is created which is the electrophile.
   • Halogenation follows the same process as the halogen isn't the electrophile. Instead a halogen carrier is required, such as AlCl3 which can be formed just by mixing aluminium with chlorine. This produces a Cl+ ion which can react with benzene.

4. Acylation

   • This is the addition of a -RCOCl group. This too requires a halogen carrier.
   • Adding acyl chloride forms an aromatic ketone.
   • Called Friedl-crafts acylation

5. Alkylation

   • Substitution of a hydrogen atom with an alkyl group. This alkyl group must have a halogen attached to it (haloalkane) and a halogen carrier such as AlCl3 is required to generate the electrophile.
   • Called Friedl-crafts alkylation.

6. Electrophilic Substitution

   • Two steps: addition and substitution.
   • Addition: An electrophile is attracted to the electron cloud. The curly arrow points to the electrophile from the circle. This forms a coordinate bond. An unstable intermediate is formed.
   • Substitution: The c-h bond breaks. Its pair of electrons restore the stable delocalised structure. A hydrogen ion is eliminated. The curly arrow points from the C-H bond to the electron cloud.

7. Activating

   • The activating group forces the additional groups to the second and fourth carbons.
   • An activator donates electrons to the Pi system.

8. Deactivating

   • The deactivating group forces the additional groups to the third and fifth carbons.
   • A deactivator accepts electrons.

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