Aromatic Compounds - Mindmap

August Kekule
1. For
  1. Benzene can be hydrogenated with 3 hydrogens to form cyclohexene. This would suggest there are 3 double bonds.
2. Against
  1. 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.
  2. 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.
  3. Benzene doesn't decolourise bromine water. Since Kekule's structure included double bonds you would expect this.
  4. Doesn't undergo electrophilic addition.
Delocalised Pi System
1. 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.
Nitration
1. Requires: Concentrated sulphuric acid, nitric acid and a temperature of 50 degrees celsius.
2. Electrophilic substitution.
3. 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.
4. 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.
Acylation
1. This is the addition of a -RCOCl group. This too requires a halogen carrier.
2. Adding acyl chloride forms an aromatic ketone.
3. Called Friedl-crafts acylation
Alkylation
1. 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.
2. Called Friedl-crafts alkylation.
Electrophilic Substitution
1. Two steps: addition and substitution.
2. 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.
3. 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.
Activating
1. The activating group forces the additional groups to the second and fourth carbons.
2. An activator donates electrons to the Pi system.
3. NH2 and OH
Deactivating
1. The deactivating group forces the additional groups to the third and fifth carbons.
2. A deactivator accepts electrons.
3. NO2

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