Flashcards in Aromatic Chemistry Deck (16):
What is the structure of benzene?
How can we describe the carbon to carbon bond length in benzene?
Between the length of a single and double bond.
How are elctrons arranged in benzene?
They are delocalised - this means they don't belong to a specific carbon atom.
How can we show that delocalisation of electrons confers stability of the molecule?
The enthalpy change when we hydrogenate benzene is less exothermic than the expected enthalpy change of hydrogenating three double bonds - as in Kekule's structure. This shows that benzene is more stable than a structure with three double bonds because more energy has been put in to break the bonds.
What type of reaction do arenes undergo, and why?
Electrophilic substitution, because a benzene ring is an area of high electron density, so attracts electrophiles. Substitution occurs because it preserves the delocalised ring of electrons - which is what makes the molecule stable.
Why are Friedel-Crafts acylation reactions important in synthesis?
They allow us to add acyl groups to benzene rings, which would be difficult otherwise because they are so unreactive. We can then modify the side chains in further reactions to give us useful products.
Describe the mechanism of Friedel-Crafts acylation.
1) React an acyl chloride with AlCl3 to produce a carbocation and AlCl4^(-).
2) Electrons in the benzene ring are attracted to the positively charged carbon atom and two electrons from the benzene ring bond to it. This partially breaks the delocalised ring of electrons and gives it a positive charge.
3) The aluminium chloride ion bonds to the hydrogen atom, forming aluminium chloride and hydrochloric acid. The electrons from the H-C bond are returned to the delocalised ring of electrons, returning it to its stable state.
How do we create a strong electrophile?
Reacting a weak electrophile with a halogen carrier.
What is the catalyst in Friedel-Crafts acylation?
Aluminium chloride (AlCl3).
What are the reaction conditions for Friedel-Crafts acylation?
Heated under reflux and in a non-aqueous envorinment.
Why are nitration reactions important in synthesis?
Nitro compounds can be reduced to form aromatic amines, which are them used to manufacture dyes and pharmaceuticals. They also decompose violently when heated, so are useful as explosives (eg. TNT).
What are the conditions for a nitration reaction?
Heat and sulfuric acid.
What is the catalyst in a nitration reaction?
Sulfuric acid (H2SO4).
How is a nitratonium ion generated for a nitration reaction?
Adding concentrated nitric acid (HNO3) to sulfuric acid will cause the nitric acid to become positively charged. This then dissociates into a nitrnium ion (NO2^(+)) and water.
Explain the mechanism for nitration of benzene.
1) The electrons in the benzene ring are attracted to the positively charged nitrogen atom of the nitronium ion.
2) The electrons from the H-C bond retreat to the benzene ring, returning it to its stable state.
3) The H+ ion released reacts with the HSO4^(-) to reform the catalyst used to make the nitronium ion.