Aromatic chemistry
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Describe Kekule’s structure of benzene in terms of electron overlap.
• Planar and cyclic • 120 degree bopnd angles • Alternating doube and single bonds • Electons in 2p overlap, forming localised Pi bonds
What chemical evidence proved that Kekule’s structure was inaccurate?
• The molecule did not undergo electrophilic substitution reactions, meaning there were no C=C double bonds. • Each bond has an intermediate length between a single and double bond • Its bonds are more stable than expected due to the enthaply change of hydrogenation being lower than expected
Describe the delocalised model of benzene in terms of electron overlap.
• Each carbon atom has 3 sigma bonds, leaving a spare electron in 2p • Planar, cyclic, 120 degree bond angles • 2 orbitals overlap above and below the ring • This forms delocalised pi bonds
What are the 3 halogen carriers?
• Iron • Iron halides • Aluminium halides
Describe the mechanism for electrophilic substutions of benzene.
• Show how the halogen carrier behaves as a catalyst by forming the electrophile • Form the unstable intermediate. 1. Draw a curly arrow going from the Pi ring to the positively charged electrophile 2. Open the benzene ring facing the carbon where the electrophile will be added 3. Add a positive charge into the horseshoe 4. Add the electrophile and a hydrogen with a curly arrow from the C-H bond to the + in the horseshoe 5. Draw the reformed ring with the electrophile added and a H+ ion next to it • Show how the catalyst reforms
What are the reagents and conditions for halogenation of benzene?
AlX3, where X is the halogen
What are the equations to form and regenerate the catalyst when performing a halogenation reaction? Assume the halogen used is Br.
• Br₂ + AlBr₃ -> AlBr₄- + Br+ • H+ + AlBr₄- -> AlBr₃ + HBr
What are the reagents and conditions for the nitration of benzene?
Nitric and sulphuric acids
What are the equations to form and regenerate the catalyst when performing a nitration reaction?
• HNO₃ + H₂SO₄ -> NO₂+ + HSO₄- + H₂O • HSO₄- + H+ -> H₂SO₄
How do you find the reagent needed for alkylation of benzene when it is not given in the question?
Take the + charge off of the electrophile and add a Cl
What are the reagents needed for the alkylation of benzene?
AlCL₃ and XCl, where X is the group already substituted to the benzene ring
What are the equations to form and regenerate the catalyst when performing an alkylation reaction? Assume the group being added is CH₃.
• CH₃Cl + AlCl₃ -> CH₃+ + AlCl₄- • H+ + AlCl₄- -> AlCl₃ + HCL
What is the 2 rules when naming aromatic compounds?
• If the group is an alkyl, halogen or nitro the prefix is the group and the suffix is benzene. (E.g. nitrobenzene) • If the group is an amine, ketone, alkene, amide or alcohol then the group added is the suffix and the prefix is phenyl (phenylamine)
What are the 2 exceptions when naming aromatic compounds?
• Benzene-carboxylic acid • Benzaldehyde
What are the reagents required for an ayclation reaction?
• The given acyl chloride • AlCl₃
What are the equations to form and regenerate the catalyst when performing an acylation reaction? Assume the group being added is CH₃COCl
• CH₃COCl + AlCl₃ -> CH₃CO+ + AlCl₄- • AlCl₄- + H+ -> AlCl₃ + HCL
Is phenol an acid or a base?
Weak acid
How can phenol be neutralised?
Phenol will neutralise by reacting with NaOH, but it will not react with carbonates
What products form when phenol reacts with an excess of bromine?
• 2-4-6-tribromophenol • The moles of Br used is the moles of HBr formed as a secondary product
Why is phenol more reactive than benzene?
The electrons in the 2p orbital of the oxygen atom, which is delocalised into the benzene ring. This increases the electron density of the ring, making it less stable and therefore easier to attack from electrophiles.
What is the differences in the nitration of phenol compared to benzene?
• Phenol reacts readily with dilute nitric acid. • A halogen carrier or concentrated sulphuric acid catalysts are unneccesary. • The reaction forms a mixture of 2 and 4 nitrophenol.
What is meant by an electron withdrawing group? Give an example.
• Decrease the electron density in the ring • NO₂
What is meant by an electron donating group? Give 2 examples.
• Increase the electron density in the ring • OH and NH₂