6.1.1 Aromatic compounds Flashcards
(18 cards)
What is the key difference between the Kekulé model and the delocalised model of benzene?
The Kekulé model suggests alternating single and double bonds, while the delocalised model proposes a π-system formed by p-orbital overlap with delocalised electrons above and below the ring.
What experimental evidence supports the delocalised model of benzene over the Kekulé model in terms of bond lengths?
X-ray diffraction shows all carbon-carbon bond lengths in benzene are equal, approximately 0.139 nm, intermediate between single and double bonds.
What experimental evidence supports the delocalised model of benzene over the Kekulé model in terms of enthalpy change of hydrogenation?
The enthalpy change of hydrogenation for benzene (-208 kJ mol⁻¹) is less exothermic than expected for three double bonds (-360 kJ mol⁻¹), indicating extra stability due to delocalisation.
What experimental evidence supports the delocalised model of benzene over the Kekulé model in terms of resistance to reaction?
Benzene does not readily undergo addition reactions, demonstrating the stability of the delocalised π-system.
How are substituted aromatic compounds named systematically?
Use IUPAC rules: name the substituents as prefixes, assign locants (numbers) to positions on the ring, and name the parent benzene structure (e.g., 2,4-dinitromethylbenzene).
What are the reagents and conditions for nitration of benzene?
Concentrated nitric acid and concentrated sulfuric acid at 50 °C.
What are the reagents and conditions for halogenation of benzene?
A halogen (e.g., Cl₂ or Br₂) in the presence of a halogen carrier such as AlCl₃ or FeBr₃.
What is a Friedel–Crafts reaction and why is it important in organic synthesis?
A reaction involving a haloalkane or acyl chloride with a halogen carrier to form a C–C bond to an aromatic ring, essential for extending carbon chains.
What is the electrophile in the nitration of benzene, and how is it formed?
The electrophile is NO₂⁺, formed by the reaction of concentrated H₂SO₄ with HNO₃.
What is the electrophile in the halogenation of benzene, and how is it formed?
The electrophile is X⁺ (e.g., Cl⁺ or Br⁺), formed by the reaction of a halogen with a halogen carrier (e.g., AlCl₃ or FeBr₃).
Why is benzene more resistant to bromination than alkenes?
The delocalised π-system in benzene has lower electron density than the localised π-bond in alkenes, making it less attractive to electrophiles.
How does phenol react with bromine?
Phenol reacts with bromine water to form 2,4,6-tribromophenol as a white precipitate.
How does phenol react with dilute nitric acid?
Phenol reacts with dilute nitric acid to form a mixture of 2-nitrophenol and 4-nitrophenol.
Why is phenol more reactive than benzene?
The lone pair of electrons on the oxygen of the OH group donates electron density into the π-system, increasing its reactivity.
What are the directing effects of electron-donating groups such as OH or NH₂?
Electron-donating groups are 2- and 4-directing, favouring substitution at these positions.
What are the directing effects of electron-withdrawing groups such as NO₂?
Electron-withdrawing groups are 3-directing, favouring substitution at this position.
What reaction demonstrates the weak acidity of phenol?
Phenol reacts with NaOH to form a phenoxide ion but does not react with carbonates.
What is the importance of directing effects in organic synthesis?
Directing effects allow prediction of substitution products, aiding in the design of synthetic pathways for complex molecules.
