Chapter 27- Aromatic Chemistry Flashcards
What is benzene?
A ring of 6 carbon atoms and 6 hydrogen atoms (C6H6) with a delocalised ring of electrons
What is the structure of benzene?
Planar hexagonal ring of carbon atoms with intermediate bond length between a single and double bond. All bonds are the same length.
Delocalised ring of electrons from the p-orbital makes it more stable than expected
How does the delocalised ring of electrons in benzene form?
An electron from the p-orbital of each carbon becomes delocalised to form the π system of electrons (two rings above and below carbon rings)
What is the evidence for the Kekulé model for benzene being wrong?
Enthalpies of hydrogenation: cyclohex-1,3-diene has x2 enthalpy of hydrogenation of cyclohexene. Benzene would be expected to be x3 however is less. Less energy released overall as more energy required to break ring of electrons hence more stable.
Bromine water: benzene does not decolourise bromine water therefore no C=C double bonds
Why are substitution reactions favoured over addition reactions for compounds with benzene?
Adding a group would cause breaking of the delocalised ring of electrons, decreasing stability
Substitution does not break the ring of electrons therefore it is favoured
What is stabilisation energy?
The energy required for benzene to react, ie overcome its extra stability
What is the reaction mechanism for reactions with aromatic compounds?
Electrophilic substitution
What is the electrophile in nitration?
nitronium ion NO2+
How is the electrophile in nitration produced… and what is the catalyst in the overall reaction?
conc. H2SO4 + conc. HNO3 –> HSO4- + H2NO3+
H2NO3+ ⇋ H2O + NO2+ producing electrophile
H+ produced from electrophilic substitution + HSO4- makes H2SO4 regenerated, so catalyst
What is nitration used for?
Manufacture of explosives and formation of amines
What are the conditions for nitration?
Temperature of 55°C and H2SO4 catalyst, NO2+ electrophile
What is the Friedel-Crafts acylation reaction?
A reaction in which the electron ring acts as a nucleophile, attacking an acyl chloride
What is the role of aluminium chloride in the Friedel-Crafts acylation reaction?
-AlCl3 reacts with acyl chloride to produce RC+O and AlCl4-. The reactive intermediate RC+O is then attacked by the electron ring…
-AlCl4- reacts with H+ produced to produced AlCl3 and HCl, so it is also a catalyst
What is the Friedel-Crafts acylation reaction used for?
Industrial production of dyes, pharmaceuticals and explosives
