Chapter 27 - Aromatic Chemistry
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describe the geometry of benzene
a flat, regular, hexagonal molecule with bond angles 120 throughout
describe the carbon carbon bond in benzene
an intermediate between a double bond and a single bond
describe the electron delocalisation in benzene
carbon forms three covalent bonds one to hydrogen and 2 to carbons. the last electron is in a p orbital which overlaps with another p orbital on adjacent carbon atoms. this creates a ring of electrons
why is the delocalised system of electrons important in benzene
because it provides an unusual stability to the molecule
give the thermochemical evidence for the stability of benzene
the enthalpy of hydrogenation of cyclohexane is -120kj/mol so the enthalpy of hydrogenation of hypothetical non delocalised benzene (cyclohexa-1,3,5-triene) should be -360kj/mol but it is actually -208kj/mol which shows that benzene is 152kj/mol more stable
why do arenes not mix with water
because the benzene ring is non polar
why is benzenes melting point higher than hexanes
because the molecules pack together very well in the solid state which makes them harder to separate
why is benzene susceptible to electrophile attack
because the delocalised ring of electrons is a region of high electron density
why do arenes burn with smoky flames
because of the high carbon to hydrogen ratio so there are often a lot of unburnt hydrocarbons when they burn
why do arenes not undergo additoin reactions
because addition would require energy to break the delocalisation and destroy the aromatic system
what conditions do nitration reactions occur in
concentrated nitric and sulfuric acids under reflux at 50 degrees
what is nitration
the substitution of an NO2 group onto a arene ring electrophilic substitution
what reaction occurs between concentrated sulfuric and nitric acids
acid base where h2so4 is the acid and donates a h+ ion to nitric acid h2no3+ then loses a molecule of water to give no2+ which is the electrophile
give the reaction between concentrated sulfuric and nitric acids
H2SO4 + NHO3 —–> NO2+ + HSO4- + H2O
give some uses of nitrobenzene
making explosives like tnt making aromatic amines which are used to make industrial dyes
how is tnt made
nitrating methylbenzene
what conditions are needed for friedel crafts acylation
acyl chloride with AlCl3 as a catalyst heated under reflux
give the dreaction to produce the electrophile in Friedel-Crafts acylation
RCOCI + AICl3 —–> RCO+ + AlCl4-