3.3.10 Aromatic Chemistry (A2)
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what is benzene’s formula and structure?
C6H6, planar hexagon with delocalised system
what is another name for arenes? how did this come about?
aromatic compounds, as first found in sweet smelling dyes
what is the most common type of reaction for benzene?
substitution (of a H for a different functional group)
what is the shape of benzene?
flat, regular hexagon, bond angle 120 degrees
what is the bond length between adjacent C atoms?
intermediate between C-C and C=C 1.39 Å (angstrommes)
what happens to the 4th electron in the p orbital of each C atom in benzene?
delocalises, forming rings of delocalised electron density above/below the planar hexagon
what is the effect on benzene’s stability of the rings of electron density?
makes it very stable, gives it aromatic stability
what is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene?
hydrogenation of cyclohexene = -120kJ/mol therefore cyclohexa-1,3,5-triene = -360kJ/mol benzene hydrogenation = -208kJ/mol so benzene is 152kJ/mol more stable
why else (other than thermochemistry) is cyclohexa-1,3,5-triene not a suitable model for benzene?
would not be symmetrical (C=C shorter then C-C) but benzene is would easily undergo nucleophilic addition reactions across the double bond - benzene does not would form two isomers on the addition of Br2 or similar - benzene does not
what is the appearance of benzene at 298K?
colourless liquid
why does benzene have a relatively high melting point?
close packing of flat hexagonal molecules when solid
is benzene soluble in water? why?
no non-polar
dangers of benzene?
carcinogen
How do you name compounds containing a benzene ring?
-benzene
why is benzene attacked by electrophiles?
high electron density above/below ring due to delocalised electrons (system)
what is delocalisation energy and what is the effect of this on benzene’s reactions?
large amount of energy that is needed to break the aromatic delocalised system apart. results in the aromatic delocalised system almost always staying intact
what is seen when benzene is combusted? why?
“dirty” (smoky) flame due to soot from unburnt carbon due to the high C:H ratio (1:1)
which ion is used to nitrate benzene?
NO2+ (+ charge is on the nitrogen) nitronium ion
how is the NO2+ ion generated?
conc. H2SO4 and conc. HNO3 H2SO4 + HNO3 –> H2NO3+ + HSO4- H2NO3+ –> H2O + NO2+ overall: H2SO4 + HNO3 –> HSO4- + NO2+ + H2O
how is the H2SO4 catalyst regenerated in the nitration of benzene?
HSO4- + H+ –> H2SO4 (H+ from benzene ring)
what are the uses of nitrated arenes?
production of explosives eg. TNT (1-methyl-2,4,6-trinitrobenzene) - releases lots of heat and gas on explosion To make aromatic amines that are used for industrial dyes
what type of catalyst is used for a Friedel-Crafts reaction?
A halogen carrier eg. AlCl3
what is happening when AlCl4- is formed in terms of electrons?
chlorine atom’s lone pair of electrons is forming a co-ordinate bond to Al
How is the AlCl3 catalyst reformed?
AlCl4- + H+ –> HCl + AlCl3 (H+ from benzene)