Aromatics Flashcards
benzene’s formula
C6H8
what is another name for arenes and what does it mean
aromatic compounds as first found in sweet smelling dyes
what is the most common type of reaction of benzene
substitution (of H for a different functional group)
what is the shape of benzene
flat
regular hexagon
bond angle = 120
what is the bond length between adjacent C atoms in benzene
intermediate between C-C and C=C
what happens to the 4th electron in the p orbital of each C atom in benzene
delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon
what is the effect on benzene’s stability of the rings of electron density
makes benzene very stable, even though it is unsaturated (aromatic stability)
what is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene
hydrogenation of cyclohexane = -120 kJmol-1
cyclohexa-1,3,5-triene = -360 kJmol-1
benzene hydrogenation = -208 kJmol-1
so benzene is 152kJmol-1 more stable
why else would cyclohexa-1,3,5-triene not a suitable model for benzene
- would not be symmetrical (C=C shorter than C-C), but benzene is
- would easily undergo addition reactions across the double bonds whereas benzene does not
- would form two isomers on the addition of Br2 or similar whereas 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 and why
no because its nonpolar
dangers of benzene
carcinogen
how do you name compounds containing a benzene ring
-benzene, or phenyl- can designate position on ring using numbers if there is more than one substituent
why is benzene attacked by electrophiles
high electron density above/below ring due to delocalised electrons
what is the delocalisation energy and what is the effect of this on benzene’s reactions
large amount of energy that is needed to break the aromatic ring apart. results in the aromatic ring almost always staying intact
what is seen when benzene is combusted
smoky flames due to soot from unburnt carbon
this is because of the high carbon:hydrogen ratio
which ion (name and formula) is used to nitrate benzene
NO2 +
(+ charge on the nitrogen)
nitronium ion or nitryl cation
how is NO2+ ion generated
concentrated H2SO4 and concentrated 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)
overall equation for the nitration of benzene
C6H6 + HNO3 –> C6H5NO2 + H2O
what are the uses of nitrated arenes
production of explosives e.g. TNT releases lots of heat and gas on explosion
to make aromatic amines that are used for industrial dyes
how do substituents with a positive inductive effect affect further substitution
they release electrons into the delocalised electron ring, increasing the electron density and making further substitution reactions more likely/quick
direct substituents to the 2,4,6 positions
how do substituents with a negative inductive effect affect further substitution
remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely/quick
direct substituents to the 3,5 position
what type of catalyst is used for a Friedel-Crafts reaction
a halogen carrier
write an equation to form an electrophile that could be used to acylate benzene starting with AlCl3 and RCOCl
AlCl3 + RCOCl –> AlCl4- + RCO+
RCO+ can attack benzene (+ on the C)
what is happening when AlCl4- is formed in terms of electrons
chlorine atom’s lone pair of electrons is forming a coordinate bond to Al
how is the AlCl3 catalyst reformed
AlCl4- + H+ –> HCl + AlCl3 (H+ from benzene)
how could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring
use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene
if you are considering cyclic compounds, what might happen if two double bonds are next to each other
C=C bonds are in close proximity, so electrons in pi cloud/p orbitals can partially delocalise and move between the two C=C double bonds
