Aromatic Chemistry Flashcards
What is benzene’s formula?
C6H6
What is another name for arenes? Why did this come about?
Aromatic compounds, as first found in sweet-smelling dyes
What is the most common type of reaction of benzene?
Substitution (of a H for a different functional group)
What is the shape of benzene?
Flat, regular hexagon. Bond angle = 120o
What is the bond length between adjacent C atoms?
Intermediate between C-C and C=C
What happens to the 4th electron in the p orbital of
each C atom in benzene?
Forms 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 cyclohexene = -120kJmol-1→
cyclohexa-1,3,5-triene = -360kJmol-1
Benzene hydrogenation = -208kJmol-1 so benzene is 152kJmol-1 more stable
Why else is 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 - 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? (why it is not used in schools)
It is a 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 delocalisation energy and what is the effect of this on benzene’s reactions?
The 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? Why?
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 is on the nitrogen). Nitronium ion or nitryl cation
How is this NO2+ ion generated? (conditions and
equations)
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
What are the uses of nitrated arenes?
Production of explosives e.g. 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
How do substituents with a positive inductive effect
(e.g. alkyl groups) 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
(e.g. NH2) 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 3,5 positions
What type of catalyst is used for a Friedel-Crafts reaction?
A halogen carrier (e.g. AlCl3)
Write an equation to form an electrophile that could be used to acylate benzene, starting with AlCl3 and RCOCl
AlCl3 + RCOCl → AlCl4- + RCO+ (+ on C)
RCO+ can attack benzene
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 2 C=C double bonds
What effect would electrons in p orbitals moving between the 2 C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation?
Makes the molecule more stable; makes enthalpy of hydrogenation more positive
