3.10 Aromatic chemistry Flashcards
What is benzene’s formula and structure
Slide 4
C6H6
What is another name for arenas? 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 = 120
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?
It 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)
Draw the skeletal structure of cyclohexa-1,3,5-triene
Slide 18
What is the Thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene?
Hydrogenation of cyclohexane = -120 JKmol-1 —> cyclohexa-1,3,5-triene = -360 KJmol-1
Benzene hydrogenation = -208kJmol-1 so benzene is 152 kJmol-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 relative 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 school)
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 substituents
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 reaction
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 sewn when benzene is combusted? Why?
Smoky flames due to soot from unburnt carbon. This is because of the high carbon:hydrogen ratio
Draw a general electrophilic substitution mechanism of benzene, using El+ to represent an electrophile
Slide 40
Which ion (name and formula) is used to nitrate Benzene?
NO2+ (+ charge is on the nitrogen). Nitronium ion or nitty 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)
Draw a mechanism and write an overall equation for the nitration of benzene
C6H6 + HNO3 —> C6H5NO2 + H20
What are the uses of nitrated arenes?
Production of explosives e.g. TNT
(1-methyl-2,4,6-trinitrobenznee) - 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 reaction more likely/quick
Direct substituents to the 2,4,6 position
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 reaction 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 RCOCI
AICI3 + RCOCI —> AICI4- + 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+ —> HCI + AICI3 (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
Draw the mechanism for the acylation of benzene from RCO+
Slide 66
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
What effect would electrons in p orbitals moving between the two C=C double bonds have on the stability of the molecule and it’s enthalpy of hydrogenation?
Makes the molecule more stable; makes enthalpy of hydrogenation more positive
