3.10 Aromatic chemistry Flashcards
What is benzenes formula and structure
C6H6
Google / hexagon with circle inside
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 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 of benzenes 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
Hexagon with 3 lines to show double bonds??
Google
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
What is the appearance of benzene at 298K
Colourless liquid
Why does benzene have a relatively high mtp
Close packing of flat hexagonal molecules when solid
Is benzene soluble in water
Why
No
Non polar
Dangers of benzene
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 benzenes 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 is used to nitrate benzene
NO2+ (+charge is on the nitrogen)
Nitronium ion or nitryl cation
Draw a mechanism and write an overall equation for the nitration of benzene
C6H6 + HNO3 –> C6H5NO2 + H2O
Google
What are the uses of nitrated arenes
Production of explosives eg TNT
To make aromatic amines that are used for industrial dyes
How do substituents with a positive inductive effect (eg alkyl groups) affect further substitution
They release electrons into the delocalised electron ring, increasing the electron density and making further substitution reactions more likely/quickly
Direct substituents to the 2,4,6 posititions
How do substituents with a negative inductive effect (eg 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 eg AlCl3
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 could you use 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+
If your are considering cyclic compounds, what might happen if 2 double bonds are next to eachother
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 its enthalpy of hydrogenation
Makes the molecule more stable; makes enthalpy of hydrogenation more positive
