10: Aromatic Compounds Flashcards
Benzene is planar - what does this mean?
Flat molecule
Benzene has a cyclic structure - what does this mean?
6 carbon atoms joined in a flat ring.
Describe the bonding in benzene.
Each carbon covalently bonded to 1 hydrogen and 2 other carbon atoms.
How many electrons remain unpaired for each carbon in benzene?
6
What orbital is the unpaired electron for each carbon atom in in benzene?
p
Describe the ring seen in benzene.
Shows the 6 delocalised electrons each from the p orbital.
Why are the electrons considered delocalised in benzene?
The electrons do not belong to a specific carbon atom.
Describe the length of bonds in benzene.
Compare this to the normal length of a C=C bond and C-C bond.
Bond length is halfway between the length of a normal C-C and C=C bond.
State the bond angle in benzene.
120
Name the molecule that would be similar to benzene, consisting of alternating double and single bonds.
Cyclohexa-1,3,5-triene
Name the process that allows us to compare the stability of theoretical cyclohexa-1, 3, 5-triene and benzene.
Hydrogenation.
Is benzene or theoretical cyclohexa-1, 3, 5-triene more stable?
Benzene
Compare the hydrogenation of cyclohexene (-120 kJmol-1) and theoretical cyclohexa-1, 3, 5-triene (-360kJmol-1) to benzene (-208 kJmol-1). Refer to the enthalpy changes.
Cyclohexene has 1 double bond with an enthalpy change of -120 kJmol-1 when hydrogenated.
If benzene had 3 double bonds, you’d expect an enthalpy of hydrogenation to be -360kJmol-1 but hydrogenation of benzene is -208kJ mol-1.
The difference in energy between theoretical cyclohexa-1, 3, 5-triene and benzene indicates benzene is of higher stability.
Explain why benzene is more stable than theoretical cyclohexa-1, 3, 5-triene.
The delocalised ring of electrons means that electron density is shared over more atoms, so the energy of the molecule is lowered and therefore it becomes more stable.
When naming compounds where the benzene ring is the secondary functional group, what do we call benzene as a prefix?
Phenyl
What type of molecule do benzene rings attract and why?
Electrophiles, they are electron pair acceptors and are usually short of electrons, so attract to regions of high electron density
Name the mechanism for the reaction of benzene with electrophiles.
Electrophilic substitution
Name 2 common electrophiles that are often involved in reactions with benzene.
H+, NO2
- Electrophilic substitution
Why do the electrophiles attract to benzene?
They are attracted to the dense region of negative charge
- Electrophilic substitution
How does the electrophile bond to the benzene?
It takes an electron pair from the centre of the molecule.
- Electrophilic substitution
Why does the benzene molecule become positively charged once the electrophile has bound?
Once the electrophile has taken the electron pair from the centre, this breaks the ring.
- Electrophilic substitution
Why is a hydrogen lost once the electrophile has bound?
To regain the stability of the molecule and ensure only 6 atoms are bound.
Give the conditions for a nitration reaction.
Concentrated H2SO4 catalyst, 55C, nitric acid
Why do we need sulfuric acid during nitration of benzene?
It helps to form the nitronium ion NO2+.
Give an equation for the formation of a nitronium (NO2+ ion)
HNO3 + H2SO4 -> HSO4- + NO2+ + H2O
During nitration, if we only want 1 NO2+ to bond then what temperature must the reaction stay below?
55C
Reduction of nitrobenzene containing compounds can form molecules with what functional group?
Aromatic amines
Give some uses for nitrobenzenes.
Pharmaceuticals, manufacturing dyes, explosives
Give an equation for the reformation of H2SO4 in nitration.
H+ + HSO4- -> H2SO4
Give the general name of the product formed after Friedel Crafts acylation with benzene.
Phenylketone (+ HCl)
Name the reaction where benzene reacts with acyl chlorides.
Friedel Crafts
How can strength of an electrophile be increased?
By reacting it with a halogen carrier catalyst.
Name a halogen carrier.
AlCl3
Name the funcional group that binds to benzene after a Friedel Crafts reaction has taken place.
RCO
In Fridel-Crafts, name the carbocation that attracts to the electron ring inside the benzene.
RC+O
In Fridel-Crafts, why does the RCO+ carbocation attract to the benzene?
The positive carbocation attracts to the region of electron density in the ring.
State the products formed after a Friedel Crafts reaction.
BenzeneRCO + HCl + AlCl3
