Aromatic Chemistry Flashcards
Describe the structure of benzene
Benzene is a cyclic, planar molecule with the formula C6H6;
Carbon has 4 valent electrons;
Each carbon is bonded to 2 other carbons and 1 hydrogen atom;
The final lone electron is in a p-orbital which sticks out above and below the planar ring
What do the lone pair of electrons in benzene combine into to form?
A delocalised ring of electrons.
Why do all the C-C bonds in benzene have the same bond length?
Due to the delocalised electron structure
Describe Kekule’s benzene structure.
Alternating double and single bonds
Why is Kekule’s structure not used?
Because we know electrons are delocalised;
There are no alternating double and single bonds
How do we measure the stability of benzene?
By comparing the enthalpy change of hydrogenation in benzene and cyclohexane-1,3,5-triene
Explain how benzene is more stable than cyclohexa-1,3,5-triene.
The hydrogenation of cyclohexene has an enthalpy chnage of -120kJmol-1;
Cyclohexene has 1 double bond;
If benzene has 3 double bonds, we would expect an enthalpy of hydrogenation 3*120=-360kJmol-1;
However, the actual enthalpy change of hydrogenation for benzene is -208kJmol-1;
This suggests more energy is required to break bonds in benzene than cyclohexane-1,3,5-triene, thus more stable.
What is the stability of benzene due to?
The delocalised electron structure.
Outline the mechanism that arenes undergo.
Electrophilic substitution
Why does benzene not undergo electrophilic addition reactions?
As this would disrupt the stable ring of electrons.
Why are aromatic compounds attacked by electrophiles?
Because the aromatic ring is very electron rich due to cloud of electrons above and below the ring.
Name the two types of electrophilic substitutions arenes undergo.
- Friedel-Crafts Acylation
- Nitration Reaction
Give the equation for the generation of electrophile in a nitration substitution.
HNO3 + 2H2SO4 –>NO2+ + 2HSO4- + H3O+
Outline and draw the mechanism when methyl benzene undergoes electrophilic substitution to make 2-nitromethylbenzene.
State the reagents and condition.
Mechanism: Nitration.
1. Generation of electrophile: HNO3 +2H2SO4 –>
NO2+ + 2HSO4- + H3O+
2. First curly arrow from benzene ring to NO2+
3. Second curly arrow from c-H bond to positively charged ring.
Reagents: concentrated HNO3 & concentrated H2SO4
Conditions: 50 C
What are the three stages when drawing a Friedel-Crafts acylation mechanism?
- Generation of electrophile
- Reaction of electrophile with benzene
- Regeneration of catalyst
How is an electrophile formed in a Friedel-Crafts acylation substitution?
Acyl chloride or acid anhydride reacts with AlCl3 to form RCO+ (electrophile)
How is the catalyst regenerated in a Friedel-Crafts acylation substitution? (acyl chloride)
AlCl4- + H+ –> AlCl3 + HCl
How is the catalyst regenerated in a Friedel-Crafts acylation substitution? (acid anhydride)
[ Cl3Al-O-C(=O)-R]- + H+ –> AlCl3 +
HO-C(=O)-R
Draw and outline the mechanism methylbenzene reacts with ethanoic anhydride and AlCl3 to make 2-methylphenylethanone.
Mechanism: Friedel-Crafts acylation
1. Generation of electrophile:
H3C - C(=O) - O - C(=O) - CH3 + AlCl3 –> H3C - C+ =O + [Cl3Al-O - C(=O) - CH3]-
2. Electrophilic substitution:
- First curly arrow from benzene ring to positive carbocation
- Second curly arrow from C-H bond to positively charged ring.
Regeneration of catalyst:
[Cl3Al - O - C(=O) - CH3 + H+ –> AlCL3 + HO - C(=O) - CH3
State the reagents and conditions for Friedel-Crafts acylation.
Reagents: Acyl Chloride or acid anhydride & AlCl3
Conditions: Anhydrous - to prevent reaction of AlCl3
State some of the uses of nitrobenzene compounds.
To make aromatic amines which can be used to make dyes.
To make explosives e.g. TNT.
