Aromatic Chemistry Flashcards
What is an aromatic compound?
A compound which contains at least 1 benzene ring
What is an aliphatic compound?
A compound which has no benzene rings
What is the kekulé structure of benzene?
Exists as C6H6
Hydrocarbon ring with alternating double bonds
Why did kekulé suggest his structure for the benzene ring?
Suggested it due to the isomers formed when groups were added to the benzene ring.
1 group added formed 1 isomer
2 groups added formed 3 isomers
Why was kekulé’s structure for the benzene ring wrong and what evidence was there to prove this?
Would expect benzene to have similar reactions to cyclic alkenes
Would have expected a different shape of benzene
Would expect the enthalpy of hydrogenation be similar to that of cyclic alkenes at -360Kjmol^-1
Kekulé structure is similar to a cyclic Alkene and would therefore expect similar reactions (addition reactions)
Why is the experimental enthalpy of kekulé structure different to the actual enthalpy?
Kekulé structure has a different enthalpy change to the experimental enthalpy of benzene. Kekulé treated his structure as a cyclic alkene and therefore would have expected the hydrogenation to be -360kjmol^-1 however the actual enthalpy was only -208kjmol^-1 meaning that the real structure was more stable than kekulé’s. This gives an explanation of the delocalised system of electrons which provides stability to the molecule
Why were the problems with the shape of benzene?
Benzene is a planar molecule (all atoms lie in one plane) which would also be true about the kekulé structure of benzene. However C-C bonds are different lengths to C=C bonds meaning that kekulé’s structure would be an irregular hexagon.
Real benzene has the same size bond lengths across all C-C bonds creating a perfect hexagon. This gives evidence for the electrons being share equally across all the carbon atoms in the delocalised system of electrons having a resonating C=C double bond.
How do we name aromatic molecules?
Smallest number combination
Prefix in alphabetical order
Ending of benzene
Eg. 1-Bromo-2-Chlorobenzene
If benzene is not the main functional group the. The prefix is Phenyl
What happens when you heat benzene in air?(combustion)
Like all hydrocarbons benzene will burn in air
C6H6 +7 1/2O2 ——> 6CO2 + H2O
Burns with a smoky flame
What happens when a halogen is added to benzene?
And give the catalyst used
Halogenation reaction ( electrophilic substitution)
An electrophile is attracted to the electron rich delocalised system
Uses an AlCl3 catalyst
What happens when we nitrate benzene?
How is the electrophile generated and what catalyst is used?
Nitration is an electrophilic substitution reaction where a nitro (NO2+) group is the electrophile
Electrophile generation:
HNO3 + H2SO4 ——> NO2+ + HSO4- + H2O
Uses the HSO4- as the catalyst
Catalyst regeneration:
H+ + HSO4- ——> H2SO4
What kind of reaction is used to add alkyl branches or carbonyl groups to aromatic molecules?
Friedel crafts reactions
How is an alkyl group added to benzene?
Using haloalkanes
This is an alkylation reaction by substituting in the haloalkane in for a hydrogen atom
How are carbonyl groups added to benzene molecules?
Using acyl chlorides to attach carbonyl groups
Type of acylation reaction
Occur under reflux
Use anhydrous conditions to ensure the electrophiles do not react with water during reactions
