Organic 10: Aromatic Chemistry Flashcards
What are arenes?
Hydrocarbons of benzene
Describe the structure of benzene.
Flat regular hexagon of carbon atoms
Each of which is bonded to a single hydrogen atom
Each carbon atom forms three covalent bonds, so one is delocalised
These delocalised electrons from a ring of electron density above and below the ring
Each carbon is an intermediate between a single and double bond
Why is benzene stable?
Aromatic stability
Due to then delocalised system
Why is the enthalpy of hydrogenation of cyclohexa-1,3-diene higher but not double that of cyclohexene?
Because the double bonds are separated by one single bond
Allowing some delocalisation of p-orbitals
Enthalpy change of cyclohexa-1,3-diene could be -200kJmol-1
Describe how enthalpy of hydrogenation changes from one cyclic compound to another.
A cyclic compound with more double bonds will have the most exothermic enthalpy of hydrogenation
Even distribution of electron density leads to less repulsion between adjacent electron clouds
Why is benzene more stable than cyclohexene?
Because benzene has a ring of delocalised electrons which provides stability to the molecule
Cyclohexene is unsaturated
Describe benzene at room temperature.
Colourless liquid
Compare the boiling point of benzene with hexane.
Boiling point of benzene is much higher than hexane’s because benzene’s flat hexagonal molecules pack together well in the solid state
They are therefore harder to separate during melting
Why are arenes insoluble in water?
Because they are non-polar
But they can mix with hydrocarbons and other non-polar solvents
Describe how arenes combust.
Burn in air with smoky flames
Arenes have a higher carbon:hydrogen ratio compared with alkanes
So there is usually unburnt carbon remaining when they burn in air, which produces soot
What is the most typical reaction for benzene and why?
Electrophilic substitution
Delocalised electron system has a high electron density so attracts positive electrophiles
Describe the general mechanism of electrophilic substitution reactions.
1) The aromatic ring attracts electrophile due to its high electron density
2) Bond forms between carbon and electrophile using electrons from the delocalised ring, destroying the aromatic system
3) The carbon must then lost a H+, and the electron in the C-H bond returns to the delocalised system
How is the electrophile generated for nitration?
Using concentrated nitric acid and concentrated sulfuric acid (catalyst)
H2SO4 + HNO3 –> HSO4- + NO2+ + H2O
How is the catalyst regenerated in nitration?
The H+ produced from the electrophilic substitution reaction reacts with HSO4- to regenerate H2SO4
What are nitrated arenes used for?
Production of TNT
Making aromatic amines to be used to make industrial dyes
Draw the mechanism for nitration of benzene.
Draw the mechanism for Friedel-Crafts acylation.
