4.2 Aromaticity Flashcards
Benzene molecular formula
C6H6
Internal C-C-C bond angle of benzene
120 degrees
Length of all C-C bond lengths in benzene
All equal
140 pm
What was kekulé’s structure
A ring of alternating single and double bonds
An alkene
Benzene’s actual structure
•sigma bonds between C and their H
• sp2 hybridised orbitals
• p-orbitals from each C atom overlap and form a delocalised ring system above and below the ring called a delocalised pi-system of electrons
Evidence against kekulé’s structure
• all bonds are the same length in reality - single and dbl bond lengths are different
• benzene reacts by substitution reactions - alkenes react by addition
• enthalpy of hydrogenation of benzene is less than calculated if it had alternate single and double bonds - about 150kjmol-1 which means benzene more stable than kekulé’s structure
What is attached to benzene if the arene is called chlorobenzene
Chlorine
What is an arene
Derivative of benzene
What is attached to benzene if the arene is called nitrobenzene
NO2
What is attached to benzene if the arene is called methylbenzene
CH3
What is attached to benzene if the arene is called phenol
OH
What is attached to benzene if the arene is called benzonitrile
C(triplebond)N
What is attached to benzene if the arene is called benzaldehyde
O=C-H
What is attached to benzene if the arene is called benzoic acid
O=C-O-H
What is attached to benzene if the arene is called Benzyl chloride
O=C-Cl
Does benzene take part in addition reaction
No
What kind of reactions does benzene take part in
Substitution reactions
Why doesn’t benzene carry out addition reactions
The stable delocalised electron ring would be disturbed and the pi-cloud lost
What does benzene substitute in substitution reactions and why
Substitution of the H atoms
To keep electron ring intact
Reactors and conditions for benzene nitration
Conc sulphuric acid
Conc nitric acid
55 degrees celcius
Product of benzene nitration
Nitrobenzene
C6H5NO2
What is intermediate called in benzene nitration
Wheland intermediate
Reactants and conditions of halogenation of benzene
Benzene and halogen(Br or Cl)
Dark- out of direct sunlight
Catalyst such as AlCl3 or FeBr3 (FeCl3)
Why does the halogenation of benzene need to be done in the dark
To avoid homolytic fission
Product of halogenation of benzene
Bromobenzene
Chlorobenzene
…
What is a Friedel-Craft reaction
The alkylation of benzene
It is the substitution of an alkyl group into the benzene ring
Commonest catalyst for Friedel-Craft reaction
Aluminium chloride
Is the C-Cl bond stronger in chlorobenzene or chloroalkane
Chlorobenzene
Why does the C-Cl bond in chlorobenzene require harsher conditions to hydrolise than that in a chloroalkane
It is stronger
Due to overlap between p-electrons on the chlorine and the delocalised pi-system in the ring to give some pi-characteristics to the bond and thus strengthening it
