Arenes Flashcards
Describe the resonance hybrid structure of benzene.
Each of the six carbon atoms in benzene is sp2 hybridized. Three sp2 hybrid orbitals, arranged in a trigonal planar manner and one unhybridised p orbital, perpendicular to the molecular plane.
Two of the three sp2 hybrid orbitals of the carbon atom overlap head-on with the sp2 hybrid orbitals of two adjacent carbons to form two C–C sigma bonds. One of the three sp2 hybrid orbitals of the carbon atom overlaps head-on with the 1s orbital of the hydrogen atom to form the C–H sigma bond. The benzene molecule is planar with all bond angles in the molecule being 120 degrees.
The unhybridised 2p orbital, which is singly occupied, and perpendicular to the plane of carbon atoms, overlaps side-on with the 2p orbital of the neighbouring carbon atoms in the ring. The continuous overlap of the unhybridised 2p orbitals results in a cyclic delocalised pi electron cloud that lies above and below the plane of the ring.
Why do the boiling points of arenes increase with an increase in relative molecular mass?
Boiling points of arenes increase with an increase in relative molecular mass due to an increase in the number of electrons and hence the polarizability of the electron cloud, thereby possessing stronger instantaneous dipole-induced dipole forces of attraction between molecules.
Why are the arenes less reactive than alkenes?
Resonance stabilisation of benzene due to delocalisation of pi electrons renders benzene less reactive (less nucleophilic) toward electrophiles than alkenes.
State the reagents and conditions needed for the nitration of benzene.
concentrated HNO3, concentrated H2SO4 catalyst and temperature <55 degrees for mononitration
State the reagents and conditions needed for the halogenation of benzene.
X2 (X = Cl or Br), Lewis acid catalyst (eg. AlX3 or FeX3), room temperature
State the reagents and conditions needed for the Fiedel-Crafts Alkylation of benzene.
RX (X=Cl or Br) and excess benzene, Lewis acid catalyst (eg AlX3 or FeX3), room temperature
State the reagents and conditions needed for the Fiedel-Crafts Acylation of benzene.
RCOX (X=Cl or Br) and benzene, Lewis acid catalyst (eg AlX3 or FeX3), room temperature
State the reagents and conditions for the catalytic hydrogenation of benzene.
H2(g) and Ni or Pd or Pt catalyst, heat
Describe how activating groups increase the rate of electrophilic substitution, relative to a hydrogen atom and state an example of an activating group.
Activating groups donate electron density to the benzene ring, increasing the availability of the pi-electron cloud, and stabilising the arenium intermediate by dispersing the positive charge, hence lowering the activation energy, rendering the benzene ring more susceptible toward electrophilic attack.
eg. CH3 group
Describe how deactivating groups decrease the rate of electrophilic substitution, relative to a hydrogen atom and state an example of a deactivating group.
Deactivating groups withdraw electron density away from the benzene ring, decreasing the availability of the pi-electron cloud, and destabilising the arenium intermediate by intensifying the positive charge hence raising the activation energy, rendering the benzene ring less susceptible toward electrophilic attack.
eg. -Cl and COOCH3
