aromatic compounds Flashcards
kekulé’s model of benzene
alternating longer C-C and shorter C=C bonds
reacts by electrophilic addition reactions across the double bonds
enthalpy change of hydrogenation is 3x that of cyclohexene
delocalised model of benzene
sideways overlap of p-orbitals
above and below the planar ring
forming a π-system
electrons are delocalised
lengths of bonds are equal
intermediate between C-C and C=C
angle is 120 around each carbon - trigonal planar
ring is planar
evidence against kekulé’s structure
bond lengths are equal, intermediate between C-C and C=C
electrophilic substitution not addition - bromine water doesn’t decolourise
enthalpy change of hydrogenation is less exothermic than expected
why is benzene less reactive than cyclohexene?
in benzene, the π-system electrons are delocalised across all 6 C atoms
in cyclohexene, the π-bond electrons are localised between 2 C atoms, making the double bond more electron dense
this allows it to induce a dipole in the Br2 molecule so it attracts electrophiles more strongly
nitration of benzene
reagents: concentrated nitric acid, benzene
conditions: heat under reflux at 50oC, H2SO4 catalyst
generation of electrophile: HNO3 + H2SO4 -> NO2+ + H2O + HSO4-
electrophilic substitution: benzene -> nitrobenzene + H+
regeneration of catalyst: H+ + HSO4- -> H2SO4
bromination of benzene
reagents: Br2, benzene
conditions: halogen-carrier catalyst (FeBr3 or AlBr3)
generation of electrophile: FeBr3 + Br2 -> Br+ + FeBr4-
electrophilic substitution: benzene -> bromobenzene + H+
regeneration of catalyst: H+ + FeBr4- -> HBr + FeBr3
chlorination of benzene
reagents: Cl2, benzene
conditions: halogen-carrier catalyst (FeCl3 or AlCl3)
generation of electrophile: FeCl3 + Cl2 -> Cl+ + FeCl4-
electrophilic substitution: benzene -> chlorobenzene + H+
regeneration of catalyst: H+ + FeCl4- -> HCl + FeCl3
-3 electron withdrawing groups
-NO2
-2 and -4 electron donating groups
-NH2
-OH
-2 and -4 electron withdrawing groups
-F
-Cl
-Br
-I
why is cyclohexene more reactive towards
electrophiles, such as bromine, than benzene?
the π-bond in an alkene is more electron dense than the π-system in benzene
because the 2 π electrons in an alkene are localised between 2 C atoms, whereas the 6 π electrons are delocalised across all 6 C
atoms in benzene
so the π bond in the alkene is sufficiently electron dense to attract electrophiles
what do curly arrows show?
movement of an electron pair
what does a halogen carrier catalyst do?
it takes one of the halogen atoms from the diatomic molecule and ‘carries’ it
through the reaction and then release it at the end in the presence of the hydrogen ion
eg AlBr3 or FeBr3
why is phenol more reactive than benzene?
in phenol the lone pair of electrons in the p-orbital of the oxygen atom is
donated and becomes partially delocalised into the π-system (8 electrons)
therefore the π-system in phenol is more electron dense than in benzene
reactions of phenol compared with benzene
phenols undergo
electrophilic substitution
reactions but are more reactive so their reactions take place more readily and under milder conditions.
