5.13 arenes Flashcards
what is the naming convention for benzene as a parent and a substituent?
parent: -benzene
substituent: phenyl- (when >6 carbon atoms chain)
compare between the reactivities of benzene and alkenes
1) benzenes are less reactive than alkenes
- since both have C=C regions of electron density, both are expected to be reactive
- however, benzene is resonance stabilised due to delocalised pi electrons, hence benzene is a weak nucleophile and needs a strong electrophile to react
2) benzenes readily undergo electrophilic substitution while alkenes undergo electrophilic addition
describe briefly the mechanism of electrophilic substitution
1) generation of electrophile
- X2 + FeX3 / AlX3 -> X+ + FeX4-
2) formation of positively-charged carbocation intermediate
- electrons transfer from inside ring to X+, forming ring with positive charge and gap in circle at H, E substitution site
3) loss of proton H+ to reform aromatic benzene ring
- electron pair on FeX4- transfer to H, H transfer back into ring to form substituted benzene + H+ + FeX3 regenerated catalyst
what are the 4 reactions benzene can undergo?
reduction, halogenation, nitration. friedel-crafts alkylation
distinguish between and give examples of lewis acid, lewis base, bronsted-lowry acid and bronsted-lowry base
lewis acid and base: electron pair acceptor / donor
- lewis acid: FeX3 and AlX3
bronsted-lowry acid and base: proton donor / acceptor
- bronsted-lowry acid: conc H2SO4
- bronsted-lowry base: conc HNO3
explain how the lewis acid catalyst generates a strong electrophiles
lewis acid catalyst is electron pair acceptor; polarises halogen molecule and accepts lone pair of electrons, generating strong X+ electrophile
state the reagents, conditions and observations for halogenation of benzene
reagents: Cl2 with anhydrous AlCl3
conditions: room temperature
observations: yellowish-green Cl2 decolourises and white fumes of HCl is observed OR
reddish-brown Br2 is decolourised and white fumes of HBr is observed
state the reagents and conditions for nitration of benzene
reagents: conc HNO3 with conc H2SO4
conditions: heat under reflux at 55C
explain how the electrophile is generated by bronsted-lowry acid and base, and state the equation
conc H2SO4 is stronger, hence it acts as bronsted-lowry acid and donates proton to conc HNO3
HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+
state the reagents, conditions and observations for friedel-crafts alkylation for benzene
reagents: CH3X with anhydrous AlX3
conditions: room temperature
observations: white fumes of HCl observed
what are the effects of substituent groups?
substituent groups affect reactivity and position of incoming electrophile
how do substituent groups affect relative reactivity?
activating groups increase reactivity by increasing electron density ; deactivating groups decrease reactivity by decreasing electron density
explain and distinguish between resonance effect and inductive effect
resonance effect donate/withdraw through overlap of p orbital/pi bond
- donating: lone pair in p orbital
- withdrawing: double bond with electronegativity diff
inductive effect: donate/withdraw through sigma bond due to electronegativity diff linking substi to benzene ring
- donating: electron-donating alkyl groups
- withdrawing: electronegative thing
compare toluene and benzene reactivity
toluene is more reactive than benzene due to the electron-donating alkyl group increasing electron density and hence increasing reactivity.
toluene is also capable of siide chain reactions in addition to ring
state the reagents and conditions for nitration of toluene
reagents: conc HNO3 and conc H2SO4
conditions: heat under reflux at 30C
