what are the criteria that an arene compund fulfils
-cyclic/ring structure
-planar
-alternating single/double bonds resulting in overlapping p orbitals
-4n+2 delocalised pi electrons in the overlapping p orbitals
why do arene compunds have added stability
delocalisation of pi electrons over the carbon atoms in the ring
why is benzene a weaker nucleophile than alkenes
pi electrons in the beneze ring are delocalised, thus the resonance stability of benzene decreases its reactivity towards electrophiles
why does benzene not undergo electrophilic addition
electrophilic addition will disrupt the side-on overlap of p orbitals throughout the ring, which changes the hybridisation of 2 carbon atoms in benzene from sp2 to sp3, leading to a loss in aromaticity and resonance stability
what is a lewis acid
electron pair acceptor
why is a lewis acid catalyst needed in the reaction of benzene with halogen, (effect of lewis acid catalyst on halogen) and what is an example of a lewis acid catalyst commonly used
w/o catalyst, halogens are not electrophilic enough to disrupt the aromatic stability of benzene ring. E.g. using anyhdrous AlCl3 will generate a stronger electrophile, as the lewis acid catalyst polarises the halogen molecule and accepts a lone pair of electrons from it.
why is the formation of a carbocation intermediate in electrophilic substitution a slow step
This is because it destroys the extra stability associated with six pi electrons delocalised in the benzene ring
in the halogenation of benzene, what does AlX3 acts as and what is its function?
lewis acid is an electron pair acceptor, polarises halogen and accepts lone pair of electrons from it, forming stronger electrophile
function of a lewis base
electron pair donor
reagents conditions for benzene react with chlorine, and type of reaction, observations
cl2, anyhydrous AlCl3 at room temperature, electrophilic substitution, greenish yellow cl2 decolourised, white fumes of hcl formed
reagents conditions for benzene react with bromine, and type of reaction, observations
br2, anyhydrous AlBr3 at room temperature, electrophilic substitution, reddish brown br2 decolourised, white fumes of hbr produced
reagents conditions for reaction benzene react with CH3Cl/halogenoalkane, type of reaction, observations
CH3Cl, anhydrous AlCl3 at room temperature, electrophilic substitution, white fumes of hcl produced
reagents conditions for nitration of benzene, type of reaction
conc HNO3, conc H2SO4, 50 degrees celcius, electrophilic substitution
what will happen if temperature is higher than 50 degrees celcius?
further substitution at the benzene ring may occur
what are the roles of conc nitric and sulfuric acid in nitration of benzene, and what do the roles do
conc nitric: bronsted-lowry base (proton acceptor), conc sulfuric: bronsted-lowry acid (proton donor)
overall equation for formation of stronger nucleophile for nitration of benzene
HNO3 + 2H2SO4 (REVERSIBLE ARROW) -> NO2+ + 2HSO4- + H3O+
characteristics of activating groups
can be deactivating also
electron donating substituents in substituted benzenes increase the elctron density of the benzene ring, making the ring a stronger nucleophile than benzene
reactivity of benzene ring towards electrophilic substitution increases
electrophilic substitution occurs faster and require a lower temperature
what is the inductive effect
effect of electron donation or withdrawal felt by polarisation of sigma bonds in molecule
when does delocalisation take place
delocalisation of pi electrons occur when there are 3 or more p orbitals overlapping side-on continuously
what is the pre-dominant effect that halogen substituents exhibit and why
the substituents are deactivating groups and while halogen substituents have a lone pair of electrons that can be delocalised into the benzene ring, pre-dominant effect is electron withdrawal from benzene ring by inductive effect, as they are deactivating groups
what is the pre-dominant effect that OH and -OR substituents exhibit and why
electronegative nature of O may result in electron withdrawal from benzene ring, delocalisation of lone pair of electrons into benzene ring is more dominant, as they are activating groups
define steric effect
repulsion of electron clouds of atoms due to proximity or arrangement in space
why is methylbenzene preferred as a solvent over benzene
less toxic
why does electrophilic substitution for methylbenzene occur faster than benzene
methyl substituent is electron donating group, increases electron density, more reactive towards ES compared to benzene
