Reactions Of Benzene Flashcards
Why does benzene not undergo electrophilic addition
As this would involve breaking up the stable delocalised ring of electrons, instead undergoes electrophilic substitution
Process of electrophilic substitution with benzene
Delocalised ring is area of high electron density so is attacked by electrophiles which partially destroys it to form positive charge in horseshoe (unstable intermediate) but is then restored to form the aromatic product and H+ ion
Equation for how Cl+ produced
AlCl3 + Cl2 ——> AlCl4- + Cl+
Explain process of chlorination of benzene
Need Cl2 and catalyst of AlCl3 which polarised the chlorine molecule making it easier for chlorine bond to break so Cl+ can act as electrophile
Catalyst and reagent for bromination of benzene and why required for halogenation
Reagent Br2 and catalyst FeBr3, requires to regenerate the electrophile
How is benzene toxic
Carcinogen
How does benzene combust and why
Combusts with a very smoky flame as high carbon to hydrogen ratio
What does nitration of benzene make and conditions needed
Nitrobenzene, need conc nitric acid in presence of conc sulfuric acid catalyst
What is the electrophile in nitration and equation for its formation
NO2+
HNO3 + 2H2SO4 —>NO2+ +2HSO4- + H3O+
How to draw horseshoe shape of intermediate
Must not extend between carbons 2 and 6
Process of nitration
NO2+ replaces hydrogen atom, hydrogen ion released and reacts with HSO4- to reproduce sulfuric acid catalyst
What is nitration of benzene used to produce
Explosives/ dye stuffs
What temp is nitration done at and why and when else can this happen
60 degrees Celsius, at higher temps multiple substitutions can occur on benzene ring, if benzene ring already has side group nitro group can join at dif positions
What is Friedel-crafts acylation
When delocalised ring in benzene acts as nucleophile and attacks acyl chlorides, requires heating under reflux as does friedel crafts alkylation
What does acylation of benzene produce and reagents needed and conditions
Phenyl ketone, need acyl chloride in presence of anhydrous aluminium chloride catalyst
-heat under reflux at 50 degrees celcius
How is reactive intermediate made in acylation and what does this act as
R-COCl +AlCl3 —->R-CO+ + AlCl4-
R-CO+ attacked by benzene ring which acts as nucleophile
Why is acylation important
As introduces reactive functional group
How is catalyst reformed in acylation and alkylation
H+ reacts with AlCl4- to reform AlCl3 catalyst and HCl
What does alkylation produce, reagents and conditions
Alkylbenzene, need chloroalkane in presence of aluminium chloride catalyst
-heat under reflux
How is electrophile formed in alkylation
AlCl3 + RCl —> R+(electrophile) + AlCl4-
Reagents and conditions for hydrogenation and what produced
Hydrogen, need nickel catalyst and 200C and 30atm, produces cyclohexane
What do side groups eg alkyl/phenol/amines do to delocalised system
Electron releasing into delocalised system which increases electron density in ring, more attractive to electrophiles so carries out substitution in milder conditions and changes properties and reactions of side geoup
What happens to properties when Cl added
C-Cl made stronger so typical halogenoalkane substitution and elimination reactions do not occur, electron rich benzene ring repels nucleophiles
What happens to properties of OH when added to benzene
Delocalisation makes C-O bond stronger and O-H weaker, phenol does not act like alcohol as more acidic and does not oxidise
What happens when NH2 added to properties of benzene
Less basic than aliphatic amines as lp delocalised and less available for accepting proton
What bond forms between Cl+ and AlCl3
Dative covalent
