Reactions Of Benzene

Question 1

Why does benzene not undergo electrophilic addition

Answer 1

As this would involve breaking up the stable delocalised ring of electrons, instead undergoes electrophilic substitution

Question 2

Process of electrophilic substitution with benzene

Answer 2

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

Question 3

Equation for how Cl+ produced

Answer 3

AlCl3 + Cl2 ——> AlCl4- + Cl+

Question 4

Explain process of chlorination of benzene

Answer 4

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

Question 5

Catalyst and reagent for bromination of benzene and why required for halogenation

Answer 5

Reagent Br2 and catalyst FeBr3, requires to regenerate the electrophile

Question 6

How is benzene toxic

Answer 6

Carcinogen

Question 7

How does benzene combust and why

Answer 7

Combusts with a very smoky flame as high carbon to hydrogen ratio

Question 8

What does nitration of benzene make and conditions needed

Answer 8

Nitrobenzene, need conc nitric acid in presence of conc sulfuric acid catalyst

Question 9

What is the electrophile in nitration and equation for its formation

Answer 9

NO2+
HNO3 + 2H2SO4 —>NO2+ +2HSO4- + H3O+

Question 10

How to draw horseshoe shape of intermediate

Answer 10

Must not extend between carbons 2 and 6

Question 11

Process of nitration

Answer 11

NO2+ replaces hydrogen atom, hydrogen ion released and reacts with HSO4- to reproduce sulfuric acid catalyst

Question 12

What is nitration of benzene used to produce

Answer 12

Explosives/ dye stuffs

Question 13

What temp is nitration done at and why and when else can this happen

Answer 13

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

Question 14

What is Friedel-crafts acylation

Answer 14

When delocalised ring in benzene acts as nucleophile and attacks acyl chlorides, requires heating under reflux as does friedel crafts alkylation

Question 15

What does acylation of benzene produce and reagents needed and conditions

Answer 15

Phenyl ketone, need acyl chloride in presence of anhydrous aluminium chloride catalyst
-heat under reflux at 50 degrees celcius

Question 16

How is reactive intermediate made in acylation and what does this act as

Answer 16

R-COCl +AlCl3 —->R-CO+ + AlCl4-
R-CO+ attacked by benzene ring which acts as nucleophile

Question 17

Why is acylation important

Answer 17

As introduces reactive functional group

Question 18

How is catalyst reformed in acylation and alkylation

Answer 18

H+ reacts with AlCl4- to reform AlCl3 catalyst and HCl

Question 19

What does alkylation produce, reagents and conditions

Answer 19

Alkylbenzene, need chloroalkane in presence of aluminium chloride catalyst
-heat under reflux

Question 20

How is electrophile formed in alkylation

Answer 20

AlCl3 + RCl —> R+(electrophile) + AlCl4-

Question 21

Reagents and conditions for hydrogenation and what produced

Answer 21

Hydrogen, need nickel catalyst and 200C and 30atm, produces cyclohexane

Question 22

What do side groups eg alkyl/phenol/amines do to delocalised system

Answer 22

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

Question 23

What happens to properties when Cl added

Answer 23

C-Cl made stronger so typical halogenoalkane substitution and elimination reactions do not occur, electron rich benzene ring repels nucleophiles

Question 24

What happens to properties of OH when added to benzene

Answer 24

Delocalisation makes C-O bond stronger and O-H weaker, phenol does not act like alcohol as more acidic and does not oxidise

Question 25

What happens when NH2 added to properties of benzene

Answer 25

Less basic than aliphatic amines as lp delocalised and less available for accepting proton

Question 26

What bond forms between Cl+ and AlCl3

Answer 26

Dative covalent