Synthesis 8

Question 1

Why doesn’t benzene undergo electrophilic addition

Answer 1

It doesn’t have any true c=c bonds as the 6 electrons in the pi orbital are delocalised
It also has a very stable structure

Question 2

Explain benzenes hybridisation

Answer 2

Sp2 hybridisation, planar shape

Question 3

What reaction does benzene usually undergo and why

Answer 3

Electrophilic substitution. 
Delocalised electrons (pi) attract electrophiles 
Addition reaction would disrupt delocalisation and decrease stability

Question 4

How does electrophilic substitution of benzene work

Answer 4

Electrophiles attacks one carbon in the ring and forms unstable intermediate ion
The positive charge in this ion is delocalised around the ring
H+ is then lost to regain stability

Question 5

What are the electrophilic substitution reactions benzene undergoes and what are its electrophile and reagent

Answer 5

1. Alkylation. CH3. CH3Cl/AlCl3
2. Halogenation. Cl+/Br+. FeCl3/FeBr3
3. Sulphonation. HOSO2. Conc H2SO4
4. Nitration. NO2+. Conc HNO3+H2SO4

Question 6

Describe Halogenation of a benzene ring

Answer 6

Catalysts = FeBr3, AlCl3, FeCl3
The catalyst accepts a pair of electrons from one atom in the halogen molecule leaving the other as electrophile X+
Electrophilic substitution continues as usual

Question 7

Describe the nitration of a benzene ring

Answer 7

Benzene reacts with nitric acid when conc nitric & sulphuric acids are used
This reacts to form electrophile NO2+
This ion continues electrophilic substitution as usual
Nitrobenzene is used to form phenylamine which can be further substituted to di and tri compounds

Question 8

Describe the sulphonation of a benzene ring

Answer 8

Benzene reacts with sulphuric acid after heated under reflux for hours
Fuming sulphuric acid (sulphuric + SO3) is used under cold condition
The sulphur atom carries a partial positive charge and can attack the benzene ring as usual

Question 9

Describe the alkylation of a benzene ring

Answer 9

AlCl3 catalyst
Haloalkanes polarised by catalyst leaving positive alkyl group R+
Substitution as normal

Question 10

What is alkylation of benzene also know as

Answer 10

Friedel-Crafts reaction

Question 11

Phenol properties

Answer 11

Benzene + OH
Weak acid and reacts like carboxylic acid
Dissociation produces phenoxide ion
Negative charge on O can be delocalised to increase stability

Question 12

Phenylamine/aniline/aminobenzene properties and what can it become through reaction

Answer 12

Aromatic primary amine
Weaker base than non-aromatic equivalent as N lone pair is delocalised through whole ring
Weak base accepts Hydrogen ions to form phenylammonium ion which has no delocalisation
Stability phenylamine>phenylammonium