Unit 4.2 - Aromaticity (Physics&mathstutor))

Question 1

Give molecular formula of benzene

Answer 1

C₆H₆

Question 2

describe bonding in benzene

Answer 2

• each carbons form a covalent bond to the carbons next to it and a hydrogen atom (meaning only 3/4 bonds are used)
• each unpaired carbon atom combine to form a delocalised ring of electrons

Question 3

describe delocalised system of benzene in terms of types of bonds involved

Answer 3

p-orbitals on each carbon atom in benzene overlap with p-orbitals either side of them ∴ producing pi bond system

Question 4

what is the shape of benzene

Answer 4

trigonal planar
regular hexagon

Question 5

bond angle in benzene

Answer 5

120 degrees

Question 6

general name for compounds containing a benzene ring:

Answer 6

arene/aromatic compounds

Question 7

does benzene undergo electrophilic addition reactions?

Answer 7

no

Question 8

why does benzene not undergo electrophilic addition reactions?

Answer 8

its ring of delocalised electrons is very stable

Question 9

what are the 2 common electrophilic substitution mechanisms for benzene

Answer 9

1. friedel-crafts acylation
2. nitration reaction

Question 10

what is the catalyst in friedel-crafts acylation of benzene

Answer 10

AlCl₃
used to generate electrophile

Question 11

what conditions are required for Friedel-crafts acylation reactions

Answer 11

heated under reflux (in non-aqueous solvent)

Question 12

why are electrophiles attracted to benzene

Answer 12

• electrophile are attracted to areas of high electron density (electron pair acceptors)
• benzene has a ring of delocalised electrons (high area of electron density) ∴ attracts electrophiles

Question 13

give the reactants required for nitration of benzene

Answer 13

benzene + conc nitric acid

Question 14

give the catalyst required for the nitration of benzene

Answer 14

conc sulfuric acid

Question 15

how is the electrophile (NO₂⁺) produced for nitration of benzene

Answer 15

conc nitric and sulfuric acid react to form NO₂⁺ intermediate

Question 16

what is mononitration

Answer 16

when only 1 NO₂ group is added to benzene

Question 17

what conditions ensure mononitration will occur in the nitration of benzene

Answer 17

temp is kept under 55°C

Question 18

why is the C-Cl bond stronger in benzene than in a chloroalkane

Answer 18

lone pair of electrons on the chlorine atom is delocalised over benzene ring, ∴ strengthens C-Cl bond

Question 19

why is chlorobenzene unreactive towards nucleophiles

Answer 19

the C-Cl bond in benzene is stronger than in halogenoalkanes.
∴ is harder to break ∴ not easily attacked by nucleophiles