Arene

Question 1

What is the bonding if benzene

Answer 1

All six carbon atoms in benzene are sp2 hybridised. The six unhybridised 2p orbitals interact to form a large delocalised pie electron cloud that spreads over the six nuclei

Question 2

Why is the benzene molecule more stable than an alkene

Answer 2

It is resonance stabilised

Question 3

Is the bond length between adjacent carbons in benzene shorter than the bond length between them in alkane

Answer 3

Yes.

Question 4

What is the resonance energy of benzene

Answer 4

149kJmol-1

Question 5

Why is benzene less reactive than an alkene

Answer 5

(i) Delocalisation of the six electrons makes benzene more energetically stable. This causes a stronger electrophile to be required.
(ii) Addition reactions will cause the disruption of delocalised pie electron cloud of benzene causing benzene to lose resonance stability. Hence benzene undergoes substitution reactions instead so as to preserve resonance stability.

Question 6

What are the reactions benzene undergo

Answer 6

(i) combustion
(ii) reduction under harsh conditions
(iii) electrophilic substitution

Question 7

Reagents needed for halogenalkane reaction with benzene

Answer 7

anhydrous AlCl3 (or FeCl3) or anhydrous AlBre (or FeBr3)
RX (where R is an alkyl group and X = Cl or Br)
Rtp

Question 8

Reagents needex for reaction of acid chloride and benzene

Answer 8

Anhydrous FeCl3 (or AlCl3)
RCOCl (where R is an alkyl or aryl group)
Rtp

Question 9

Reagents needed for halogen and benzene reaction

Answer 9

Cl2 or Br2
Anhydrous AlCl3 or FeCl3 or anhydrous AlBr3 or FeBr3
Rtp

Question 10

Reaction of concentrated nitric acid with benzene

Answer 10

Concentrated HNO3
Concentrated H2SO4
Water bath 60°C

Question 11

Obtaining phenylamine from nitrobenzene

Answer 11

1. Sn, HCl, heat

2. NaOH (aq), rtp

Question 12

What is the electrophilic substitution mechanism

Answer 12

For the reaction with Br2,

1. Lewis acid catalyst reacts with Br2 to generate Br+
2. The pie electrons cloud attracts tge Br+ to form the positively charged arenium ion (slow step)
3. A hydrogen atom is abstracted from the areniym ion by AlBr4- to restore aromaticity of the benzene ring and to form the product as well as to regenerate the AlBr3 catalyst

Question 13

How is the reactivity of the benzene ring affected by the substituent groups

Answer 13

(i) an electron-donating substituene makes the benzene ring more electron-rich than the benzene molecule and hence, the benzene is more reactive towards electrophilic substitution. Thus, the reaction conditions used for electrophilic substitution are milder. This leads rate of reaction to be faster

Question 14

How can electrons be donated into the benzene ring

Answer 14

(i) through the sigma bond - inductive effect
[Viable when there is a difference in electronegativity]
(ii) through the pie bond - resonance effect
[Viable when there is overlapping of p orbitals between the sp2 hybridised C atom of the benzene ring and the substituent it is bonded to]

Question 15

To distinguish alkylbenzenes from alkanes and benzenes

Answer 15

KMnO4 with dilute H2SO4, heat
Alkanes and benzene - no decolourization of purple KMnO4
Methylbenzene - decolourization of purple KMnO4
Ethylbenzene - decolourization of purple KMnO4 with effervescence. Gas evolved gives a white precipitate with limewater