Arenes

Question 1

Physical properties of benzene

Answer 1

1. Colourless liquid with a characteristic odour
2. Non-polar, insoluble in water, and less dense than water

Question 2

Resonance of benzene ring

Answer 2

Each carbon atom has an unhybridised P orbital occupied by a single electron.

Each P orbital overlaps sideways with adjacent P orbital on either side

Continuous overlap of P orbitals results in a cloud of cyclic delocalised Pi electrons.

Question 3

Electrophilic substitution mechanism

Answer 3

Step 1: This is the slow step (rate-determining step) which involves the destruction of the aromaticity in the benzene ring. Carbocation is formed as electrophile attacks the electron rich benzene, taking two electron of six in the electron cloud.

Step 2: This is the fast step which involves the carbocation in losing a proton from the carbon atom containing the electrophile. Two electrons bonded to the proton becomes part of the delocalised Pi electron system.

Question 4

Reaction of benzene rings: ES with nitric acid

Answer 4

Reagent & condition: Concentrated nitric acid, concentrated sulfuric acid and refluxed in water bath 55-60’c

Question 5

Reaction of benzene rings: ES with halogens

Answer 5

Reagent & conditions: Fe/FeX3/AlX3 anhydrous, room temperature

Question 6

Reaction of benzene rings: Freidel-crafts alkylations

Answer 6

Reagent & conditions: Anhydrous AlCl3 as catalyst

Question 7

Reaction of benzene rings: Reduction

Answer 7

Raney nickel, 150’c OR Ni catalyst, high temp & pressure

Question 8

Reaction of alkyl benzene: Free radical substitution

Answer 8

X2, sunlight/UV light

Question 9

Reaction of alkyl benzene: Oxidation

Answer 9

KMnO4 (aq), dilute H2SO4, heat
OR
KMnO4 (aq), dilute NaOH, followed by H2SO4

Note:
Product is a benzoic acid, CO2, H2O
K2Cr2O7 (aq) unable to oxidise

Question 10

Effects of -CH3 group in methylbenzene, on reactivity and orientation

Answer 10

1. Lower temperature required to undergo ES as -CH3 is an activating group
2. -ch3 group is 2,4 directing

Question 11

Effect of activating group on benzene ring

Answer 11

Activating group is electron donating. Increase in electron density in benzene ring makes ring more susceptible to electrophilic attack.
AG helps to disperse the positive charge on the intermediate carbocation and leads to stabilisation of carbocation.

Question 12

Effect of deactivating group on benzene ring

Answer 12

Deactivating group is electron withdrawing
Decrease in electron density in benzene ring makes ring less susceptible to electrophilic attack.
DG tends to intensify the positive charge on the intermediate carbocation and leads to destabilisation of carbocation