arenes

Question 1

Why are benzene less reactive towards electrophiles as compared to alkenes?

Answer 1

• Due to the presence of pi electrons, both alkenes and benzenes are e-rich and are expected to react w E
• due to its delocalised pi e cloud around the ring, benzene is resonance stabilised = weak Nu
• this results in decreased reactivity towards E. So, strong E required to react with benzene

Question 2

why benzene readily undergoes electrophilic substitution while alkenes readily undergo electrophilic addition?

Answer 2

electrophilic addition=> change in hybridisation of 2 C atoms in benzene - from sp2 to sp3

p-orbitals no longer overlap throughout the ring. benzene will lose its aromaticity and its resonance stability

(H2 (g) w Ni catalyst only addition reaction it reacts)

Question 3

why the slow step is the
step 1– formation of +vely-charged carbocation intermediate?

Answer 3

cuz it destroys the extra stability associated with the delocalisation of the 6 e in benzene ring. high Ea.

Question 4

why benzene only reacts with halogens in the presence of a catalyst?

Answer 4

halogen itself not a strong E to distrupt the stable aromatic ring

Question 5

Cl2 with anhydrous FeCl3/AlCL3/Fe?

Answer 5

presence of water in the reaction mixture will render the catalyst useless as ( ) catalyst will react with water to form other products

Question 6

why hcl exist as white fumes after chlorination of benzene?

Answer 6

hcl not soluble in most organic solvents. but when water is used as a solvent, white fumes will not be observed

Question 7

what are activating groups?

Answer 7

e-donating substituents increase the e density of the benzene ring, making it a stronger nucleophile. thus, it increases the reactivity of the benzene ring

(can allow benzene ring to occur at a faster rate, and also can occur at lower temperature)

Question 8

what are de-activating groups?

Answer 8

e-withdrawing substituents decrease the e density of the benzene ring, making it a weaker nucleophile. thus, it decreases the reactivity of the benzene ring

(allow benzene ring to occur at a slower rate, and only can occur at higher temperature)

Question 9

what is resonance effect?

Answer 9

donation or withdrawal of e through the overlap of p orbital on the substituent w a p orbital on the carbon atom of benzene ring

Question 10

what is lewis acid

Answer 10

e-pair acceptor

Question 11

what is lewis base

Answer 11

e-pair donor

Question 12

what is bronsted-lowry acid

Answer 12

proton (H+) donor

Question 13

what is bronsted-lowry base

Answer 13

proton (H+) acceptor

Question 14

why nitration of methylbenzene only need a lower temperature of 30C than benzene (55C) for electrophilic substitution?

Answer 14

this is due to the activating methyl group which makes the ring more susceptible to electrophilic substitution

Question 15

what happens when more nitro groups are introduced into the methylbenzene?

Answer 15

the benzene ring is further deactivated.

s subsequent nitration will require more vigorous conditions