4. Arenes

Question 1

are electron donating groups usually 2,4 or 3- directing?

Answer 1

2,4-

Question 2

arenes undergo what kind of reactions?

Answer 2

electrophilic substitution

Question 3

state reagents and conditions for electrophilic substitution of NO2 to benzene

Answer 3

conc HNO3, conc H2SO4, 55°C

Question 4

nitrobenzene is what colour?

Answer 4

yellow liquid with almond smell

Question 5

state reagents and conditions for electrophilic substitution of halogen to benzene

Answer 5

Br2, anhydrous FeBr3/AlBr3 [change accordingly to halogen]

Question 6

state reagents and conditions for electrophilic substitution of alkyl groups to benzene

Answer 6

RX, anhydrous AlX3/FeCl3

Question 7

during electrophilic substitution of benzene, why must catalyst eg. AlBr3/FeBr3 be anhydrous?

Answer 7

the catalyst can undergo partial hydrolysis in water

Question 8

state briefly the 3 steps of electrophilic substitution of benzene

Answer 8

1. generation of electrophile
2. electrophilic attack of benzene ring to form carbocation intermediate
3. loss of H+ from carbocation to restore aromaticity + regenerate catalyst

Question 9

if an activating group is alr bonded to benzene, condition for electrophilic addition will be harsher/milder?

Answer 9

milder, as benzene will be more reactive

Question 10

physical properties of benzene

Answer 10

benzene is trigonal planar with respect to each C atom at the corners of a regular hexagon (planar thus symmetrical) & exists as non-polar simple molecule
1. colourless liquid
2. bp 80°C; mp 5.5°C
3. sparingly soluble in water (forms upper layer)

Question 11

chemical reactions of arenes

Answer 11

1. oxidation
2. addition
3. combustion
4. electrophilic substitution

Question 12

state reagents and conditions for electrophilic substitution of acyl groups to benzene

Answer 12

(R-C=O)X, anhydrous AlX3/FeX3

Question 13

describe a methylbenzene

Answer 13

methylbenzene has a methyl group attached to the benzene ring, where C atom of methyl uses sp3 orbitals to form 4 σ bonds, hence 4 bp of e- form a tetrahedral shape with bond angle 109.5°

Question 14

state the physical properties of methylbenzene

Answer 14

1. colourless liquid
2. bp 111°C ; mp -95°C
3. less toxic than benzene & more useful as solvent

Question 15

chemical reactions of methylbenzene

Answer 15

1. side chain reactions (free rad sub & oxidation)
2. electrophilic substitution of benzene rings

Question 16

reagents and conditions of free radical substitution of side chain of methylbenzene

Answer 16

X2, uv light, heat

Question 17

reagents and conditions of acidic & alkaline oxidation of methylbenzene

Answer 17

KMnO4(aq), H2SO4(aq) acidic / NaOH(aq) alkaline , heat

Question 18

side chain oxidation only occurs if there is at least ____ present

Answer 18

a benzylic H

Question 19

chemical reactions of electrophilic substitution of methylbenzene

Answer 19

1. nitration
2. halogenation

Question 20

is methylbenzene/benzene more reactive?

Answer 20

methylbenzene. methyl group donates electrons into the ring, making it more susceptible to electrophilic attack (activating)

Question 21

describe halogenation of methylbenzene

Answer 21

methylbenzene chlorinated in the presence of catalyst such as anhydrous halogen carrier & in the dark to present FRS of halogen with the alkyl groups on the benzene