18a: arenes - benzenes

Question 1

what are arenes?

Answer 1

ring of hydrocarbons / ring with carbon atoms in which has delocalised e-

Question 2

what does kekule’s model suggest in terms of the structure of benzene?

Answer 2

3 double bonds & 3 single bonds alternating in a ring

Question 3

what is the shape of a benzene?

Answer 3

planar hexagon

Question 4

what is the bond angle in benzenes?

Answer 4

120

Question 5

what does the delocalised model of benzene suggest in terms of structure?

Answer 5

all C-C bonds are the same

Question 6

how does the orbitals from carbon atoms interact to form the bonds in a benzene ring?

Answer 6

1- end-on-end overlapping between p-orbitals from neighbouring C atoms to form a sigma bond
2- 6 delocalised e- were shared equally

Question 7

how does the difference in enthalpies of hydrogenation supports the delocalised structure?

Answer 7

real enthalpy is less than predicted value
-> due to 6 pi-orbital e- are delocalised & not arranged in double bonds -> more thermodynamically stable

Question 8

list all intermolecular forces in benzenes.

Answer 8

weak London forces

Question 9

what type of flame is observed when benzenes combust? why?

Answer 9

sooty/smokey flame due to low C:H ratio

Question 10

what are the reagents, conditions & mechanism of bromination of benzenes?

Answer 10

mechanism: electrophilic substitution
reagent: Br2
conditions: room temp. & FeBr3 catalyst

Question 11

what are the reagents, conditions & mechanism of chlorination of benzenes?

Answer 11

mechanism: electrophilic substitution
reagent: Cl2
conditions: room temp. & FeCl3 / AlCl3 catalyst

Question 12

draw the mechanism of electrophilic substitution of benzenes with bromine. *

Question 13

draw the mechanism of nitration of benzenes. *

Question 14

what are the reagents, conditions & mechanism of nitration of benzenes?

Answer 14

mechanism: electrophilic substitution
reagents: conc. HNO3 & conc. H2SO4 catalyst
conditions: below 55C

Question 15

what is the mechanism of friedel-crafts reactions?

Answer 15

electrophilic substitution

Question 16

what are the reagents and conditions of friedel-crafts alkylation?

Answer 16

reagent - RCl (chloroalkyl compound)
conditions - anhydrous AlCl3 catalyst
- heat under reflux

Question 17

draw the mechanism of friedel-crafts alkylation. *

Question 18

what are the reagents & conditions of friedel-crafts acetylation?

Answer 18

reagents - acyl chloride
conditions - anhydrous AlCl3 catalyst
- heat under reflux at 50C

Question 19

draw the mechanism of friedel-craft acetylation. *

Question 20

describe how the reactivity of the OH group & ring is changed in a phenol group.

Answer 20

lone pair of e- on O is delocalised with charge cloud

Question 21

why does phenol reacts with NaOH?

Answer 21

weakly acidic

Question 22

describe the reagents and conditions of bromination of phenols.

Answer 22

reagents - bromine water
conditions - room temp.

Question 23

draw the mechanism of bromination of phenols. *

Question 24

why does phenols not require a halogen carrier?

Answer 24

lone pair on O on -OH group is partially delocalised in the ring & activates the ring
-> higher e- density & Br2 is more polarised

Question 25

how could 2,4,6-tribromophenol be identified?

Answer 25

white solid, antiseptic smell