O- Aromatics

Question 1

proposed structure of benzene by Kekule

Answer 1

cyclohexa-1,3,5-triene
alternating single + double bonds

Question 2

Evidence that benzene is not a triene

Answer 2

1. all bond lengths equal (X-ray diffraction)- lengths in betw C-C single & double
2. enthalpy of hydrogenation ↓ than expected- thermodynamically, 152kJmol-1 ↑ stable than theoretical cyclohexa-1,3,5-triene
3. x react by electrophilic addition (like alkenes)

Question 3

Why are more reactive electrophiles needed to react with aromatic rings than alkenes?

Answer 3

• involves initial disruption to ring
• electrophiles must be ↑ powerful

Question 4

Reagents for nitration

Answer 4

1. conc nitric acid (HNO3)
2. conc sulfuric acid (H2SO4)- catalyst

Question 5

condition for nitration

Answer 5

reflux at 55C

Question 6

uses of nitrated arenes

Answer 6

• explosives
• aromatic amines- industrial dyes

Question 7

equation for generation of NO2+ electrophile

Answer 7

H2SO4 + HNO3 –> NO2+ + HSO4- + H2O

Question 8

examples of electron-releasing groups

Answer 8

CH3, OCH3, OH, NH2

Question 9

examples electron-withdrawing groups

Answer 9

NO2, COCl

Question 10

state of benzene at room temp

Answer 10

colourless liquid

Question 11

Why does benzene have a higher melting point than hexane?

Answer 11

• flat molecule
• pack tgt better- solid

Question 12

What observation during combustion shows that the compound is an aromatic compound?

Answer 12

smoky flames
∵ high C:H ratio
- unburnt C remaining

Question 13

Is phenol or ethanol a better nucleophile?

Answer 13

CH3CH2OH = better nuclephile
- alkyl group- slightly e- donating (O ↑ -ve)
- lone pair ↑ accessible to attack H in H2O
- delocalised e- in phenol –> ↑ stable
- ↓ available to nucleophile/ base

Question 14

reagents for Friedel-Crafts Acylation

Answer 14

1. anhydrous aluminium chloride AlCl3
2. acyl chloride RCOCl

Question 15

conditions for Friedel-Crafts Acylation

Answer 15

1. reflux 50C
2. dry inert solvent (ether)

Question 16

RCOCl + AlCl3 –> RCO+ + AlCl4-
why does this reaction take place?

Answer 16

• Al atom only has 6 e- in outer shell
• readily accepts a lone pair
• AlCl3- acid; RCOCl- base

Question 17

Reagents and conditions for reduction of nitrobenzene to phenylamine.

Answer 17

reagent- Sn (tin) in conc. HCl
conditions- heat + reflux

Question 18

equation for reduction nitrobenzene to phenylamine

Answer 18

C6H5NO2 + 6[H] –> C6H5NH2 + 2H2O