Aromatics

Question 1

who discovered the molecular formula of benzene and what is it

Answer 1

faraday

C6H6

Question 2

what structure did Kekule dream about the electrons in benzene

Answer 2

he suggested the 6 carbon ring contained alternating double and single bonds

Question 3

what are the 2 problems of Kekules hypothetical model

Answer 3

1) since his model was unsaturated, then it should be able to undergo electrophillic addition but it rarely does

it doesn’t decolourise bromine water

2) we know from electron density spectra that the carbon atoms are all equivalent and therefore that means all the carbon to carbon bonds are the same length

Question 4

pauling proposed the idea that the electrons in benzene were in resonance but now it is accepted that…

Answer 4

that the pi electrons are delocalised in a ring in the centre of benzene

Question 5

what’s the structure of benzene

Answer 5

• the carbon atoms form 3 bonds in the triagonal planar structure
• the 4th outer electron (2p) forms a pi bond by overlapping with the 2p electrons on the adjacent carbon atoms resulting in a ring

Question 6

there’s 2 pieces of evidence of kekules structure being wrong :

• all c-c bonds are the same length
• benzene doesn’t decolourise bromine water

what’s the other thermochemical evidence

Answer 6

the enthalpy of hydrogenation of benzene provides further evidence of kekules structure being incorrect because :

*the expected 🔺H is -360kJmol-1

• however the actual 🔺H is -152kJmol-1 less exothermic
• because benzene is more stable than Kekules structure
• due to delocalised ring of electrons

Question 7

when naming aromatic compounds what 4 ways is the ending called “benzene”

Answer 7

when the group attached to the benzene ring is :

1) an alkyl group
2) a halogen
3) a nitro group (NO2)
4) a carboxyllic acid

Question 8

how are aromatic compounds named when the group attached to the benzene ring is not a halogen,alkyl group, carboxyllic acid or nitro group

Answer 8

the start of the name is phenyl

Question 9

what’s the only mechanism for benzene

Answer 9

electrophillic substitution

Question 10

what are the 2 electrophiles that benzene reacts with

Answer 10

NO2 +( nitronium ion )

RCO +( acylium ion )

Question 11

how is benzene nitrated

how many steps are there

Answer 11

the nitration of benzene occurs when concentrated HNO3 and concentrated H2SO4 ( the catalyst) is heated to 50°C to form nitrobenzene

3

Question 12

explain the 3 steps for nitration of benzene

Answer 12

1) generation of electrophile (NO2+)

• H2SO4 protonates HNO3 as it is a stronger acid

H2SO4 + HNO3 —-> HSO4- + H2NO3+

then the protonated nitric acid breaks down and forms the nitronium ion

H2NO3+ ——> NO2+ + H2O

Step 2 : electrophilic substitution for nitration of benzene

step 3: regeneration of catalyst

HSO4- + H+ ——> H2SO4

Question 13

what’s a synthetic use of nitrobenzene

what’s a non synthetic use of nitrated arenes

Answer 13

nitrobenzene can be used in the preparation of azo dyes.

the nitro group is reduced to an amine

reagent - conc HCl
condition - reflux and Tin catalyst

C6H5NO2 + 6[H] ——> C6H5NH2 + 2H2O

2) Making TNT

Question 14

how does benzene become phenyl ketone

what’s the electrophile

Answer 14

1) by the acylation of benzene

( benzene reacts with acyl chlorides in the presence of the catalyst AlCl3 to produce phenyl ketones )

2) acylium ion ( RCO+)

Question 15

name steps of forming phenyl ketone

Answer 15

1) formation of electrophile

• AlCl3 + CH3COCl —> CH3CO+ + AlCl4-

2) electrophillic addition

3) regeneration of catalyst

• AlCl4- + H+ —> AlCl3 + HCl