Arenes

Question 1

Physical properties Benzene

Answer 1

1. Boiling point 80.1C
2. Colourless, highly flammable
3. Characteristic pleasant smell
4. Non polar (soluble in non polar solvents and insoluble in polar solvents)
5. Toxic/Carcinogenic

Question 2

Nomenclature

Answer 2

Monosubstituted benzenes
Polysubstituted benzenes

Question 3

Monosubstituted benzenes

Answer 3

benzene- parent chain (suffix)
substituents (alkyl groups, halogens) -prefix

if substituents more complex then benzene becomes prefix and treated as a substituent instead and is named as ‘phenyl’ instead

Question 4

Polysubstituted benzenes

Answer 4

Position of substituents assigned should be as small as possible

Identical substituents
write numbered position of the substituents and use ‘di’ ‘tri’ at prefix followed by the substituent name

Different substituents
substituents arranged according to alphabetical order according to first letter of the prefix of the substituents

indicate using number the position of each substituent with first prefix on first carbon (using dashes)

Question 5

Enthalpy change of hydrogenation of benzene

Answer 5

The enthalpy change of hydrogenation of benzene is in actual 3 times lower in magnitude than if kekule’s structure is correct

This is as benzene has a resonance struc. which gives it additional stability leading to a smaller difference in the energy level of benzene and cyclohexane

Question 6

Bond length in benzene

Answer 6

C-C (147pm)
C=C(134pm)

expected alternation of these two bond lengths in benzene but

bond length is 139pm in benzene due to partial double between carbons

Question 7

Resonance structure

Answer 7

All carbon atoms in benzenes are sp2 hybridised. Unhybridised p orbital of each C atom contains 1 electron
there is continuous side way overlap of unhybridised p orbitals

causing 6pi electrons to be delocalised throughout the entire ring structure. No alternating single-double bonds giving rise to aromatic stability

Question 8

Chemical properties of benzene

Answer 8

Electrophilic substitution
Combustion

Question 9

Difference in reactivity between alkene and benzene

Answer 9

Br2 in CCl4: alkene decolourises bromine from orange-red to colourless , but the colour remains with benzene

Cold KMnO4, NaOH: alkene -purple KMnO4 decolourises, Brown ppt of MnO2 is formed, but colour remains with benzene

Hot KMnO4, H2SO4: alkene- decolourisation of purple KMnO4 but colour remains with benzene

Question 10

Combustion

Answer 10

In sufficient oxygen, benzene undergoes complete combustion to produce CO2 and H2O but in incomplete combustion produces soot

due to the high C:H ratio benzene containing compounds burns with a sooty flame because of insufficient oxygen

Question 11

Benzene formula

Answer 11

C6H6

Question 12

Electrophilic substitution

Answer 12

HAAN
halogenation
alkylation
acylation
nitration

Question 13

Why addition reactions are unfavourable for benzene

Answer 13

Benzene has resonance structure with a ring of 6pi delocalised electrons above and blow the plane as a result of the continuous overlap of unhybridised p orbitals giving it additional aromatic stability. Making it less succeptible to electrophiles. Addition reactions which destroy the aromatic ring requires an additional input of energy and is unfavourable

Question 14

Mechanism of electrophilic substitution

Answer 14

1. Generation of strong electrophile
2. Electrophilic attack on benzene forming reactive arenium carbocation intermediate(slow step)
3. Loss of H+ to regain aromaticity and regeneration of catalyst(fast step)
   all C sp2 become hybridised and benzene ring has restored aromatic stability

remember curly arrows and partial circle within benzene during step 2 and the positive charge

Question 15

Halogenation

Answer 15

forming (halogen)benzene
Condition: Br2 with anhydrous FeBr3 OR Cl2 with anhydrous FeCl3/AlCl3

the halogen in the catalyst depends on the halogen reacting with benzene

AlCl3—> lewis acid catalyst

Question 16

Alkylation

Answer 16

Condition: RX (alkyl-R, X-halogen) and anhydrous FeX3/AlX3

Question 17

Acylation

Answer 17

Condition: Anhydrous FeX3/AlX3 and R-C=O R is an alkyl group

Question 18

Nitration

Answer 18

formation of nitrobenzene
Condition: Concentrated HNO3 and concentrated H2SO4 and heat

Step 1 is the generation of the strong +NO2 electrophile
HNO3 +H2SO4 (acts as acid) —-> H2O + +NO2 +HSO-4
(acts
as base)

Step 2 is the electrophilic attack of +NO2 on benzene forming arenium carbocation intermediate (slow step)

Step 3 loss of H+ to regain aromaticity and regeneration of catalyst (fast step)

remember curly arrows and partial circle within benzene during step 2 and the positive charge

Question 19

Step 2 of the mechanism

Answer 19

When electrophile attacks benzene, benzene forms dative bond with electrophile hence 2 of the 6pi delocalised electrons leaving 4pi delocalised electrons over remaining 5 Cs in positively charged carbocation intermediate
The carbon atom which forms the bond with the electrophile becomes sp3 hybridised . No longer has the unhybridised p-orbital to form a continuous p orbital overlap with other unhybridised p orbitals of remaining C atoms . Aromatic stability disrupted