4.2 - Aromaticity

Question 1

What is the structure of benzene

Answer 1

• C6H6 with planar hexagonal shape
• Sp2 hybridisation
• 120 degrees bond angles
• Sideways overlap of p orbitals causing a ring of delocalised electrons above and below the plane
• C-C bond lengths are equal, sigma bonds between C-H and C-C from s-s and s-p overlap

Question 2

What is the evidence to support the current benzene structure

Answer 2

1. All C to C bond lengths are equal with length being halfway between C=C and C-C
2. Electrons distribution is symmetrical, there are no areas of high e- density from C=C
3. One isomer formed by chlorination indicating no C=C bond otherwise two isomers would form
4. Enthalpy of hydrogenation values
   Cyclohexene + H2 is -120 kJmol-1
   Kekule states benzene had 3 C=C bonds so enthalpy of hydrogenation should be -360 kJmol-1

However it was -208 kJmol-1 so a difference of -152 kJmol-1 shows that benzene is stable due to its delocalisation stabilisation energy

Question 3

What are the characteristics of benzene

Answer 3

• an aromatic compound
• unsaturated and burns with a very sooty flame
• has a distinctive colour

Question 4

What is the mechanism for reactions of benzene

Answer 4

Electrophilic substitution
Electrophile: an electron deficient species that accepts a lone pair of electrons

• not an addition reaction to protect the delocalisation stabilisation energy

Question 5

What is the reaction for nitration of benzene

Answer 5

Conditions: less than 55 degrees, concentrated HNO3 and H2SO4
Electrophile: HNO3 + H2SO4 > NO2+ + SO4- + H2O

H2SO4 catalyst is reformed

Above 55 degrees further nitration occurs and the compound becomes analogous

TNT is formed ( 2,4,6 - trinitrotoewn)

Question 6

What is the reaction for bromination

Answer 6

Conditions: Br2, heat, anhydrous, AlBr3 (Lewis acid catalyst)
Electrophile: Br - Br |||| Al+Br3- > Br+ + AlBr4-

HBr and AlBr3 formed

Question 7

What is the friedel craft alkylation reaction

Answer 7

Conditions: halogenoalkane, heat, anhydrous AlCl3
Electrophile: /\/\Cl- ——— Al+Cl3- > /+ + AlCl4

HCl + AlCl3 formed as products

Question 8

What is the difference between the C-Cl bond in chloroalkanes and chlorobenzene

Answer 8

C-Cl bond in chlorobenzene is stronger than the C-Cl bond in chloroalkanes from the overlap of the lone pair of electrons and pi bond in the benzene ring which increases the bond strength so chlorobenzene is less reactive than chloroalkanes

Question 9

What are the two ways that benzene can react with chlorine

Answer 9

1. Attached directly to the ring
   Cl2, heated, anhydrous, AlCl3 catalyst and no UV light
2. Attached to a group on the ring
   .Cl- free radical, low temperature and UV light

1 will not react further with NaOH because the lone pair of electrons on the chlorine is delocalised into the ring, which shortens and strengthens the bond so resists hydrolysis