6.1.1 - Aromatic Compounds

Question 1

Name the 3 problems with Kekule’s model for benzene

Answer 1

• Enthalpy change of hydrogenation
• Carbon-Carbon bond length
• Resistance to reaction with Bromine

Question 2

Kekule vs delocalised model: Enthalpy change of hydrogenation

Answer 2

• The expected enthalpy change of hydrogenation for kekule’s model is -360KJ/mol-1
• It is actually less exothermic (-208KJ/mol-1)
• benzene is more stable than Kekules model suggests due to the pi-bond system

Question 3

Kekule vs delocalised model: carbon-carbon bond length

Answer 3

• kekule’s model suggests the carbon-carbon bond lengths should be of alternating lengths
• however the lengths of the carbon bonds are all the same

Question 4

Kekule vs delocalised model: resistance to reaction with bromine

Answer 4

• kekule’s model suggests benzene should undergo addition reactions with Bromine
• It doesn’t

Question 5

Explain the bonding in the delocalised model of benzene

Answer 5

• each carbon forms 3 sigma bonds (2x C, 1x H) forming a cyclic molecule
• the remaining p-orbital overlap sideways with eachother forming a pi-bonding system above and below the plane of the molecule
• all the electrons in the pi-bond system are delocalised
• benzene has a low electron density

Question 6

Describe the shape of a benzene molecule

Answer 6

• 120 degree bond angles
• planar
• equal c-c bond lengths

Question 7

forming the electrophile: halogenation of benzene

Answer 7

X2 + AlX3 —> X+ + AlX4-

Question 8

Regeneration of the catalyst: halogenation of benzene

Answer 8

AlX4- + H+ —-> AlX3 + HX

Question 9

Conditions for the halogenation of benzene

Answer 9

Halogen carrier catalyst

Question 10

examples of halogen carriers

Answer 10

• iron
• iron halides
• aluminium halides

Question 11

What reactions can benzene undergo

Answer 11

electrophillic substitution

Question 12

conditions for the nitration of benzene

Answer 12

conc. nitric acid
conc. sulfuric acid catalyst

Question 13

Forming the electrophile: nitration of benzene

Answer 13

HNO3 + H2SO4 —> NO2+ + HSO4- + H2O

Question 14

what is NO2+

Answer 14

nitronium ion

Question 15

regenerating the catalyst: nitration of benzene

Answer 15

HSO4- + H+ —> H2SO4

Question 16

Products of the nitration of benzene

Answer 16

nitrobenzene + H2O

Question 17

Products of the halogenation of benzene

Answer 17

halobenzene + hydrogen halide

Question 18

freidel-crafts acylation reagent

Answer 18

acyl chloride

Question 19

Acyl chloride

Answer 19

R-C(=O)-Cl

Question 20

Products of acylation

Answer 20

pentyl…one + HCl

Question 21

Forming the electrophile: acylation

Answer 21

CH3COCl +AlCl3 –> CH3CO+ + AlCl4-

Question 22

Regenerating the catalyst: acylation

Answer 22

AlCl4- + H+ –> HCl + AlCl3

Question 23

conditions for acylation

Answer 23

acyl chloride
halogen carrier catalyst

Question 24

conditions for alkylation

Answer 24

haloalkane
halogen carrier catalyst

Question 25

forming the electrophile: alkylation

Answer 25

XR + AlX3 --> AlX4- + R+

Question 26

Regenerating the catalyst: alkylation

Answer 26

AlX4- + H+ ---> HX + AlX3

Question 27

Why doesn't benzene react with bromine without a halogen carrier catalyst

Answer 27

- arenes have delocalised electrons spread above and below the carbon plane in the pi-bonding system - there is a lower electron density - benzene is unable to polarise bromine to act as an electrophile

Question 28

Naming when benzene is the substituant

Answer 28

Prefix: phenyl

Question 29

When is benzene a substituant

Answer 29

When it is attached to an alkyl group that is either 7+ carbons long or has a functional group

Question 30

Explain the electrons in the intermediate of benzen

Answer 30

- more unstable - carbon 1 is bonded to a hydrogen and the electrophile and has no unbounded electrons disrupting the pi-bonding system - there are 4 electrons spread over 5 carbons so they have restricted movement around the molecule

Question 31

Reactivity of arenes vs. Alkenes

Answer 31

- benzene is more stable as the electrons in the pi-bond system are delocalised - the electron density around a carbon-carbon bond is too low to induce a dipole in bromine - the electrons in the carbon- carbon double bonds of alkenes are concentrated between the 2 atoms (not delocalised ) so a higher electron density to polarise bromine - benzene is more stable so more energy required