3.10 Aromatic Chemistry

Question 1

What is the molecular formula of benzene?

Answer 1

C₆H₆

Question 2

State the structure of benzene

Answer 2

planar cyclic

(6 carbons are joined together in a flat ring)

Question 3

Draw the skeletal formula for benzene

Answer 3

Question 4

Describe the structure of benzene

Answer 4

• Each C atom forms single covalent bonds to C on either side and to 1 H
• Final unpaired electron on each C atom is located in p-orbital that sticks out above and below the plane of ring
• P-orbitals on each C atom combine to form ring of delocalised electrons

Question 5

Describe the carbon-carbon bonds in benzene

Answer 5

All carbon-carbon bonds in ring are same = so same length (140 pm)

Lies between the length of a single C-C bond (154 pm) and a double C=C bond (135 pm)

Question 6

Why is benzene very stable?

Answer 6

∵ delocalised ring of electrons

Question 7

State how you can prove that benzene is far more stable than theoretical compound cyclohexa-1,3,5-triene (where ring would be made up of alternating single and double bonds)

Answer 7

By comparing enthalpy change of hydrogenation for benzenes with enthalpy change of hydrogenation for cyclohexene

Question 8

Cyclohexene has 1 double bond and when it’s hydrogenated, the enthalpy change is -120 kJ mol^-1. State what the theoretical enthalpy of hydrogenation would be if benzene had 3 double bonds.

Answer 8

-360 kJ mol^-1

Question 9

Describe how the experimental enthalpy of hydrogenation of benzene is different to the theoretical value of -360 kJ mol^-1

Answer 9

• Amount of energy is less
  
  • far less exothermic than expected
• (Experimental enthalpy of hydrogenation of benzene is -208 kJ mol^-1)

Question 10

Explain the difference between the theoretical enthalpy of hydrogenation of benzene and the experimental value.

Answer 10

1. Energy is put in to break bonds and released when bonds are made
2. More energy must have been put in to break bonds in benzene than would be needed to break bonds in a theoretical cyclohexa-1,3,5-triene molecule
3. Difference indicates benzene is more stable than cyclohexa-1,3,5-triene would be
   
   • Thought to be due to delocalised ring of electrons

Question 11

What are aromatic compounds or arenes?

Answer 11

Compounds containing a benzene ring

Question 12

Name the compound

Answer 12

Question 13

Name the compound

Answer 13

Question 14

Name the compound

Answer 14

Question 15

Name the compound

Answer 15

Question 16

Name the compound

Answer 16

In other molecules the benzene ring can be regarded as a substituent side group on another molecule, like alkyl groups are. The C₆H₅ - group is known as the phenyl group

Question 17

Explain why arenes attract electrophiles

Answer 17

Benzene ring is a region of high electron density so it attracts electrophiles

Question 18

Explain why arenes undergo electrophilic substitution opposed to electrophilic addition

Answer 18

• As benzene ring’s so stable, doesn’t undergo electrophilic addition reactions, which would destroy delocalised ring of electrons
• Instead undergoes electrophilic substitution reactions where one of H atoms (or another functional group) is substituted for electrophile

Question 19

Give 2 examples of useful chemicals that contain benzene rings

Answer 19

Dyes and pharmaceuticals

Question 20

Why is it tricky to make chemicals that contain benzene

Answer 20

∵ benzene is so stable = fairly unreactive

Question 21

What does Friedel-Crafts acylation do to benzene rings?

Answer 21

• Adds acyl group (RCO-) to benzene ring
• Once acyl group has been added, side chains can be modified using further reactions to make useful products

Question 22

Why can’t most electrophiles attack a benzene ring?

Answer 22

Most aren’t polarised enough

Question 23

How can electrophiles be made into stronger electrophiles?

Answer 23

By using a catalyst called a halogen carrier

Question 24

State what electrophile Friedel-Crafts acylation uses

Answer 24

Acyl chloride

Question 25

Give an example of halogen carrier that Friedel-Crafts acylation could use

Answer 25

AlCl₃

Question 26

Describe how AlCl₃ makes the acyl chloride electrophile stronger

Answer 26

* AlCl₃ accepts lone pair of electrons from acyl chloride * As lone pair of electrons is pulled away, polarisation in acyl chloride increases and it forms a carbocation * Makes it stronger electrophile and gives it a strong enough charge to react with benzene ring

Question 27

Draw a diagram showing how AlCl₃ makes an acyl chloride electrophile stronger

Answer 27

Question 28

Draw the mechanism for Friedel-Crafts acylation (electrophilic substitution)

Answer 28

Question 29

State the conditions for Friedel-Crafts acylation to occur

Answer 29

Reactants need to heated under reflux in non-aqueous solvent (like dry ether) for reaction to occur

Question 30

Describe nitration briefly

Answer 30

When you warm benzene with concentrated nitric and sulfuric acids = nitrobenzene

Question 31

State the catalyst used in nitration

Answer 31

Sulfuric acid

Question 32

Nitration Why is sulfuric acid used as a catalyst?

Answer 32

Helps to make nitronium ion NO₂⁺ = electrophile

Question 33

Nitration Write equations showing how the nitronium ion, electrophile, is made

Answer 33

Question 34

Draw the electrophilic substitution mechanism for nitration of benzene

Answer 34

Question 35

Nitration State what you can do, if you want only 1 NO₂ group (mononitration) substituted?

Answer 35

Need to keep temperature below 55°C Above this temperature = get lots of substitutions

Question 36

Name 2 uses of nitration reactions

Answer 36

1. Nitro compounds can be reduced to form aromatic amines * Use to manufacture dyes and pharmaceuticals 2. Some nitro compounds can be used as explosives * e.g. 2,4,6-trinitromethylbenzene (TNT)

Question 37

What happens when methylbenzene is nitrated with a mixture of nitric and sulfuric acid?

Answer 37

Produces mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene (More vigorous conditions + more nitro-groups substituted)

Question 38

How can a mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene be separated?

Answer 38

By thin layer chromatography

Question 39

Describe how methylbenzene reacts compared to benzene

Answer 39

Reacts similarly to benzene but is more reactive than benzene towards electrophilic substitution

Question 40

Explain why methylbenzene is more reactive than benzene towards electrophilic substitution

Answer 40

∵ methyl group releases electrons onto benzene ring = electrophile attracted more

Question 41

Explain why it's harder to substitute the chlorine atom in 1-chloromethylbenzene than in phenylchloromethane

Answer 41

* C-Cl bond in 1-chloromethylbenzene is stronger than in phenylchloromethane * ∵ electrons in filled p-orbitals on Cl atom interact with delocalised electrons on benzene ring

Question 42

How do substituents affect the reactivity of benzene ring towards electrophiles?

Answer 42

By releasing electrons onto ring or by withdrawing them

Question 43

Friedel-Crafts Acylation Write an equation to show the role of aluminium chloride as a catalyst in this reaction (with ethanoyl chloride)

Answer 43

CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄^-

Question 44

Friedel-Crafts Acylation State the product formed

Answer 44

Phenylketone

Question 45

Explain why methylbenzene does not react easily with nucleophiles such as ammonia (2)

Answer 45

* Delocalised ring is electron rich * Repels nucleophiles

Question 46

The formula of the ester is (CH₃)C₆H₄COOCH₂CH₂CH₃. State why this is not a structural formula. (1)

Answer 46

the CH₃ group can be in different positions on the (aromatic) ring