CD deg: Aromatic compounds & their reactions

Question 1

Describe and draw the Kekulé structure of benzene.

Answer 1

Six-membered ring of carbon atoms with alternating single + double bonds.

Question 2

Name 2 characteristics of benzene which were accurately predicted by the Kekulé model.

Answer 2

• Bond angles 120^o
• Planar

Question 3

Name three issues with the Kekulé model of the structure of benzene.

Answer 3

• Unexpected unreactivity with bromine water
• Discrepancy between observed and expected bond lengths (shapes)
• Discrepancy between observed and expected enthalpies of hydrogenation

Question 4

Explain why benene’s unreactivity with bromine water was not accounted for by the Kekulé model of its structure. State the conclusion that this issue pointed towards.

Answer 4

• Kekulé model contains 3 C=C bonds, so benzene was expected to react with + decolourise bromine water
• Unreactivity suggests either that benzene has no double bonds, or that something makes those present less reactive

Question 5

Explain why benene’s shape was not accounted for by the Kekulé model of its structure. State the conclusion that this issue pointed towards.

Answer 5

• Kekulé structure contains C-C + C=C bonds. C=C bonds are shorter so bonds in benzene expected to be different lengths
• All bonds in benzene are same length, so Kekulé bonding is incorrect

Question 6

Explain why benzene’s enthalpy of hydrogenation was not accounted for by the Kekulé model of its structure. State the conclusion that this issue pointed towards.

Answer 6

• Enthalpy of hydrogenation of cyclohexa-1,3-diene is approximately twice that of cyclohexene, since it has twice as many double bonds, so that of benzene was expected to be 3 x that of cyclohexene
• Enthalpy of hydrogenation of benzene is lower than that of cyclohexa-1,3-diene
• Benzene’s structure is stabler than suggested by Kekulé model

Question 7

Describe and draw the delocalised model of benzene.

Answer 7

• Carbon p-orbitals perpendicular to ring overlap, so electrons delocalise
• Forms circular π-orbitals above and below ring, containing 6 electrons overall

Like a bagel sandwich. Each carbon has 4 valence e^-: 1 goes to C-H bond, 2 go to C-C bonds, 1 left over. 6 spare e^- delocalise

Question 8

Explain how the issues with the Kekulé model of benzene’s structure were rectified by the delocalised model.

Answer 8

Unreactivity with bromine water: carbon p-orbitals perpendicular to ring overlap, forming circular π-orbitals above and below ring. Delocalisation makes benzene stable, so doesn’t readily react by addition

Discrepancy between observed + expected shape: all bonds are equal length due to delocalisation

Discrepancy between observed + expected enthalpies of hydrogenation: delocalisation makes benzene stable, so hydrogenation doesn’t make benzene much more stable, so is less exothermic than predicted

Question 9

Name this molecule:

Answer 9

1-ethyl-4-methyl benzene

Minimisation of numbers takes priority over alphabetical order (in this case, it would be 1-4 either way, so is in alphabetical order)

Question 10

Name the following molecules:

Answer 10

Chlorobenzene, nitrobenzene, benzoic acid, benzene sulfonic acid

-NO₂ = nitro group, -SO₃H = sulfonic acid group

Question 11

Name this molecule:

Answer 11

2-phenylbut-2-ene

Phenyl prefix is used to represent benzene when suffix is occupied

Question 12

Name this molecule:

Answer 12

Phenylamine

Phenyl prefix is used to represent benzene when suffix is occupied

Question 13

Name the following:

• C₆H₅OOCCH₃
• C₆H₅COOCH₂CH(NH₂)CH₃

Answer 13

• Phenyl ethanoate (molecular formula was written backwards)
• 2-amino propylbenzoate

Question 14

What is an arene?

Answer 14

A molecule containing a benzene ring.

All arenes are aromatic, but this isn’t reversible

Question 15

What are the conditions required for aromaticity?

Answer 15

Molecule must:

• Be cyclic
• Be planar
• Contain 4n + 2 delocalised π-electrons (where n = non-zero integer)

Question 16

How can the number of delocalised electrons in a molecule be easily calculated?

Answer 16

No. double bonds x 2

Question 17

Describe the bonding in aromatic molecules.

Answer 17

• All ring atoms have an available p-orbital perpendicular to ring
• P-orbitals overlap, so electrons delocalise

Question 18

Is this molecule aromatic?

Answer 18

Yes, because:

• Cyclic
• Planar
• 4n + 2 delocalised π-electrons, where n = 3

Question 19

Is this molecule aromatic?

Answer 19

Yes, because:

• Cyclic
• Planar
• 4n + 2 delocalised π-electrons, where n = 1

Question 20

State and explain whether aromatic compounds undergo addition or substitution.

Answer 20

• Undergo substitution, since this maintains the stable delocalisation of electrons
• Don’t undergo addition since this would give 2 double bonds, which would be energetically unfavourable

Question 21

• State the name of the mechanism by which aromatic compounds replace hydrogen atoms with other groups
• Draw a general mechanism to represent this

Answer 21

Electrophilic substitution

Initial attack of electrophile is slow + often unfavourable, so this step is catalysed

Question 22

• Write the equation, using structural formulae, for the bromination of benzene
• State the conditions required
• Name the organic product

Answer 22

• FeBr₃ (or form in situ from Fe), reflux
• Bromobenzene

​Catalyst used to form Br⁺, which is a good electrophile

Question 23

During the bromination of benzene, iron filings can be used to form the required catalyst in situ.

Using equations, describe the role of this catalyst in the reaction. Include an overall equation.

Answer 23

Formation of electrophile: Fe + 1.5Br₂ → FeBr₃

FeBr₃ + Br₂ → Br⁺ + FeBr₄^-

Reaction with benzene: Br⁺ + C₆H₆ → C₆H₅Br + H⁺

Regeneration of catalyst: FeBr₄^- + H⁺ → FeBr₃ + HBr

FeBr₃ → Fe + 1.5Br₂

Overall: Br₂ + C₆H₆ → C₆H₅Br + HBr

Question 24

• Write the equation, using molecular formulae, for the chlorination of benzene
• State the conditions required
• Name the organic product

Answer 24

• C₆H₆ + Cl₂ → C₆H₅Cl + HCl
• Anhydrous AlCl₃
• Chlorobenzene

Question 25

Using equations, describe the role of the AlCl₃ catalyst used for the chlorination of benzene. Include an overall equation.

Answer 25

Formation of electrophile: Cl₂ + AlCl₃ → Cl⁺ + AlCl₄^-

Reaction with benzene: C₆H₆ + Cl⁺ → C₆H₅Cl + H⁺

Regeneration of catalyst: AlCl₄^- + H⁺ → AlCl₃ + HCl

Overall: C₆H₆ + Cl₂ → C₆H₅Cl + HCl

Question 26

Suggest why halogenated arenes are important in industry.

Answer 26

Halogenation makes arenes more reactive, allowing the substitution of benzene into other molecules.

More reactive since halogens are good leaving groups

Question 27

• Write an equation, using structural formulae, for the nitration of benzene
• State the conditions required
• Name the organic product

Answer 27

• c. HNO₃ (reactant), c.H₂SO₄, below 55^oC
• Nitrobenzene

At temperatures above 55^oC, the 1,3-dinitro and 1,3,5-trinitro products form, which are highly explosive (TNT is trinitrotoluene, a.k.a. trinitro methylbenzene)

Question 28

Using equations, describe the role of sulfuric acid in catalysing the nitration of benzene. Include an overall equation.

Answer 28

Formation of electrophile: HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄^- + H₂O + H⁺

Reaction with benzene: C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺

Regeneration of catalyst: 2HSO₄^- + 2H⁺ → 2H₂SO₄

Overall: HNO₃ + C₆H₆ → C₆H₅NO₂ + H₂O

Question 29

• Write the equation, using structural formulae, for the sulfonation of benzene
• State the conditions required
• Name the organic product

Answer 29

• H₂SO₄ (reactant), reflux
• Benzenesulfonic acid

Question 30

• Describe how the electrophile in the sulfonation of benzene is formed
• Explain why it is a good electrophile

Answer 30

• Conc sulfuric acid dissociates: H₂SO₄ → SO₃ + H₂O
• 3 polar S=O bonds give central sulfur a large partial positive charge, making it susceptible to attack

Question 31

Draw the mechanism for the sulfonation of benzene.

Answer 31

Question 32

• Write a general equation for the Friedel-Crafts alkylation of benzene
• State the conditions required

Answer 32

• C₆H₆ + RCl → C₆H₅R + HCl
• Anhydrous AlCl₃, reflux

Cl is replaced by benzene ring

Question 33

• Write a general equation for the Friedel-Crafts acylation of benzene
• State the conditions required

Answer 33

• C₆H₆ + RCOCl → C₆H₅COR + HCl
• Anhydrous AlCl₃, reflux

Cl is replaced by benzene ring

Question 34

• Write equations to show how the electrophiles are formed in the Friedel-Crafts alkylation and acylation of benzene.
• Explain why they are good electrophiles.

Answer 34

Alkylation: AlCl₃ + RCl → AlCl₄^- + R⁺

Acylation: AlCl₃ + RCOCl → AlCl₄^- + RC⁺O

Both contain carbons with 1+ charge, which makes them susceptible to attack

Question 35

Suggest why Fridel-Crafts substitution is important in the chemical industry.

Answer 35

It is a way of forming C-C bonds, and thereby extending chains or adding side chains.

Question 36

• Write an equation showing the formation of the electrophile during the reaction between 2-chloropropane and benzene.
• Draw the mechanism for the reaction between benzene and the electrophile.
• Write an equation showing the reformation of the catalyst.
• Name the product.

Answer 36

Product: methyl ethylbenzene / 2-phenylpropane

Question 37

• Write an equation showing the formation of the electrophile during the reaction between 2-ethanoyl chloride and benzene.
• Draw the mechanism for the reaction between benzene and the electrophile.
• Write an equation showing the reformation of the catalyst.
• Name the product.

Answer 37

Product: 1-phenyl propanone

Question 38

Fill in the table summarising the substitution reactions of benzene.

Answer 38

Question 39

Fill in the mechanism summary; in each box:

• Describe briefly what the mechanism applies to
• Draw the general mechanism

Answer 39