Unit 3.4.6 - Aromatic Chemistry

Question 1

What are aromatic compounds?

Answer 1

Compounds that contain a benzene ring

Question 2

What is another name for aromatic compounds?

Answer 2

Arenes

Question 3

What is the molecular formula for benzene?

Answer 3

C₆H₆

Question 4

What is the shape of the structure of benzene

Answer 4

Cyclic structure

Question 5

The benzene ring is said to be what shape? And why?

Answer 5

Planar - all the hydrogens stick out in the same plane

Question 6

How many models are there to explain the structure of benzene?

Answer 6

Two

Question 7

What are the name of the two models to explain the structure of benzene?

Answer 7

Kekulé model and the delocalised model

Question 8

Which is the accepted model for the structure of benzene?

Answer 8

The delocalised model

Question 9

What is the Kekulé model?

Answer 9

There is a ring of carbon atoms with alternating single and double bonds

Question 10

How was the Kekulé model later adapted

Answer 10

There are two isomers of the benzene and the benzene molecule constantly flips between them

Question 11

What is the delocalised model?

Answer 11

There is a ring of carbon atoms and each carbon donates one electron from its p orbital to form a ring of delocalised electrons

Question 12

What are the two main pieces of evidence to support the delocalised model?

Answer 12

1.) If the Kekulé model was correct you’d expect alternating short and long bonds however all the bonds are the same length 2.) The enthalpy of hydrogenation of benzene is 152 kjmol⁻¹ more than you’d expect it to be

Question 13

Explain why the enthalpy of hydrogenation of benzene is 152 kjmol⁻¹ more than you’d expect it to be if it had the Kekulé structure.

Answer 13

The enthalpy of hydrogenation of one double bond takes -120 kjmol⁻¹, so if it has the Kekulé structure then it would require -360 kjmol⁻¹, however it only requires -208 kjmol⁻¹ and so it 152 kjmol⁻¹ more than you’d expect it to be - it is less exothermic than predicted

Question 14

How do you name aromatic compounds if the benzene ring was the main functional group?

Answer 14

The suffix -benzene is used and then prefixes of any other functional group of the benzene ring

Question 15

How do you name aromatic compounds if benzene isn’t the main functional group?

Answer 15

Use the prefix -phenyl and then the suffix comes from the other functional groups than the benzene group was attached to

Question 16

What makes benzene more stable than the Kekulé model predicted?

Answer 16

The ring of delocalised electrons means the electron density is shared over more atoms, which means the energy of the molecule is lowered and so it becomes more stable

Question 17

What does benzene attract? And why?

Answer 17

Electrophiles because it is a region of high electron density

Question 18

What is an electrophile?

Answer 18

Electron pair acceptor

Question 19

Name three common electrophiles?

Answer 19

H⁺, NO₂⁺ and polar molecules (like carbonyl compounds)

Question 20

What type or mechanism does benzene most often go under and why?

Answer 20

Electrophilic substitution to preserve the delocalised ring

Question 21

Give the five steps in the mechanism for the reaction between benzene and an electrophile.

Answer 21

1.) The ring of delocalised electrons attracts an electrophile 2.) The electrophile takes a pair of electrons from the centre of the benzene ring and forms a bond with one of the carbons 3.) This partially breaks the delocalised electron ring and gives the molecule a positive charge 4.) To regain stability of the benzene ring the carbon bonded to the electrophile loses a hydrogen 5.) A H⁺ had now been substituted for an electrophile

Question 22

Name two examples of reactions where benzene cause under electrophilic substitution.

Answer 22

Nitration and Frieden-Crafts Acylation

Question 23

What are the conditions for nitration of benzene?

Answer 23

You warm it with concentrated nitric acid and concentrated sulphuric acid

Question 24

What is the reaction called that produces nitrobenzene from benzene?

Answer 24

Nitration

Question 25

What acts as catalyst during nitration?

Answer 25

Sulphuric acid

Question 26

How does sulphuric acid do during nitration?

Answer 26

It makes to make the NO₂⁺ which acts as an electrophile

Question 27

What are the equations for how the NO₂⁺ is produced to use as an electrophile in nitration?

Answer 27

HNO₃ + H₂SO₄ → HSO₄⁻ + NO₂⁺ + H₂O HSO₄⁻ + H⁺ → H₂SO₄

Question 28

What is mononitration?

Answer 28

When only one NO₂ group is added to benzene

Question 29

How do you ensure you only carry out mononitration?

Answer 29

Temperature below 55ºC

Question 30

Name one thing nitro benzene can be reduced to.

Answer 30

Aromatic amines

Question 31

What are two uses of aromatic amines?

Answer 31

Making dyes and making pharmaceuticals

Question 32

What s one use of nitro compounds?

Answer 32

As explosives like TNT

Question 33

Why is Friedel-Crafts acylation used?

Answer 33

Because benzene is so stable it is fairly unreactive, and so it is used to add an acyl group to make it easier modify the side chains in further reactions

Question 34

What is added to to benzene during Friedel-Crafts acylation?

Answer 34

An acyl group

Question 35

How do you carry out Friedel-Crafts acylation?

Answer 35

Heat an acyl chloride and benzene under reflux in a non aqueous environment

Question 36

What is produced at the end of Friedel-Crafts acylation?

Answer 36

HCl and a phenylketone

Question 37

What is used to make an electrophile stronger for Friedel-Crafts acylation?

Answer 37

Halogen carrier

Question 38

What electrophile and halogen carrier are use in Friedel-Crafts acylation?

Answer 38

Acyl chloride and AlCl₃

Question 39

How is an acyl chloride made a stronger catalyst by AlCl₃

Answer 39

The AlCl₃ accepts a lone pair of electrons from the chlorine in the acyl chloride, the polarisation in the acyl chloride increases and forms a carbocation, which makes a much stronger electrophile

Question 40

Give the seven steps in the mechanism for the reaction between benzene and AlCl₃ and an acyl chloride.

Answer 40

1.) The ring of delocalised electrons attracts the positively charged carbocation 2.) The carbocation takes a pair of electrons from the centre of the benzene ring and forms a bond with one of the carbons 3.) This partially breaks the delocalised ring and gives the molecule a positive charge 4.) The AlCl₄⁻ is attracted to the positively charged ring 5.) One chloride ion breaks away from the aluminium chloride ion and bonds with the hydrogen 6.) This removes hydrogen that is on the carbon bonded to the electrophile and HCl is produced 7.) A H⁺ has been substituted for an acyl group

Question 41

Draw the Kekulé model.

Answer 41

Question 42

Draw the delocalised model.

Answer 42

Question 43

Draw the eqaution for the nitraion of benzene.

Answer 43

Question 44

Draw the structure of TNT

Answer 44

Question 45

Draw the equation for Freidel-Crafts acylation of benzene.

Answer 45

Question 46

Draw the equation for the foramtion of the stronger electrophile used in Friedel-Crafts acylation.

Answer 46