25 - Aromatic Compounds

Question 1

What is the formula for benzene?

Answer 1

C6H6

Question 2

What are the two ways of representing the benzene ring?

Answer 2

The Kekulé model and the delocalised model.

Question 3

Explain what the Kekeulé model of Benzene looks like.

Answer 3

Made up of planar (flat) ring of carbon atoms with alternating single and double bonds between them, one hydrogen is attached to each carbon.

Question 4

Explain the delocalised model for benzene.

Answer 4

The p-orbitals of all six carbons overlap to create a π-system.
The π-system is made up of two ring-shaped clouds of electrons above and below the plane of the six carbons.
All the bonds are the same length as all the bonds are the same.
The electrons are delocalised because they do not belong to a specific carbon atom, represented as a circle.

Question 5

Why is the kekule structure not completely right from X-ray diffraction studies?

Answer 5

If Kekule structure was correct you’d except three bonds with length of 154 pm of a C—C bond and three bonds with a length of 134 pm of a C=C bond.

X-ray studies show that the carbon to carbon bonds in benzene have the same length of 140 pm, between a length of a single bond and double bond of carbon.

Question 6

Explain enthalpy change of hydrogenation with an alkene.

Answer 6

If you react an alkene with hydrogen gas, two atoms of hydrogen add across a double bond, this called hydrogenation, and the enthalpy change of this is the enthalpy change of hydrogenation.

Question 7

Explain the evidence for the delocalised model instead of the kekule model, using Enthalpy Changes of Hydrogenation.

Answer 7

Cyclohexene has one double bond, when hydrogenated the enthalpy change is -120 kj mol-1. So if benzene had three double bonds you would expect the enthalpy of hydrogenation to be (3x120=-360).

But the actual enthalpy of hydrogenation of benzene is -208 kJ mol-1, which is far less Exothermic.

Question 8

What do we learn from benzene’s enthalpy change of hydrogenation being less Exothermic then expected from the kekule model?

Answer 8

More energy is required to break the bonds in benzene than would be need to be put in to break the kekule structure.

This difference indicates benzene is more stable than the kekule structure, due to the resistance to reaction. This is thought to be from the delocalised ring of electrons.

Question 9

What is the experimental evidence for a delocalised rather than Kekulé?

Answer 9

1. bond lengths
2. enthalpy change of hydrogenation
3. resistance to reaction

Question 10

What is the experimental evidence of benzene’s resistance to reaction to prove to favour model of benzene?

Answer 10

Alkenes usually react easily with bromine water at room temperature, they decolourises by adding across the double bond. However with benzene it is a struggle to react with bromine water, you need hot benzene and ultraviolet light.

Reluctance of undergoing addition reaction proves this.

Question 11

Why is it a struggle for benzene to react with bromine water?

Answer 11

Due to the π system in benzene of the delocalised electron rings above and below the plane of carbon atoms. The spread out negative charge makes the benzene ring stable, so benzene is reluctant to undergo addition reactions which would destroy the stable ring.

In Alkenes Alkenes the π bond in C=C is a area of high electron density which strongly attracts electrophiles but in benzene this attraction is reduced to the negative charge being spread.

Question 12

Draw out nitrobenzene.

Answer 12

nitrobenzene

Question 13

Name this molecule

Answer 13

Chlorobenzene

Question 14

Name this molecule.

Answer 14

Ethylbenzene

Question 15

Name this molecule.

Answer 15

Phenol

Question 16

Draw phenylamine.

Answer 16

Question 17

Name this molecule.

Answer 17

Benzoic Acid

Question 18

Name this molecule.

Answer 18

Nitrobenzene

Question 19

Name this molecule.

Answer 19

Phenylamine

Question 20

Name this molecule.

Answer 20

Benzaldehyde

Question 21

If we have more than one functional group attached to the benzene ring how do you number the functional groups on the carbons?

Answer 21

• If the functional groups are all the same, pick any group to start from and count round the way which gives the smallest numbers.
• If the functional groups are different, start from which functional group gives the the molecule the suffix (e.g -OH group giving phenol) and count round the way that gives the smallest numbers.

Question 22

What groups are prefixes (in front) to benzene? What

Answer 22

Alkyl (CH3, C2H5 up until seven or more carbons)
Halogens (F,Br,Cl)
Nitro (NO2)

Question 23

When does benzene become a prefix and what is the prefix called?

Answer 23

When the benzene ring is attacked to an alkyl group with a functional group of an alkyl chain site seven or more carbon atoms.

The prefix of benzene in phenyl.

Question 24

Name this molecule.

Answer 24

2 - phenyloctane

Question 25

What are the exception for the prefix phenyl for benzene and name them?

Answer 25

Benzoic acid - (benzene carboxylic acid) Phenylamine Benzaldehyde

Question 26

Name this molecule.

Answer 26

4-Chloro-3,5-dimethylphenol

Question 27

Draw 2,4,6-trinitrophenol

Answer 27

Question 28

Name this molecule.

Answer 28

2-phenylphenol

Question 29

Draw out 2-benzyl-4-chlorophenol

Answer 29

Question 30

Draw out an phenylethanone

Answer 30

Question 31

Draw out phenyl ethanoate

Answer 31

Question 32

What type of reaction does benzene usually undergo?

Answer 32

Electrophilic Substitution Reaction

Question 33

What is a result of an electrophilic substitution reaction of benzene?

Answer 33

Hydrogen atom being substituted by an electrophile.

Question 34

Draw out the two step general mechanism for a benzene and electrophile, electrophilic substitution reactions?

Answer 34

Question 35

Why do you need such a strong electrophile for benzene?

Answer 35

Need a strong positive charge to be able to attack the stable benzene ring.

Question 36

What do you need to make a strong electrophile to attack benzene?

Answer 36

A halogen carrier like AlCl3, FeBr3

Question 37

Explain how a halogen carrier carrier to make a good electrophile.

Answer 37

A halogen carrier accepts a lone pair of electrons from a halogen atom on an electrophile. As the lone pair of electrons is pulled away, the polarisation in the molecule and forms carbocation.

Question 38

Draw out the equation and mechanism for halogen carriers making an electrophile.

Answer 38

Question 39

How does the nitration of Benzene happen?

Answer 39

When you warm benzene with concentrated nitric acid and concentrated sulfuric acid, you get nitrobenzene

Question 40

How does a nitration of benzene work?

Answer 40

A

Question 41

Draw out the three step process with mechanisms for the nitration of benzene.

Answer 41

Question 42

Write out the equation for creating the NO2+ ion for the nitration of benzene by reacting Concentrated Nitric Acid and Concentrated Sulfuric acid.

Answer 42

Question 43

What is the catalyst in the nitration of benzene and how is it regenerated?

Answer 43

Sulfuric Acid, H2SO4 An HSO4- is created in the reaction of nitric acid and sulfuric acid to create the NO2+ ion. This HSO4- molecule will then reacts with the H+ displaced by the NO2 group on the benzene to form H2SO4 again.