Aromatic Compounds and Carbonyls

Question 1

What is the formula of a benzene?

Answer 1

C6H6

Question 2

Describe the structure and number of carbons in a benzene molecule

Answer 2

It has a cyclic structure of 6 carbon atoms bonded in a ring

Question 3

Is benzene planar or non-planar?

Answer 3

Planar

Question 4

What is the Kekulé model?

Answer 4

A model proposed by the chemist Friedrich Kekulé. He came up with the idea of a ring of C atoms with alternating single and double bonds between them

Question 5

What is the Kekulé model used to explain?

Answer 5

The structure of a benzene ring

Question 6

What was in the updated Kekulé model after he made some adjustments to the model?

Answer 6

He said that the benzene molecule was constantly flipping between 2 isomers by switching over the double and single bonds

Question 7

What evidence is there that suggests the Kekulé model is incorrect?

Answer 7

If it was correct, you’d expect there to be 3 bonds that are 154pm in length (single bonds) and 3 bonds that are 134pm in length (double bonds). Instead, all of the bonds are 140pm in length, so the Kekulé model can’t be right

Question 8

Why is the Kekulé model still used today?

Answer 8

Because it’s useful for when drawing reaction mechanisms

Question 9

What is the delocalised model used to explain?

Answer 9

The structure of a benzene ring

Question 10

Describe the delocalised model for the benzene ring

Answer 10

The p-orbitals of all six carbons overlap to create a π-system. This π-system is made up of 2 ring-shaped clouds of electrons - one above and one below the plane of 6 carbon atoms

Question 11

In the delocalised model, why are the electrons said to be delocalised?

Answer 11

Because they don’t belong to a specific carbon atom

Question 12

What is hydrogenation?

Answer 12

A reaction where a hydrogen has been added

Question 13

What are the 3 pieces of evidence supporting the delocalised model instead of the Kekulé model?

Answer 13

Bond length data
Enthalpy change data
Benzene is relatively unreactive

Question 14

How does bond length data support the delocalised model of the benzene ring?

Answer 14

Because the bonds are the same, they have the same length which supports the observations

Question 15

How does enthalpy change data support the delocalised model of the benzene ring?

Answer 15

When cyclohexane (1 double bond) is hydrogenated, the enthalpy change is -120kj mol^-1. In the Kekulé model, you’d expect the enthalpy of hydrogenation to be -360kj mol^-1, due to the 3 double bonds. Instead, it’s actually more exothermic with an enthalpy of hydrogenation of -208kj mol^-1. This shows that more energy must have been put in to break the bonds in benzene than the Kekulé model

Question 16

How does the stability of the benzene ring support the delocalised model of the benzene ring?

Answer 16

The difference in enthalpy of hydrogenation between the actual and expected values indicates that benzene is more stable than the Kekulé model would be

Question 17

Why is the delocalised model of the benzene ring more stable than the Kekulé model?

Answer 17

Because of the delocalised ring of electrons. In a ring, the electron density is shared over more atoms, making the molecule more stable

Question 18

What is the name of non-aromatic molecules?

Answer 18

Aliphatic molecules

Question 19

What are the 2 possible names given to compounds containing a benzene ring?

Answer 19

Arenes or Aromatic Compounds

Question 20

If the benzene ring is the functional group of an aromatic compound, how do you name them?

Answer 20

The suffix is benzene and there are prefixes to represent any other functional group (e.g. Methyl)

Question 21

If the benzene ring is not the functional group of an aromatic compound, how do you name them?

Answer 21

The molecule is named as having a phenyl group, so phenyl- is the prefix and suffix comes from other functional groups on the molecule

Question 22

Why do alkenes react with bromine but benzene doesn’t?

Answer 22

Due to the different π-system in benzene. The delocalised rings are above and below the plane of carbon atoms, meaning that the molecule is very stable and the negative charge is spread out. This means that benzene doesn’t react with something that’ll destroy the stable ring

Question 23

Which reaction does benzene prefer to undergo, electrophilic substitution or electrophilic addition?

Answer 23

Electrophilic substitution

Question 24

Why do benzene rings attract electrophiles?

Answer 24

A benzene ring is an area of high electron density

Question 25

Why are electrophiles attracted to areas of high electron density?

Answer 25

Because they’re electron deficient

Question 26

Why does benzene prefer electrophilic substitution instead of electrophilic addition?

Answer 26

It preserves the stable delocalised ring

Question 27

Describe the 5 steps in the electrophilic substitution mechanism on a benzene ring

Answer 27

• The electron dense region at the centre of the benzene ring attracts an electrophile
• The electrophile steals a pair of electrons from the centre of the benzene ring and forms a bond with one of the carbons
• This partially breaks the benzene ring resulting in a positively charged molecule
• To regain stability of the benzene ring, the carbon which the electrophile has bonded to loses a hydrogen
• As a result you get a substitution of a hydrogen for an electrophile

Question 28

What is a halogen carrier?

Answer 28

A molecule which can accept a halogen atom

Question 29

What are halogen carriers used in?

Answer 29

Used to add a halogen atom onto a benzene ring via an electrophilic substitution

Question 30

Why are halogen carriers used in the electrophilic substitution of benzene?

Answer 30

The electrophile doesn’t have a strong enough positive charge to attack the stable benzene ring

Question 31

How do halogen carriers work in the electrophilic substitution of benzene?

Answer 31

They make the electrophile stronger by accepting a lone pair of electrons from the electrophile. As the lone pair of electrons are pulled away, the the electrophile becomes more polar and a permanent dipole forms (making it a lot stronger)

Question 32

What is the overall word equation for the halogenation of benzene using a halogen carrier?

Answer 32

Halogen carrier + Bromine —> Halobenzene + Hydrogen halide

Question 33

What is an intermediate?

Answer 33

A molecule that is formed from two or more reactants and then reacts further to give products

Question 34

When drawing the intermediate in the halogenation of benzene, what must you do to avoid losing marks in an exam?

Answer 34

Draw the horseshoe shape of the benzene ring more than half way up (draw a segment of a circle of more than 180°)

Question 35

Describe the nitration reaction of benzene

Answer 35

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

Question 36

What is the overall equation for the nitration of benzene?

Answer 36

Nitric acid + Benzene —> Nitrobenzene + Water

Sulfuric acid acts as catalyst

Question 37

Why is sulfuric acid used in the nitration of benzene?

Answer 37

It acts as a catalyst - it helps to make the nitronium ion (NO2+) which is the electrophile

Question 38

What is the equation for the formation of a nitronium ion?

Answer 38

HN03 + H2SO4 —> HSO4- + NO2+ + H2O

Question 39

In the nitration of benzene, what do you have to do if you only want one NO2 group (mononitration)?

Answer 39

Keep the temperature under 55°C, otherwise lots of substitutions will take place