Chapter 25 - Aromatic compounds

Question 1

Formula of benzene

Answer 1

C6H6

Question 2

What type of hydrocarbon is benzene?

Answer 2

Aromatic - it is an arene

Question 3

What is Kekule’s model of benzene?

Answer 3

The six carbon ring is joined by alternating single and double bonds

Question 4

What are the three pieces of evidence to disprove Kekule’s model?

Answer 4

Benzene’s lack of reactivity
The length of the c-c bonds
Hydrogenation enthalpy of benzene

Question 5

Why does benzene’s lack of reactivity disprove Kekule’s model?

Answer 5

Benzene doesn’t undergo addition reactions or decolourise bromine

Question 6

Why does the length of benzene’s c-c bonds disprove kekule’s model?

Answer 6

The c-c bond is between the length of the single and double bond

Question 7

Why does the hydrogenation enthalpy of benzene disprove kekule’s model?

Answer 7

Its expected enthalpy would be triple the enthalpy of cyclohexene but it doesn’t

Question 8

What is the delocalised model of benzene?

Answer 8

Each carbon uses three of its four electrons to bond to 2 x C and 1 x H
The overlapping of unused p-orbitals creates a system of pi-bonds which spread over the six atoms

Question 9

When will the prefix ‘phenol’ be used?

Answer 9

When the alkyl group attached to the benzene has more than 7 carbon atoms

Question 10

What happens when benzene is nitrated?

Answer 10

One of the hydrogen atoms on the benzene ring is replaced with a NO2 group

Question 11

What is used to catalyse the reaction between benzene and a nitrate group?

Answer 11

Sulfuric acid

Question 12

How is the nitronium ion produced?

Answer 12

Nitric acid reacts with sulfuric acid

Question 13

What must be present for a halogen to react with benzene?

Answer 13

A halogen carrier, e.g. AlCl3

Question 14

How does benzene reacts with alkyl groups?

Answer 14

A hydrogen atom in the benzene ring is replaced by an alkyl group

Question 15

Conditions needed for alkylation?

Answer 15

React benzene with a haloalkane in the presence of a halogen carrier

Question 16

Why is alkylation useful?

Answer 16

Increases the length of the carbon chain

Question 17

What is an acylation reaction?

Answer 17

When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst

Question 18

What is produced when benzene is acylated?

Answer 18

An aromatic ketone

Question 19

Why won’t benzene react with bromine unless a halogen carrier is present?

Answer 19

Benzene has delocalised electrons above and below the c-c ring
Lower electron density
When a non-polar molecule like benzene approaches, this means there are insufficient electrons to polarise the bromine

Question 20

What is a phenol?

Answer 20

An OH group bonded to a benzene ring

Question 21

What is the difference between an alcohol and a phenol?

Answer 21

An alcohol involves a OH group bonded to a carbon side chain, not to benzene itself

Question 22

Why is phenol less soluble than alcohol?

Answer 22

The benzene ring is non-polar

Question 23

Why is phenol a weak acid?

Answer 23

Phenol partially dissociates when dissolved in water

Question 24

Will ethanol react with sodium hydroxide or sodium carbonate?

Answer 24

No

Question 25

Will phenols and carboxylic acids react with NaOH?

Answer 25

Yes

Question 26

Will carboxylic acids react with sodium carbonate?

Answer 26

Yes

Question 27

What happens when phenol is brominated?

Answer 27

Bromine is added to the benzene ring - no halogen carrier needed

Question 28

Why do halogens and nitric acid react more readily with phenol than benzene?

Answer 28

A lone pair of electrons is donated from the oxygen atom to the pi-system
This increases the electron density so it can attract electrophiles more readily

Question 29

What is activation?

Answer 29

The aromatic ring reacts more readily with electrophiles

Question 30

What is deactivation?

Answer 30

The aromatic ring reacts less readily with electrophiles

Question 31

What type of directing is -NH2?

Answer 31

2 or 4

Question 32

What type of directing is -NO2?

Answer 32

3