Chapter 25 Aromatic Compounds (benzene)

Question 1

What is the molecular formula of benzene?

Answer 1

C6 H6

Question 2

Describe the structure of the delocalised model of benzene.

Answer 2

Carbon, as an atom, has 4 valent electrons.
In benzene, each carbon is bonded to two other carbons and a hydrogen.
The remaining lone electron is in a p-orbital that sticks out above and below the planar ring.
The p-orbitals from each carbon in the ring overlaps to form a delocalised ring of electrons- a Pi system.

Question 3

How does Kekule Model of Benzene structure differ to the delocalised model?

Answer 3

Kekule model still has a six membered carbon ring. However, in this ring, there is alternating single and double bonds. In the delocalised model, only single bonds are present.

Question 4

Why did scientists not accept Kekule’s model of benzene?

Answer 4

The structure of Kekule’s benzene did not match with the chemical and physical properties it portrayed.

Question 5

One piece of evidence to disprove Kekule’s model of benzene was its reactivity with bromine. Explain this.

Answer 5

In accordance to Kekule’s model of Benzene, there are some double bonds in the structure. Hence, like alkenes, it was hypothesised that benzene would undergo electrophilic addition with bromine, and decolourise the bromine water.
However, when this was tested out, benzene does not undergo electrophilic addition and does not decolourise bromine water under normal conditions.
This suggests there is no double bonds present in benzene.

Question 6

Another piece of evidence to disprove Kekule’s model of benzene was the length of carbon-carbon bonds. Explain this.

Answer 6

By using a technique called x-ray diffraction, it is possible to measure the length of bonds. When this was carried out on benzene, it was found that all the carbon-carbon lengths were the same, meaning they could only be one type of bond. In order for each carbon to have the correct amount of bonds, the type of bond must have been a sigma bond.

Question 7

Another evidence to disprove Kekule’s model was the enthalpy of hydrogenation reactions. Explain this.

Answer 7

If benzene had Kekule’s structure, then the hydrogenation enthalpy would be three times than that of cyclohexene. Cyclohexene has a hydrogenation enthalpy of -120 kJ/mol. Hence, it was hypothesised that benzene would have a hydrogenation enthalpy of -360kJ/mol if it has 3 double bonds.
However, when carrying out the hydrogenation of benzene, it was seen that the enthalpy was -208 kJ/mol. Since the enthalpies did not match up, it also disproves that benzene had Kekule’s structure.

Question 8

What are arenes?

Answer 8

Arenes are compounds containing a benzene ring.

Question 9

In nomenclature, where is the parent compound found in the name of the whole compound?

Answer 9

The parent compound is found at the end of the name.

Question 10

In what cases is benzene not the parent compound, but as the substituent?

Answer 10

If there is an alkyl chain bonded to the benzene ring and this alkyl chain contains a functional group.
OR
If the alkyl chain bonded to the benzene is made of 7 or more carbons.

Question 11

What is the name of the molecule, where there is a -COOH bonded directly to benzene?

Answer 11

Benzoic acid.

Question 12

What is the name of the molecule, where there is a -NH2 bonded directly to the benzene?

Answer 12

Phenylamine.

Question 13

What is the name of the molecule, where there is a -CHO bonded directly to the benzene?

Answer 13

Benzaldehyde.

Question 14

What is the name of the molecule, where there is a -OH bonded directly to the benzene?

Answer 14

Phenol.

Question 15

What type of mechanisms is benzene involved in (for halogenation, nitration, acylation, alkylation)?

Answer 15

Electrophilic substitution.

Question 16

What are the reagents and products of a nitration reaction of benzene?

Answer 16

Benzene + nitric acid > Nitrobenzene + water

Question 17

What are the conditions needed for the nitration reaction of benzene?

Answer 17

A sulfuric acid catalyst.
Temperature of 50 degrees Celsius.

Question 18

What happens when the temperature exceeds 50 degrees Celsius in the nitration of benzene?

Answer 18

Multiple substitutions could occur, resulting in dinitrobenzene.

Question 19

What could nitrobenzene be used in?

Answer 19

Dyes for clothing
Explosives
Pharmaceutical- Preparation for paracetamol

Question 20

State the steps in the electrophilic substitution mechanism of the nitration of benzene.

Answer 20

Step 1:
The electrophile is formed.
This is done by reacting the nitric acid with sulfuric acid, to produce the nitronium ion (the electrophile), HSO4- ion and water.
Step 2:
The nitronium ion is attracted to the electron dense Pi system, and the Pi system gives its electrons to the nitronium ion; it has formed a bond with it.
Now, one of the carbons in the benzene ring has 4 bonds (to 2 other carbons, the nitronium ion, and a hydrogen). Hence, the stable state of the benzene ring has been disrupted.
To re-enter a stable state, the hydrogen that is bonded to the carbon with 4 bonds, donates its electrons back into the Pi system, and the hydrogen now becomes a free hydrogen ion.
What is left is nitrobenzene, which is stable.
Step 3:
The hydrogen ion formed in step 2, reacts with the HSO4- ion formed in step 1. Via this reaction, sulfuric acid is reformed. Due to the reformation, sulfuric aid is considered a catalyst.

Question 21

State the reagents and products of the halogenation of benzene.

Answer 21

Benzene + halogen > halobenzene + hydrogen halide

Question 22

What is the conditions needed for the halogenation of benzene? Why are these conditions needed?

Answer 22

For the halogenation of benzene, a halogen carrier is needed (iron bromide, aluminium bromide, iron chloride, aluminium chloride) . The halogen carrier acts as a catalyst and helps to generate the nucleophile. This nucleophile is now electron deficient enough to be attracted to the Pi system in benzene.

Question 23

State the electrophilic substitution mechanism for the halogenation of benzene.

Answer 23

Step 1:
An electrophile needs to be generated.
A bromine molecule reacts with iron bromide, to produce a bromide ion and FeBr4-. The bromide ion produced is the electrophile. (Iron bromide acts as a catalyst. Alternatively, aluminium bromide can be used).
Step 2:
The bromide ion is attracted to the Pi system in the benzene ring. The Pi system donates a pair of electrons to the bromide ion; the benzene has now formed a bond with the bromine.
However, the benzene has now entered an unstable state. This is because the carbon that has just bonded to the bromine has 4 bonds (with 2 other carbons, the bromine and a hydrogen. In benzene, carbons normally has 3 bonds and has a lone electron in the p-orbital).
To re-enter a stable state, the hydrogen bonded to the carbon that has a bromine atom, gives its electrons to the Pi system. The hydrogen becomes a hydrogen ion.
Though, the halobenzene is now stable.
Step 3:
The hydrogen ion formed in step 2, reacts with the FeBr4- ion. This produces hydrogen bromide and FeBr3. As FeBr3 is reformed, we consider FeBr3 as a catalyst.

Question 24

What is an alkyl group?

Answer 24

An alkyl group is a chain of carbon and hydrogen atoms ( in a chain structure).

Question 25

What is an acyl group?

Answer 25

An acyl group is an alkyl group or aryl group that also contains a carbonyl group (C=O).

Question 26

In the alkylation of benzene, what is the reagents and products?

Answer 26

Benzene + haloalkane > alkylbenzene + hydrogen halide

Question 27

What are the conditions of an alkylation of benzene? Why is this needed?

Answer 27

The alkylation of benzene requires a halogen carrier as a catalyst. The halogen carrier helps to generate a nucleophile.

Question 28

What are the products in the reaction of benzene with chloroethane?

Answer 28

ethylbenzene and hydrochloric acid.

Question 29

What is an acyl chloride?

Answer 29

An acyl chloride is an acyl group containing a chlorine atom.

Question 30

What are the reagents and products of the acylation of benzene?

Answer 30

Benzene + acyl chloride (or acyl bromide) > aromatic ketone + hydrogen halide

Question 31

What are the conditions needed for the acylation of benzene?

Answer 31

A halogen carrier is required for the reaction as a catalyst.

Question 32

Compare the reactivity of alkenes with bromine to benzenes with bromine.

Answer 32

Alkenes react more readily with bromine than benzene because:
The Pi bond in alkenes contains localised electrons.
The Pi system in benzene has delocalised electrons. As these electrons are delocalised, benzene has a lower electron density.
The insufficient electron density means benzene is unable to attract electrophiles and induce a dipole (electrophiles like bromine).

Question 33

In a molecule containing a benzene ring and a hydroxyl group, in what cases in the molecule an alcohol and not a phenol?

Answer 33

It is an alcohol, when there is a side carbon chain bonded to the benzene, and in this carbon chain there is a hydroxyl group. (The hydroxyl group should not be directly bonded to benzene).

Question 34

Why is phenol less soluble in water than alcohols?

Answer 34

Phenol is less soluble in water due to the non-polar benzene ring. Hence, when dissolved in water, phenol only partially dissociates to form a phenoxide ion and hydrogen ion. Hence, phenol is a weak acid.

Question 35

Can:
Phenol react with strong bases?
Carboxylic acids react with strong bases?
Ethanol react with strong bases?
An example of a strong base is aqueous sodium hydroxide.

Answer 35

Phenol is able to react with a strong base.
Carboxylic acids are able to react with a strong base.
Ethanol is unable to react with a strong base.

Question 36

Can:
Phenol react with weak bases?
Carboxylic acids react with weak bases?
Ethanol react with weak bases?
An example of a weak base is sodium carbonate.

Answer 36

Phenols cannot react with weak bases.
Ethanol is unable to react with weak bases.
Carboxylic acids can react with weak bases. In the reaction of a carboxylic acid with sodium carbonate, there will be effervescence due to the production of carbon dioxide.

Question 37

Are there any conditions for the bromination and nitration of phenol?

Answer 37

The bromination and nitration of phenol is done at room temperature and pressure?? It does not require any catalysts.

Question 38

Phenol reacts more readily with bromine and nitric acid with no catalysts, than when benzene reacts with bromine and nitric acid. Why is this?

Answer 38

In the structure of phenol:
A lone pair of electrons from the oxygen p-orbital in the -OH group is donated to the Pi system in phenol.
Due to this, there is an increased electron density in the benzene ring of phenol.
Increased electron density attracts more electrophiles more strongly than benzene would, so phenol is more susceptible to attack from electrophiles, like bromine and nitric acid.

Question 39

What is produced when phenol reacts with sodium hydroxide?

Answer 39

Sodium phenoxide and water.