3.10 Aromatic Chemistry

Question 1

Explain the bonding in and the shape of a benzene molecule.

Answer 1

Stage 1 :Bonding
1a) Each C has three (covalent) bonds
1b) Spare electrons (in a p orbital) overlap (to form a π cloud) 1c) delocalisation
Stage 2 :Shape
2a) Planar
2b) Hexagon/6 carbon ring/120° bond angle
2c) C–C bonds equal in length / C–C bond lengths between single and double bond

Question 2

Formation of NO2+

What is the base?

Answer 2

HNO3 + 2H2SO4 –> NO2+ + 2HSO4- + H3O+

Question 3

The product of step 1 was purified by recrystallisation as follows.
The crude product was dissolved in the minimum quantity of hot water and the hot solution was filtered through a hot filter funnel into a conical flask. This filtration removed any insoluble impurities. The flask was left to cool to room temperature.
The crystals formed were filtered off using a Buchner funnel and a clean cork was used to compress the crystals in the funnel. A little cold water was then poured through the crystals.
After a few minutes, the crystals were removed from the funnel and weighed.
A small sample was then used to find the melting point.
Give reasons for each of the following practical steps.
The minimum quantity of hot water was used
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The flask was cooled to room temperature before the crystals were filtered off
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The crystals were compressed in the funnel
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A little cold water was poured through the crystals

Answer 3

The minimum quantity of hot water was used:
To ensure the hot solution would be saturated / crystals would form on cooling
1
The flask was left to cool before crystals were filtered off:
Yield lower if warm / solubility higher if warm
1
The crystals were compressed in the funnel:
Air passes through the sample not just round it
Allow better drying but not water squeezed out
1
A little cold water was poured through the crystals:
To wash away soluble impurities

Question 4

What are the reagents required to converts benzene to nitrobenzene?
What sort of reaction is this?

Answer 4

conc nitric acid in the presence of concentrated sulfuric acid (catalyst)
Electrophilic substitution.

Question 5

What are the conditions and reagents needed to convert benzene to phenylketone?

Answer 5

Acyl chloride in the presence of anhydrous aluminium chloride catalyst.
heat under reflux (50*C)

Question 6

What is the reaction to produce the electrophile in freidal crafts acylation?

Answer 6

AlCl3 + CH3COCl —> CH3CO+AlCl4-

Question 7

How do anti bumping granules prevent bumping during reflux?

Answer 7

By allowing small bubbles to form and preventing the formation of large bubbles.

Question 8

Describe and explain the bonding in benzene.

Answer 8

Each carbon has 3 covalent bonds.
The spare electrons in the p orbital overlap forming a pi cloud.
These electrons are delocalised.

Question 9

Describe and explain the shape in benzene.

Answer 9

Planer molecule.
Hexagon with 120 bond angle.
C-C bonds are equal in length.

Question 10

The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexane.
Will the value be higher or lower, justify your answer.

Answer 10

Higher,

Because the double bond is separated by one single bond which allows some delocalisation.