Benzene

Question 1

Describe the structure and bonding of benzene.

Answer 1

Structure:

1) Hexagonal ring of 6 C atoms
2) Ring is planar i.e. All C atoms lie in the same plane
3) All bond angles 120°

Bonding:
Each C atom is sigma bonded to 2 other C atoms and 1 C atom

Question 2

How are sigma bonds formed?

Answer 2

By overlap of atomic orbitals with electron density along axis between the nuclei.

Question 3

How are delocalised pi bonding electron clouds formed?

Answer 3

By sideways overlap of ‘p’ orbitals above and below the plane of the molecule.

Question 4

What accounts for the decrease in chemical potential energy of the benzene molecule compared to the Kekulé structure?

Answer 4

Benzene has a delocalised pi bonding system formed by the overlap of ‘p’ orbitals which delocalises the p electrons, spreading negative charge and reducing repulsion forces within the molecule.

Question 5

What is meant by an electrophile and give an example.

Answer 5

Electron deficient species that will accept a pair of electrons to form a dative covalent bond.
Example: H+, Br2, HBr

Question 6

Why does benzene resist addition reactions?

Answer 6

Addition reactions would result in a loss of the stabilising, delocalised pi system and the formation of a less stable addition product.

Question 7

Give the conditions needed for the nitration of benzene.

Answer 7

1. Concentrated nitric (V) acid
2. Concentrated sulphuric (VI) acid
3. 50-60°C

Question 8

What is the name of the electrophile involved in nitration of benzene?

Answer 8

NO2+ ion (nitryl ion / nitronium ion)

Question 9

After electrophilic attack on the benzene ring, what forms and what does it do?

Answer 9

Carbocation intermediate forms; positive charge is spread around the ring, increasing stability.

Question 10

State the reaction conditions for halogenation of benzene.

Answer 10

1. Absence of light
2. Room temperature
3. Anhydrous conditions
4. In the presence of a halogen carrier

Question 11

Give an example of a halogen carrier and how does it play a part in halogenation / alkylation of benzene.

Answer 11

• Aluminium chloride, AlCl3
• Al is electron deficient, has only 6 valency electrons around it
• dative bonds to halogen molecule (e.g. Cl2)
• polarises significantly, creating attacking nucleophile Cl+
• regenerated/reformed at the end; acts as a catalyst

Question 12

State the reaction conditions for alkylation of benzene.

Answer 12

1) Appropriate halogenoalkane
2) Halogen carrier
3) Anhydrous conditions
4) Reflux

Question 13

Give the similarities between ethene and benzene in terms of their mode of reaction.

Answer 13

Both molecules have a highly exposed pi bonding electron cloud. This negative electron density will be attacked by electrophiles.

Question 14

Why does ethene undergo electrophilic addition?

Answer 14

The relatively weak pi bond makes ethene a high energy molecule. Ethene undergoes addition reactions to give lower energy, saturated and more stable products.

Question 15

Why does benzene prefer electrophilic substitution rather than electrophilic addition?

Answer 15

Addition reactions would cause a loss in the stabilising delocalised pi bonding system and so, less energetically stable product would result.

Electrophilic substitution is preferred in which the stabilising pi bonding system is retained.

Question 16

Why doesn’t chlorobenzene undergo alkaline hydrolysis with OH- (aq)?

Answer 16

Electronegative chlorine atom causes the electron cloud to extend by overlap of orbitals.
This:
1) Shields C1 from the attacking nucleophile, OH-
2) Strengthens the C-Cl bond

Question 17

What is meant by the term delocalisation energy?

Answer 17

The decrease in chemical potential energy achieved by spreading (delocalising) the pi electrons around the ring, reducing repulsion forces within the molecule.