Substitution reactions of benzene 12-13

Question 1

Draw the general reaction mechanism of electrophilic substitution of benzene.

Answer 1

• The electron-dense ring attracts an electrophile- the electrophile accepts a pair of the pi electrons from the delocalised ring to form a covalent bond.
• An intermediate forms that that contains both the electrophile and the hydrogen atom that is being substituted. The delocalised pi-electron cloud has been disrupted and the intermediate is less stable than benzene.
• The unstable intermediate rapidly loses the hydrogen as an H* ion. The delocalised ring of electrons reforms and stability is restored.

Question 2

Write the equation for the formation of NO₂⁺.

Answer 2

HNO₃ + H₂SO₄ —–> NO₂⁺ + HSO₄^- + H₂O

Question 3

What are the reagents needed for the nitration of benzene and why are they needed in the reaction?

Answer 3

• A mixture of concentrated nitric acid and concentrated sulfuric acid is used.
• The sulfuric acid is needed to generate an electrophile from the nitric acid.
• The electrophile is the nitryl cation, nitronium ion, and has the formula NO₂⁺.

Question 4

Draw the mechanism for the nitration of benzene.

Answer 4

Question 5

Explain how the sulfuric acid acts as a catalyst in the nitration of benzene.

Answer 5

The H⁺ reacts with the HSO₄^- from the first step to reform H₂SO₄. So the sulfuric acid is acting as a catalyst.

H⁺+ HSO₄^- —–> H₂SO₄

Question 6

Write how the equation for the formation of Br⁺(bromonium ion, reacts with benzene).

Answer 6

Br₂ + FeBr₃ ——> Br⁺ + FeBr₄^-

Question 7

Draw out the mechanism for the bromination of benzene.

Answer 7

Question 8

Show how the iron(III) bromide used as a halogen carrier in the bromination of benzene is a catalyst.

Answer 8

The H⁺ reacts with FeBr₄^- from the first step to form the other product of the overall reaction, HBr, and to reform FeBr₃. So the iron(III) is acting as a catalyst.

H⁺ + FeBr₄^-——-> FeBr₃ + HBr

Question 9

For bromination what halogen carriers could you use?

Answer 9

• For bromination, the halogen carrier is AlBr₃, FeBr₃or Fe.
• You can use Fe for bromination because Fe reacts with the halogens to form the iron (III) halide.