Unit 5- arenes

Question 1

Explain a benzene combustion reaction 5.4.1.b.i

Answer 1

Burn in air or oxygen. Carbon dioxide and water is produced if enough Oxygen Complete combustion of benzene requires a large amount of oxygen if incomplete combustion, particles of carbon is formed and a yellow sooty flame

Question 2

Explain the reaction of benzene when treated with bromine in the presence of light 5.4.1.b.ii

Answer 2

ultraviolet light/ bright sunlight forms 1,2,3,4,5,6 hexabromocycloxane free radical reaction due to light energy needed to overcome activation energy and rapidity of reaction same reaction with chlorine addition reactions

Question 3

explain the reaction of benzene when treated with bromine in the dark

Answer 3

electrophilic substitution reaction

Dry benzene

Liquid Bromine

Iron catalyst or Iron (III) bromide

If iron used it reacts with bromine to form:

Fe + 3Br₂ —> 2FeBr₃

Br₂ + FeBr₃ —> Br⁺ + [FeBr₄]^-

steamy fumes of HBr given of under reflux

catalyst is regenerated by:

[FeBr₄]^- + H⁺ —> FeBr₃ + HBr

Question 4

Explain the nitration of benzene

5.4.1.b.iii

Answer 4

• Concentrated Nitric acid and sulfuric acid used to overcome the activation energy of the benzene ring
• temperature must be kept below 50-55 degrees as dinitrobenzene is formed
• H₂SO₄ + HNO₃ –> NO₂⁺ + H₂O + HSO₄^-
• NO₂⁺ is the attacking electrophile
• H₂O produced

Question 5

Explain Sulfonation of benzene

5.4.1.b.iv

Answer 5

• Reacts with fuming sulfuric acid
• fuming sulfuric acid is produced from concentrated sulfuric acid and sulfur trioxide
• benzenesulfonic acid produced
• electrophile is SO₃

Question 6

Explain the halogenoalkane reaction with benzene- Alkylation (Friedel-Crafts)

5.4.1.b.v

Answer 6

• anhyrdrous aliminium chloride catalyst
• alkyl group with a X
• reaction mixture must be dry
• electrophile produced by :
• CH₃CH₂X + AlCl₃ –> CH₃CH₂⁺ + [AlCl₃X]^-
• products are Arylbenzene + HX
• reformation of a catalyst is :
• [AlCl₄]^- + H⁺ —> HCL + AlCl₃

where X is a halogen

Question 7

Explain the acylation of benzene (Friedel-Crafts)

5.4.1.b.v

Answer 7

• refluxed with an acyl chloride
• produces a ketone
• with anhydrous aluminium chloride
• reagents must be dry
• Electrophile produced by : ( ethanoyl chloride in this example)
• CH₃COCl + AlCl₃ + —> CH₃C⁺O + [AlCl₄]^-
• catalyst regenerated by :
• H⁺+ [AlCl₄]^- —> HCl + AlCl₃

Question 8

Explain the addition of Hydrogen to benzne

5.4.1.b.vi

Answer 8

• double bond in benzene not as susceptible to addition
• reacts with Hydrogen in the presense of a Raney nikel catalst
• 150 degrees
• forms C₆H₁₂

Raney catalst - finely divided with a very large surface area and a very high catalyctic activity

Question 9

Why is phenol more activated?

Answer 9

The lone pair of electrons on the oxygen in the OH group becomes part of the delocalised Pi system.

Question 10

Explain the reaction with bromine and phenol

5.4.1.e

Answer 10

• electron rich ring in phenol attacked by bromine water
• electrophilic substituion
• bromine water decolourised
• white precipiate of 2,4,6 tribromophenol and solution of HBr formed
• no catalyst

Question 11

Explain the reaction with nitric acid and phenol

5.4.1.e

Answer 11

• Ring is sufficently activated to use dilute nitric acid
• room temp
• forms mixture of 2-nitrophenol and 4-nitrophenol

if mixture is headted, 2,4 and 2,6 dinitrophenol formed

concentrated acid formes 2,4,6 trinitrophenol