Benzene

Question 1

What is the molecular formula of benzene

Answer 1

C6H6

Question 2

What can be used as a test for benzene

Answer 2

Benzene burns with a smoky flame due to high carbon:hydrogen ratio

Question 3

Structure of benzene

Answer 3

Hexagonal planar
6 carbons bonded in a ring by sigma bonds leaving 1 electron on each carbon in a P orbital = P orbitals overlap to form delocalised P orbital

6 P orbitals overlap = 3 pi bonds = clouds of electrons above + below plane of ring

Question 4

What 3 pieces of evidence disprove the Kekule model for benzene

Answer 4

1- all carbon - carbon lengths are the same
2- bond angles = 120• = structure must be planar
3- Enthalpy of hydrogenation to cyclohexane is less exothermic than expected = Benzene must be more stable than Kekule model

Question 5

What type of mechanism does benzene usually go through

Answer 5

Electrophilic substitution

Question 6

Why does benzene need a catalyst to react with halogens

Answer 6

Electron density is not high enough in benzene to induce a dipole on halogen for electrophilic addition

Question 7

Describe the mechanism of electrophilic substitution

Answer 7

1- 2 delocalised electrons from ring form dative covalent bond with electrophile

2- leaves positive charge on carbon electrophile bonded to

3- Hydrogen Ion is eliminated to regain stable structure

Question 8

What catalyst is used for benzene reaction with bromine

Answer 8

AlBr3 or FeBr3

Question 9

Show how the catalyst AlBr3 works in the producing an electrophile

Answer 9

Br2 + AlBr3 = BrAlBr3(-) + Br(+)

Catalyst is reformed through the eliminated proton

BrAlBr3(-) + H(+) = AlBr3 + HBr

Catalyst is reformed

Question 10

What could be used instead of AlBr3 in the halogenation of benzene and why isn’t it

Answer 10

Al

Al is not reformed at end so is not a catalyst

Question 11

What is phenol

Answer 11

A benzene with a hydroxyl -OH
Group instead of a hydrogen on one carbon

Question 12

Why is Phenol more reactive than Benzene

Answer 12

Lone pair of electrons on oxygen atom delocalises into delocalised ring of P orbitals = increases electron density

Question 13

Why does Phenol not need a catalyst when reacting with a halogen

Answer 13

High electron density of Phenol means that it can Induce a dipole on Halogen

Question 14

Does phenol decolourise bromine water

Answer 14

Yes
Adds 3 bromine atoms via electrophilic ADDITION ( OH group does not come off)
Orange = colurless

Question 15

What is the elctrophile in the nitration of benzene

Answer 15

NO2+ ion

Question 16

How is the NO2 + ion produced for the Nitration / Elctrophillic substitution to produce Nitrobenzene

Answer 16

Mix of Con Nitric acid + sulphuric acid

HNO3 + H2SO4 = H2NO3(+) + HSO4 (-)

Then

H2NO3(+) = NO2(+) + H20

Sulphuric acid protonates/ gives H+ ion to nitric acid that decomposes to produce electrophile/ NO2 +

Question 17

How is sulphuric acid remade to be a catalyst in the nitration of benzene

Answer 17

HSO4(-) + H(+) = H2SO4

Hydrogen ion eliminated when NO2 + was added

Question 18

Conditions for nitration of benzene

Answer 18

HNO3
H2SO4
50•

Question 19

What happens in alkylation

Answer 19

Hydrogen atom on benzene substituted for alkyl group - CH3 for example

Question 20

How is the alkyl electrophile produced

Answer 20

CH3Cl + AlCl3 = [AlCl4]- + CH3(+)

Question 21

What happens in acylation

Answer 21

Hydrogen atom swapped for Acyl group- C=O bond eg : COCH3

Question 22

How is the acycl electrophile produced

Answer 22

CH3COCl + AlCl3 = [AlCl4]- + CH3CO(+)

Question 23

What does the prefix -phenyl mean

Answer 23

A benzene ring without a hydrogen atom + with new group

Question 24

What conditions are needed for Freidel-crafts reactions - acylation/alkylation

Answer 24

1- Alimiminium Halides - AlCl3
2- Heat under Reflux
3- Anhydrous conditions

Question 25

Why are anhydrous conditions needed In Friedel-crafts reactions

Answer 25

Reactants would react with water