1. Delocalisation of Benzene Halogenation of Benzene
A2 Chemistry Chapter 2
Chapter 2 Objectives
Properties of Phenol
Delocalisation of Benzene
Phenol + NaOH
Nitration of Benzene
Phenol + Na(s)
Halogenation of Benzene
Phenol + Br2
SAQ 2.1
SAQ 2.3
SAQ 2.6

2. A2: Arenes Chapter 2 Objectives (1-8)
Show understanding of the concept of delocalisation of electrons as used in a model of benzene
Describe electrophilic substitution of arenes with concentrated nitric acid in the presence of concentrated sulphuric acid, a halogen in the presence of a halogen carrier, and a halogenoalkane such as chloromethane in the presence of a halogen carrier (Friedel-Crafts reaction).
Describe the mechanism of electrophilic substitution in arenes, using the mononitration of benzene as an example
Understand that reactions of arenes, such as those in point 2 above, are used by industry during the synthesis of commercially important materials, for example explosives, pharmaceuticals and dyes (from nitration), and polymers such as polystyrene (from alkylation),

3. Chapter 2 Objectives (continued)
Explain the relative resistance to bromination of benzene, compared with cyclohexene, in terms of delocalisation of the benzene ring.
Describe the reactions of phenol with bases and sodium to form salts and with bromine to form 2,4,6-tribromophenol
Explain the relative ease of bromination of phenol, compared with benzene, in terms of activation of the benzene ring
State the uses of phenols in antiseptics and disinfectants

4. C Consider only the carbon ring that lies in a plain.
The structure of benzene can be represented in a variety of ways C
If you imagine the lobes being large enough to overlap, the image changes to:
Add the  bonds noticing they are adding perpendicular to the plain of the ring
, then taking away the lower lobes for clarity

5. Again for clarity, the electron clouds above and below the plain of the ring have been reduced in size
, the green being above the plain of the ring and pink below. e- C
Because of the delocalisation, a benzene ring does not attract electrophiles with the same force as aliphatic double bond molecules. e- e- e- e- e-
Three electrons are added to each cloud area.
Because of the overlapping orbitals, they are able to move over the entire perimeter of the ring.
This is known as delocalisation.
Replay slides

6. Electrophilic Substitution of Arenes
Example 1: A mixture of concentrated nitric acid (HNO3) and concentrated sulphuric acid (H2SO4)
The sulphuric acid catalyst provides protons
H2SO4 (l)  HSO4- H+
The proton from H2SO4 adds to HNO3 creating a new arrangement
Rearrangement then occurs
Nitric acid has this spatial arrangement.
HNO3
This is the electrophile which will add molecules with double bonds.
Loss of H2O occurs
NO2+
Nitronium ion
To replay the sequence, right click on the screen and select previous.

7. Substitution on the benzene ring continues as illustrated
nitronium ion
NO2+
The nitronium ion moves toward the benzene ring
A pair of electrons moves from the ring toward the nitronium ion…
giving the intermediate
Loss of H+ occurs finally giving nitro benzene
To replay the sequence, right click on the screen and select previous.

8. Electrophilic Substitution of Arenes
Example 2: A halogen (X2) in the presence of a halogen carrier. (Br2 with FeCl3)
Iron (III) chloride arrives as the catalyst to help the reaction.
Benzene…
In the presence of the catalyst, bromine’s polarity changes from…
Initially, bromine does not become polarized enough to react with benzene. _  Fe Cl _  _   +  +
FeCl3 Br _ 
It spatial arrangement creates the following dipoles…
Electon density shifts…
Into…
Bromine’s electron density is immagined as… Cl Br
Br2 +
Br+ Br Fe Cl
To create…
…is going to react with bromine Cl

9. Br+ + HBr FeCl3 H Br Cl Br Fe Cl Cl
Answer SAQ 2.1, 2.2 & 2.3
Br+ ion is a strong enough electrophile to add to the benzene ring
The hydrogen atom is substituted by the bromine atom
Br+ H Br
+ HBr Cl -
FeCl3 Br Fe Cl + Cl
The catalyst is regenerated
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10. Phenols and their properties
Phenol occurs widely in nature but the effects differ remarkably.
Estradiol
Phenol
An important female sex hormone
Maintains female sexual characteristics
Stimulates RNA synthesis and therefore promotes growth.
Found in seed pods of vanilla orchid
Used as flavouring additive in ice cream and chocolate
Cholesterol
Vanillin

11. Phenol reacting with Alkali (NaOH)- H + +
Na+
NaOH
OH- -
Na+
The ring draws electron density toward it
This weakens the O-H bond allowing it dissociate more easily than other alcohols. It is therefore slightly acidic.
Sodium phenoxide
An alkali will react with phenol in the expected way producing a salt and water. +
H2O
pH ~ 6

12. Phenol reacting with sodium metal (Na(s))2
Na(s) reacts similarly with all alcohols…
…but phenol is somewhat more reactive.
Because density is drawn into the ring…
…the hydrogen comes off more readily (H+)
Reduction: gain of electrons
Sodium metal and hydrogen ions undergo oxidation and reduction.
Na(s) + 2H+(aq) Na+(aq) + H2(g)
Oxidation: loss of electrons

13. 1 1 2 2 3 3

14. phenol
bromine
Hydrogen bromide
2,4,6-tribromophenol
Answer SAQs 2.4, 2.5 & 2.6

15. SAQ 2.1
Draw and name three isomers which might be produced following electrophilic substitution of NO2+ for one hydrogen atom in methylbenzene.
TNT has the systematic name 1-methyl-2,4,6-trinitrobenzene. Draw the structural formula of TNT

16. SAQ 2.3
Suggest a suitable halogen carrier to use in the reaction of benzene with chloromethane.
Anhydrous FeCl3 or AlCl3
Suggest suitable reactants which might lead to the formation of the following compound in the presence of a halogen carrier.
Benzene + 2-chloro-2-methyl propane
Write a balanced equation using your suggested reactants

17. SAQ 2.6 _ 
How does bromine in aqueous solution become sufficiently polar to achieve electrophilic substitution _   +
FeCl3 Br _ 
Bromine has a symmetrical electron density arrangement
In the presence of FeBr3, the electron density shape changes