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A2 Chemistry Chapter 2. Chapter 2 Objectives. Properties of Phenol. Phenol + NaOH. Delocalisation of Benzene. Phenol + Na (s). Nitration of Benzene. Halogenation of Benzene. Phenol + Br 2. SAQ 2.1. SAQ 2.3. SAQ 2.6. A2: Arenes Chapter 2 Objectives (1-8). - PowerPoint PPT Presentation
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A2 ChemistryChapter 2
Chapter 2 Objectives
Delocalisation of Benzene
Nitration of Benzene
Halogenation of Benzene
Properties of Phenol
Phenol + NaOH
Phenol + Na(s)
Phenol + Br2
SAQ 2.1 SAQ 2.3 SAQ 2.6
A2: Arenes Chapter 2 Objectives (1-8)
1.1. Show understanding of the concept of delocalisation of Show understanding of the concept of delocalisation of electrons as used in a model of benzeneelectrons as used in a model of benzene
2.2. Describe electrophilic substitution of arenes with concentrated Describe electrophilic substitution of arenes with concentrated nitric acid in the presence of concentrated sulphuric acid, a nitric acid in the presence of concentrated sulphuric acid, a halogen in the presence of a halogen carrier, and a halogen in the presence of a halogen carrier, and a halogenoalkane such as chloromethane in the presence of a halogenoalkane such as chloromethane in the presence of a halogen carrier (Friedel-Crafts reaction).halogen carrier (Friedel-Crafts reaction).
3.3. Describe the mechanism of electrophilic substitution in Describe the mechanism of electrophilic substitution in arenes, using the mononitration of benzene as an examplearenes, using the mononitration of benzene as an example
4.4. Understand that reactions of arenes, such as those in point 2 Understand that reactions of arenes, such as those in point 2 above, are used by industry during the synthesis of above, are used by industry during the synthesis of commercially important materials, for example explosives, commercially important materials, for example explosives, pharmaceuticals and dyes (from nitration), and polymers such pharmaceuticals and dyes (from nitration), and polymers such as polystyrene (from alkylation),as polystyrene (from alkylation),
Chapter 2 Objectives(continued)
5.5. Explain the relative resistance to bromination of benzene, Explain the relative resistance to bromination of benzene, compared with cyclohexene, in terms of delocalisation of the compared with cyclohexene, in terms of delocalisation of the benzene ring.benzene ring.
6.6. Describe the reactions of phenol with bases and sodium to form Describe the reactions of phenol with bases and sodium to form salts and with bromine to form 2,4,6-tribromophenolsalts and with bromine to form 2,4,6-tribromophenol
7.7. Explain the relative ease of bromination of phenol, compared with Explain the relative ease of bromination of phenol, compared with benzene, in terms of activation of the benzene ringbenzene, in terms of activation of the benzene ring
8.8. State the uses of phenols in antiseptics and disinfectantsState the uses of phenols in antiseptics and disinfectants
C
C C
C
CC
The structure of benzene can be represented in a variety of ways
Consider only the carbon ring that lies in a plain.
Add the bonds noticing they are adding perpendicular to the plain of the ring, then taking away the lower lobes for clarity
If you imagine the lobes being large enough to overlap, the image changes to:
HC
HC CH
CH
CHHC
e-
C
C C
C
CC
e-
e-
e-e-
e-
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.
•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.
Because of the delocalisation, a benzene ring does not attract electrophiles with the same force as aliphatic double bond molecules.
Replay slides
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+
HNO3
Loss of H2O occursNO2
+
Nitronium ion
This is the electrophile which will add molecules with double bonds.
The proton from H2SO4 adds to HNO3 creating a new arrangementNitric acid has this
spatial arrangement.
Rearrangement then occurs
To replay the sequence, right click on the screen and select previous.
NO2+
Loss of H+ occurs finally giving nitro benzene
Substitution on the benzene ring continues as illustrated
The nitronium ion moves toward the
benzene ringA pair of electrons moves from the ring toward the nitronium ion…
giving the intermediate
benzenenitronium ion
To replay the sequence, right click on the screen and select previous.
Electrophilic Substitution of Arenes• Example 2: A halogen (X2) in the presence of a
halogen carrier. (Br2 with FeCl3)
FeCl3Fe
Cl
Cl
Cl+
_
_
_
Br2Br Br
Br Br
Benzene…
…is going to react with bromine
Bromine’s electron density is immagined as…
Initially, bromine does not become polarized enough to react with benzene.
Iron (III) chloride arrives as the catalyst to help the reaction.
It spatial arrangement creates the following dipoles…
In the presence of the catalyst, bromine’s polarity changes from…
Into…
_+
Electon density shifts…
Fe
Cl
Cl
ClBr To create…
+ Br+
Br
Br+
Br+ ion is a strong enough electrophile to add to the benzene ring
+ HBr
Fe
Cl
Cl
ClBr FeCl3
H
+
-
The hydrogen atom is substituted by the bromine atom
The catalyst is regenerated
To replay the sequence, click
Answer SAQ 2.1, 2.2 & 2.3
Phenol
Vanillin
Estradiol
Phenols and their properties
Phenol occurs widely in nature but the effects differ remarkably.
•Found in seed pods of vanilla orchid•Used as flavouring additive in ice cream and chocolate
•An important female sex hormone•Maintains female sexual characteristics•Stimulates RNA synthesis and therefore promotes growth.
OH
OH
O
CH3
C
H OH2C
HO
H2C
CH2
H2C
CH
CH3
CH3
OH
HO
Cholesterol
NaOHNa+OH
The ring draws electron density toward it
+ OH--
H2O
O+
This weakens the O-H bond allowing it dissociate more easily than other alcohols. It is therefore slightly acidic.
An alkali will react with phenol in the expected way producing a salt and water.
- Na+
Sodium phenoxide
+
pH ~ 6
Phenol reacting with Alkali (NaOH)
Phenol reacting with sodium metal (Na(s))
OH + 2Na O Na + H2 (g)
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+)
Na(s) + 2H+(aq) Na+
(aq) + H2(g)
Reduction: gain of electrons
Oxidation: loss of electrons
Sodium metal and hydrogen ions undergo oxidation and reduction.
2
OH
Br2Br Br
OH
Br
H
Br
HBr
OH
Br
Br Br
OH
Br
Br
HBr
OH
Br
Br
Br Br
HBr
HBr
OH
Br
Br
Br
H
Br 1
3
1
2
3
2
OH
+ 3Br2+
OH
Br
Br
Br
3HBr
phenol bromine
2,4,6-tribromophenolHydrogen bromide
Answer SAQs 2.4, 2.5 & 2.6
SAQ 2.1CH3
NO2
2-nitro methyl benzene
CH3
NO2
2,4,6-trinitro methyl benzene
NO2O2N
CH3
NO2
3-nitro methyl benzene
A. Draw and name three isomers which might be produced following electrophilic substitution of NO2
+ for one hydrogen atom in methylbenzene.
B. TNT has the systematic name 1-methyl-2,4,6-trinitrobenzene. Draw the structural formula of TNT
CH3
NO2
4-nitro methyl benzene
SAQ 2.3
A. Suggest a suitable halogen carrier to use in the reaction of benzene with chloromethane.
B. Suggest suitable reactants which might lead to the formation of the following compound in the presence of a halogen carrier.
Anhydrous FeCl3 or AlCl3
Benzene + 2-chloro-2-methyl propane
+ H3C C CH3
Cl
CH3
B. Write a balanced equation using your suggested reactants
+ Cl C
CH3
CH3
CH3CH3
CH3
CH3
Fe Br
Br
Br
SAQ 2.6
A. How does bromine in aqueous solution become sufficiently polar to achieve electrophilic substitution
Br Br
Bromine has a symmetrical electron density arrangement
In the presence of FeBr3, the electron density shape changes
FeCl3 +
_
_
_