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A2 Chemistry Chapter 2

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

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

Page 2: A2 Chemistry Chapter 2

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),

Page 3: A2 Chemistry Chapter 2

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

Page 4: A2 Chemistry Chapter 2

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

Page 5: A2 Chemistry Chapter 2

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

Page 6: A2 Chemistry Chapter 2

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.

Page 7: A2 Chemistry Chapter 2

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.

Page 8: A2 Chemistry Chapter 2

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+

Page 9: A2 Chemistry Chapter 2

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

Page 10: A2 Chemistry Chapter 2

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

Page 11: A2 Chemistry Chapter 2

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)

Page 12: A2 Chemistry Chapter 2

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

Page 13: A2 Chemistry Chapter 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

Page 14: A2 Chemistry Chapter 2

OH

+ 3Br2+

OH

Br

Br

Br

3HBr

phenol bromine

2,4,6-tribromophenolHydrogen bromide

Answer SAQs 2.4, 2.5 & 2.6

Page 15: A2 Chemistry Chapter 2

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

Page 16: A2 Chemistry Chapter 2

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

Page 17: A2 Chemistry Chapter 2

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 +

_

_

_


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