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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).

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a2 chemistry chapter 2

A2 ChemistryChapter 2

Chapter 2 Objectives

Properties of Phenol

Phenol + NaOH

Delocalisation of Benzene

Phenol + Na(s)

Nitration of Benzene

Halogenation of Benzene

Phenol + Br2

SAQ 2.1

SAQ 2.3

SAQ 2.6

slide2

A2: ArenesChapter 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),
chapter 2 objectives continued
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
slide4

C

C

C

C

C

C

Consider only the carbon ring that lies in a plain.

The structure of benzene can be represented in a variety of ways

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

slide5

C

C

C

C

C

C

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-

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

electrophilic substitution of arenes
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 selectprevious.

slide7

Substitution on the benzene ring continues as illustrated

benzene

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 selectprevious.

slide8

Cl

Br

Br

Fe

Cl

Br

Br

Cl

+

+

_

_

_

_

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.

FeCl3

It spatial arrangement creates the following dipoles…

Electon density shifts…

Into…

Bromine’s electron density is immagined as…

Cl

Br2

+

Br+

Br

Fe

Cl

To create…

…is going to react with bromine

Cl

slide9

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+

Br

H

+ HBr

Cl

-

FeCl3

Br

Fe

Cl

+

Cl

The catalyst is regenerated

To replay the sequence, click 

slide10

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

slide11

O

O

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

slide12

Na(s) + 2H+(aq) Na+(aq) + H2(g)

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.

Oxidation: loss of electrons

slide13

1

1

2

2

3

3

slide14

phenol

bromine

Hydrogen bromide

2,4,6-tribromophenol

Answer SAQs 2.4, 2.5 & 2.6

saq 2 1
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
slide16

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
slide17

SAQ 2.6

Br

Br

_

_

_

+

  • How does bromine in aqueous solution become sufficiently polar to achieve electrophilic substitution

FeCl3

Bromine has a symmetrical electron density arrangement

In the presence of FeBr3, the electron density shape changes

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