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

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),

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

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

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

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.

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.

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

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 To replay the sequence, click 

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

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

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

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

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

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

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