Presentation on theme: "Delocalisation of Benzene Halogenation of Benzene"— Presentation transcript:
1 Delocalisation of Benzene Halogenation of Benzene A2 Chemistry Chapter 2Chapter 2 ObjectivesProperties of PhenolDelocalisation of BenzenePhenol + NaOHNitration of BenzenePhenol + Na(s)Halogenation of BenzenePhenol + Br2SAQ 2.1SAQ 2.3SAQ 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 benzeneDescribe 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 exampleUnderstand 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-tribromophenolExplain the relative ease of bromination of phenol, compared with benzene, in terms of activation of the benzene ringState 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 waysCIf 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-CBecause 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 protonsH2SO4 (l) HSO4-H+The proton from H2SO4 adds to HNO3 creating a new arrangementRearrangement then occursNitric acid has this spatial arrangement.HNO3This is the electrophile which will add molecules with double bonds.Loss of H2O occursNO2+Nitronium ionTo replay the sequence, right click on the screen and select previous.
7 Substitution on the benzene ring continues as illustrated nitronium ionNO2+The nitronium ion moves toward the benzene ringA pair of electrons moves from the ring toward the nitronium ion…giving the intermediateLoss of H+ occurs finally giving nitro benzeneTo 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._FeCl__++FeCl3Br_It spatial arrangement creates the following dipoles…Electon density shifts…Into…Bromine’s electron density is immagined as…ClBrBr2+Br+BrFeClTo create……is going to react with bromineCl
9 Br+ + HBr FeCl3 H Br Cl Br Fe Cl Cl Answer SAQ 2.1, 2.2 & 2.3Br+ ion is a strong enough electrophile to add to the benzene ringThe hydrogen atom is substituted by the bromine atomBr+HBr+ HBrCl-FeCl3BrFeCl+ClThe catalyst is regeneratedTo replay the sequence, click
10 Phenols and their properties Phenol occurs widely in nature but the effects differ remarkably.EstradiolPhenolAn important female sex hormoneMaintains female sexual characteristicsStimulates RNA synthesis and therefore promotes growth.Found in seed pods of vanilla orchidUsed as flavouring additive in ice cream and chocolateCholesterolVanillin
11 Phenol reacting with Alkali (NaOH) -H++Na+NaOHOH--Na+The ring draws electron density toward itThis weakens the O-H bond allowing it dissociate more easily than other alcohols. It is therefore slightly acidic.Sodium phenoxideAn alkali will react with phenol in the expected way producing a salt and water.+H2OpH ~ 6
12 Phenol reacting with sodium metal (Na(s)) 2Na(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 electronsSodium metal and hydrogen ions undergo oxidation and reduction.Na(s) + 2H+(aq) Na+(aq) + H2(g)Oxidation: loss of electrons
15 SAQ 2.1Draw 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 2.3Suggest a suitable halogen carrier to use in the reaction of benzene with chloromethane.Anhydrous FeCl3 or AlCl3Suggest suitable reactants which might lead to the formation of the following compound in the presence of a halogen carrier.Benzene + 2-chloro-2-methyl propaneWrite a balanced equation using your suggested reactants
17 SAQ 2.6_How does bromine in aqueous solution become sufficiently polar to achieve electrophilic substitution_+FeCl3Br_Bromine has a symmetrical electron density arrangementIn the presence of FeBr3, the electron density shape changes