Presentation on theme: "A2 Chemistry Chapter 2 Chapter 2 Objectives Delocalisation of Benzene Nitration of Benzene Halogenation of Benzene Properties of Phenol Phenol + NaOH Phenol."— Presentation transcript:
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 2.1SAQ 2.3SAQ 2.6
A2: Arenes Chapter 2 Objectives (1-8) 1.Show understanding of the concept of delocalisation of electrons as used in a model of benzene 2.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). 3.Describe the mechanism of electrophilic substitution in arenes, using the mononitration of benzene as an example 4.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) 5.Explain the relative resistance to bromination of benzene, compared with cyclohexene, in terms of delocalisation of the benzene ring. 6.Describe the reactions of phenol with bases and sodium to form salts and with bromine to form 2,4,6-tribromophenol 7.Explain the relative ease of bromination of phenol, compared with benzene, in terms of activation of the benzene ring 8.State the uses of phenols in antiseptics and disinfectants
C CC 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:
e-e- C CC C CC e-e- e-e- e-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 (HNO 3 ) and concentrated sulphuric acid (H 2 SO 4 ) The sulphuric acid catalyst provides protons H 2 SO 4 (l) + HSO 4 - H+H+ HNO 3 Loss of H 2 O occurs NO 2 + Nitronium ion This is the electrophile which will add molecules with double bonds. The proton from H 2 SO 4 adds to HNO 3 creating a new arrangement Nitric acid has this spatial arrangement. Rearrangement then occurs To replay the sequence, right click on the screen and select previous.
NO 2 + Loss of H + occurs finally giving nitro benzene Substitution on the benzene ring continues as illustrated The nitronium ion moves toward the benzene ring A pair of electrons moves from the ring toward the nitronium ion… giving the intermediate benzene nitronium ion To replay the sequence, right click on the screen and select previous.
Electrophilic Substitution of Arenes Example 2: A halogen (X 2 ) in the presence of a halogen carrier. (Br 2 with FeCl 3 ) FeCl 3 Fe Cl + _ _ _ Br 2 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 Br To create… + Br +
Br Br + Br + ion is a strong enough electrophile to add to the benzene ring + HBr Fe Cl Br FeCl 3 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. Cholesterol
NaOHNa + O H The ring draws electron density toward it +OH - - H2OH2O 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) ) 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) + H 2(g) Reduction: gain of electrons Oxidation: loss of electrons Sodium metal and hydrogen ions undergo oxidation and reduction. 2
SAQ 2.1 A.Draw and name three isomers which might be produced following electrophilic substitution of NO 2 + for one hydrogen atom in methylbenzene. B.TNT has the systematic name 1-methyl-2,4,6- trinitrobenzene. Draw the structural formula of TNT
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 FeCl 3 or AlCl 3 Benzene + 2-chloro-2-methyl propane B.Write a balanced equation using your suggested reactants
SAQ 2.6 A.How does bromine in aqueous solution become sufficiently polar to achieve electrophilic substitution Br Bromine has a symmetrical electron density arrangement In the presence of FeBr 3, the electron density shape changes FeCl 3 + _ _ _