VJC 2024 Arenes Lesson Tutorial

23 5 Summary (a) Mechanism Similarities and differences between alkene and benzene Like alkenes, benzene possesses high π electron density and is hence attacked by electrophiles. However, benzene differs from alkenes in that: (1) delocalisation of π electrons renders the benzene ring less nucleophilic compared to the C=C bond in alkenes. Hence, benzene can only be attacked by strong electrophiles. (2) benzene undergoes substitution instead of addition to preserve its resonance stabilised ring system. Hence, typical reaction of benzene is electrophilic substitution. Step 1: Halogenation / Friedel-Crafts alkylation Catalyst acts as a Lewis acid by accepting a lone pair of electrons from Cl2 / RCl, thereby generating a strong electrophile, Cl+ / R+. Cl2 + FeCl3 → Cl+ + FeCl4– RCl + AlCl3 → R+ + AlCl4– Nitration H2SO4 catalyst acts as a Br ønsted-Lowry acid by donating a proton which leads to generation of the strong electrophile, NO2+. 2H2SO4 + HNO3 → NO2+ + H3O+ + 2HSO4– Step 2: + Cl+ + H Cl + R+ + H R + NO2 + + H NO2 Step 3: Cl + H Cl + + HCl + FeCl3FeCl4 - R + H R + + HCl + AlCl3AlCl4 - NO2 + H NO2 + + H2SO4HSO4 -

24 (b) Effect of substituents on benzene ring reactions (given in the Data Booklet) 2, 4–directing 3–directing activating deactivating deactivating –R (alkyl group) –Cl –CHO, –COR –OH or –OR (highly activating) –Br –CO2H, –CO2R –NH2, –NHR or –NR2 (highly activating) –I –NH3+ –NHCOR –NO2, –CN (c) Complete the following tables. • Reactions common to both benzene and methylbenzene Type of reaction Reagents and conditions Products for benzene methylbenzene Cl2, anhydrous FeCl3 catalyst Cl CH3CH2Br, anhydrous AlBr3 catalyst, excess benzene / methylbenzene (for monosubstitution) Concentrated HNO3, concentrated H2SO4 catalyst, 55 oC for benzene / 30 oC for methylbenzene • Reactions for alkylbenzene only (side-chain reactions) Type of reaction Reagents and conditions Reactant Products Br2, uv light or heat / high temperature (excess organic compound for monosubstitution) CH3

25 Type of reaction Reagents and conditions Reactant Products KMnO4(aq), H2SO4(aq), heat Note: K2Cr2O7(aq), H2SO4(aq), heat cannot affect this reaction CH3 Side chain with benzylic H but not alkene: CH(CH3)2 CH2CH2OH CHClCH2CH3 Side chain with no benzylic H: C CH2CH3 CH3 CH3 Side chain with alkene: CH2CH=CHCH2CH3 Side chain with alkene: CH2CH C CH3 CH3 N.B. For side chain with C=C bond, consider oxidative cleavage of alkene followed by oxidation to benzoic acid due to presence of benzylic hydrogen. Note: Alkaline KMnO 4 gives the salt of carboxylic acid which must be first acidified before solid benzoic acid can be obtained. CH2CH2OH CO2 heat KMnO4(aq), OH (aq) H+ (aq) CO2H

26 Victoria Junior College JC2 H2 Chemistry Arenes (Aromatic Hydrocarbon) Tutorial Nomenclature 1 Write the structural formulae for the following benzene derivatives. (a) 2,4,6-trinitrophenylamine (b) 2-hydroxybenzoic acid 2 Name the following compounds. (a) CHO Br (b) OHCl Electrophilic Substitution 3 (a) Nitrobenzene is prepared by reacting benzene with a mixture of concentrated sulfuric acid and concentrated nitric acid. The electrophile in this reaction is the NO2+ cation. (i) Explain the meaning of the term electrophile. (ii) Write an equation to show how NO 2+ is formed from the reagents used in the reaction. (iii) Describe the mechanism for the formation of nitrobenzene from benzene. (iv) Explain why benzene undergoes substitution reactions rather than addition reactions. (b) (i) Iodine monochloride, ICl, can be prepared by passing chlorine gas over iodine crystals and it is collected as a dark brown liquid. Draw the structure of the likely organic product of the reaction of I Cl with benzene in the presence of a Lewis acid catalyst. Explain your answer. (ii) Explain why benzene reacts with I Cl in the presence of a Lewis acid catalyst unlike an alkene which reacts with ICl in the absence of a catalyst. 4 Which statement is true regarding the formation of chlorobenzene from benzene and chlorine? A A catalyst is not required for the reaction to take place. B The six carbon atoms in benzene remain sp 2 hybridised throughout the reaction. C The relative molecular mass of the organic product is 35.5 greater than that of benzene. D The reaction involves an intermediate which contains four delocalised electrons.

27 5 Consider the followi ng electrophilic substitution reaction, in which a compound of beryllium, BeCl2, is involved. O BeCl2 OCH2CH2Cl (a) BeCl2 has two roles during this reaction. State and describe the roles of BeCl2 during the reaction. (b) Describe the mechanism of the reaction. You should include curly arrows indicating the movement of electrons, and all charges in your answer. Integrated Questions 6 N1992/IV/22 (modified) The reaction between boiling methylbenzene and chlorine takes place in a number of steps to give several products. Which of the following could be one of the steps? A CHCl + Cl2 CHCl2 + Cl B CH2 + HCl CH2Cl + H C CH3 + CH3 + H+ ClH Cl D CH3 + Cl+ CH3 + H Cl 7 N 1997/I/6 Methylbenzene can be used as an additive in unleaded petrol. Some chemical transformations of methylbenzene are given below. CH3 CH3 Br CH2Br CO2H I II III A

28 (a) Suggest reagents and conditions for each of the reactions I, II and III, and describe as fully as you can the type of reaction undergone in each case. (b) There are two other positional isomers of A. Draw their structural formulae, and suggest which one is more likely to be formed along with A in reaction III. 8 Compound A has the molecular formula C 10H12. When A was treated with bromine in CC l4 in the absence of light, compound B with the molecular formula C 10H12Br2 was formed. A can also be heated under reflux with acidified KMnO4 to produce compound C as shown below. Deduce possible structures for compounds A and B. Explain the types of reaction involved and the functional groups present. Effect of Substituent Groups 9 De vise syntheses of the following compounds from the given starting material. Your proposed syntheses should allow as high a yield of the products as possible. Identify the reagents and conditions at each step, and draw the structural formulae of the intermediates. (a) Br NO2 (2-step synthesis) (b) CO2HCH3 Cl O2N (3-step synthesis) CO2H CH3 O C (C 9H8O3)

29 Victoria Junior College Arenes (Aromatic Hydrocarbon) Supplementary Questions Electrophilic Substitution 1 N 1998/III/22 Which compound can be obtained by reacting a hydrocarbon with bromine in the presence of a halogen carrier? A CH2=CHCH2Br B CH3CHBrCH3 C CH2Br D CH2CH3 Br 2 N 1999/III/23 (modified) What is the correct set of conditions for the conversion of benzene into nitrobenzene? acid temperature A dilute HNO3 100 oC B concentrated HNO3 50 oC C concentrated HNO3 and concentrated H2SO4 50 oC D concentrated HNO3 and concentrated H2SO4 120 oC 3 During the nitration of benzene, a nitro group substitutes at a carbon atom. What is the arrangement of the bonds at this carbon atom during the reaction? at the start of the reaction in the intermediate complex at the end of the reaction A planar planar planar B planar tetrahedral planar C planar tetrahedral tetrahedral D tetrahedral tetrahedral tetrahedral 4 N2015/II/5 (a) Compound Y has a molecular formula of C 6H3Cl3. Suggest three possible structures for Y . State the name of each structure you have drawn. (b) Mononitration of compound Y produces just one compound rather than a mixture of isomers. Use this additional information to identify which