VJC 2024 Arenes Lesson Student Notes

1 VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT Mrs Mah Ai Ling / poh.ailing@vjc.edu.sg JC1 H2 CHEMISTRY ARENES (AROMATIC HYDROCARBON) Lecture 1 2 Pages 1 – 11 12 – 21 Complete by T3W10 T3W10 Tutorial questions 1 – 5 6 – 9 ______________________________________________________________________________ LECTURE OUTLINE 1 Introduction 1.1 Structure of Benzene 1.2 Nomenclature 2 Physical Properties of Arenes 3 Benzene 3.1 Preparation 3.2 Electrophilic Substitution Reactions 3.2.1 Halogenation 3.2.2 Nitration 3.2.3 Friedel-Crafts Alkylation 3.3 Addition of Hydrogen Reaction (Reduction) 4 Methylbenzene 4.1 Structure of Methylbenzene 4.2 Preparation 4.3 Reactions 4.3.1 Effect of Substituent Groups 4.3.2 Electrophilic Substitution Reactions (similar to benzene) 4.3.3 Addition of Hydrogen Reaction (Reduction) (similar to benzene) 4.3.4 Free Radical Substitution Reaction (side-chain reaction similar to alkanes) 4.3.5 Oxidation Reaction (side-chain reaction) 5 Summary REFERENCES 1 Modern Organic Chemistry by R.O.C. Norman and D.J.Waddington 2 Principles of Organic Chemistry by Peter R.S. Murray 3 A-Level Chemistry by E.N. Ramsden 4 Chemistry in Context by Hill and Holman

2 LEARNING OUTCOMES Candidates should be able to: (a) explain, in terms of delocalisation of π electrons, the difference between benzene and alkene: (i) reactivity towards electrophiles (ii) preference of benzene to undergo substitution rather than addition reaction (b) describe the chemistry of the benzene ring as exemplified by the following reactions of benzene and methylbenzene: (i) electrophilic substitution reactions with chlorine and with bromine (recognise the use of Lewis acid as catalysts) (ii) nitration with concentrated nitric acid (recognise concentrated sulfuric acid as a Br ønsted- Lowry acid catalyst) (iii) Friedel-Crafts alkylation with halogenoalkanes (recognise the use of Lewis acid as catalysts) (c) (i) describe the mechanism of electrophilic substitution in arenes, using the mono - bromination of benzene as an example (ii) describe the effect of the delocalisation of electrons in arenes in such reactions (d) describe the chemistry of the alkyl side- chain of benzene ring as exemplified by the following reactions of methylbenzene: (i) free-radical substitution by chlorine and by bromine (ii) complete oxidation to give benzoic acid (e) predict whether halogenation will occur in the side- chain or aromatic nucleus in arenes depending on reaction conditions (f) apply the knowledge of positions of substitution in the electrophilic substitution reactions of mono-substituted arenes

3 1 Introduction 1.1 Structure of Benzene • The molecular formula of benzene is C6H6. • The six carbon atoms of benzene are arranged in a hexagonal ring. Each carbon atom of the benzene molecule is sp 2 hybridised and uses two of the three sp 2 hybrid orbitals to form σ bonds with the sp 2 hybrid orbitals of the adjacent carbon atoms. The remaining sp 2 hybrid orbital overlaps with the 1s orbital of hydrogen atom to form a C–H σ bond. • Each carbon atom in benzene has an unhybridised 2p orbital that is perpendicular to the hexagonal plane of carbon atoms. Since these six 2p orbitals are parallel to one another, each of the 2p orbital can overlap sideways and equally with the adjacent two 2p orbitals to form π bonds. Each of the six 2p unhybridised orbital contains an electron. Hence, the sideways overlap of the 2p orbitals results in a doughnut-shaped delocalised π electron cloud above and below the hexagonal plane of carbon atoms. • Due to delocalisation of the π electrons, benzene molecule exists as a resonance hybrid of the resonance structures (I) and (II). Note: resonance structures (I) and (II) do not exist. The structural formula of benzene is shown below. The hexagon represents the six carbon atoms arranged in a hexagonal ring and the circle represents the delocalised six π electrons. resonance structures of benzene resonance hybrid of benzene (I) (II) • The delocalisation of the π electrons conferred extra stability to the benzene structure which is said to be resonance–stabilised. doughnut-shaped delocalised π electron cloud pi bond formation

4 • The following evidences show that benzene exists as a resonance hybrid. (a) Benzene has the same carbon-carbon bond lengths of 0.139 nm which are intermediate in length between C– C bond and C=C bond. Hence, carbon–carbon bond in benzene is stronger than C–C bond but weaker than C=C bond. bond bond length / nm C–C 0.154 C=C 0.132 (b) Heat of hydrogenation (i.e. heat evolved when 1 mol of unsaturated compound is hydrogenated) of benzene has a lower magnitude than expected. E.g. Compare the heat of hydrogenation of the following molecules. cyclohexene cyclohexatriene benzene (non-existent) The product formed after hydrogenation of the above molecules is cyclohexane, . Energy level diagram: Energy / kJ mol–1 (l) + 3H2(g) (l) + 3H2(g) (l) + H 2(g) (l) Benzene evolves 151 kJ mol –1 less energy than predicted. Hence, benzene is more stable by 151 kJ mol –1 than we would have expected cyclohexatriene to be. Thus, benzene is resonance–stabilised with resonance energy = 151 kJ mol–1. (c) Unlike alkenes, b enzene undergoes substitution reactions readily rather than addition reactions. This is to preserve its resonance– stabilised ring structure as addition reactions would destroy it. E.g. Unlike alkenes, benzene does not decolourise orange-red liquid bromine. Instead, it undergoes substitution reaction with bromine in the presence of anhydrous FeBr 3 catalyst. + Br2 anhydrous FeBr3 catalyst Br + HBr –120 (observed) –209 (observed) resonance energy = 151 kJ mol–1 3(–120) = –360 (calculated) ring preserved catalyst required

5 Exercise Which of the following could be a result of the delocalised π electron cloud in the benzene molecule? (You may choose more than one correct answer.) 1 B enzene is a good conductor of electricity. 2 Benzene prefers to undergo substitution rather than addition reaction. 3 All the carbon-to-carbon bonds are equal in length. 1.2 Nomenclature • P henyl group, C6H5–, is where a hydrogen atom is removed from benzene. • For monosubstituted benzene, (a) if benzene is regarded as a substituent, the name of the compound will have the prefix “phenyl”. NH2 CH=CH2 CH2CH2OH CHO E.g. phenylamine phenylethene 2- phenylethanol phenylmethanal ( b) if the compound is regarded as a derivative of benzene, the name of the substituent appears as the prefix of “benzene”. NO2 Cl Br CH3 E.g. nitrobenzene chlorobenzene bromobenzene methylbenzene (c) there are some special names such as phenol, benzoic acid and benzaldehyde. OH CO2H CHO phenol benzoic acid benzaldehyde (= phenylmethanal) • For two or more substituents attached to the benzene ring, the c arbon atoms in the ring are numbered from 1 to 6, beginning with the carbon atom that is bonded to the first substituent and continuing in such a direction as to lead to the lowest numbers for the rest of the substituents. OH Br CH3 CH3 NH2 Cl E.g. 2- bromophenol 1,2-dimethylbenzene 3-chlorophenylamine NO2 BrCl CH3 NO2O2N 1- bromo-3-chloro-5-nitrobenzene 2- methyl-1,3-dinitrobenzene

6 Exercise Draw the structural formula for each of the following compounds. (a) 4-hydroxybenzoic acid (b) 2-phenyloctane You are now ready to attempt Tutorial Questions 1 and 2. 2 Physical Properties of Arenes • Arenes are l iquids or low melting point solids with characteristic ‘aromatic ’ odours. Their vapours are toxic and should avoid inhaling them. N