JPJC 2026 Arenes Lecture Notes

2026 JPJC JC1 H2 Chemistry 1 Topic 11: Arenes Jurong Pioneer Junior College 2026 JC1 H2 Chemistry (9476) Topic 11: Arenes Recommended References : 1. Chemistry for advanced level – Peter Cann & Peter Hughes 2. Organic Chemistry – John McMurry Learning Outcomes (L.O) Candidates should be able to: (h) explain in terms of delocalisation of  electrons, the difference between benzene & alkene: (i) reactivity towards electrophiles; (ii) preference of benzene to undergo substitution rather than addition reaction (i) 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 the use of concentrated sulfuric acid as a Brønsted–Lowry acid catalyst) (iii) Friedel–Crafts alkylation with halogenoalkanes (recognise the use of Lewis acid as catalysts) (j) (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 (k) describe the chemistry of the alkyl side –chain of benzene ring as exemplified by the following reactions of methylbenzene: (i) free–radical substitution of chlorine and by bromine (ii) complete oxidation to give benzoic acid (l) predict whether halogenation will occur in the side–chain or aromatic nucleus in arenes depending on reaction conditions (m) apply the knowledge of positions of substitution in the electrophilic substitution reactions of mono– substituted arenes (n) recognise the environmental consequences of: (i) carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the internal combustion engine and of their catalytic removal (ii) gases that contribute to the enhanced greenhouse effect (o) recognise petroleum, a chemical feedstock, is a finite resource & importance of recycling Section Content L.Os Page 1 Introduction to Benzene 1.1 Proposed Structure of Benzene 2 1.2 Actual Structure of Benzene (h) 4 1.3 Nomenclature recall 6 1.4 Physical Properties of Benzene recall 6 2 Chemical Properties of Benzene 2.1 Combustion 7 2.2 (Catalytic) Reduction by H2 7 2.3 Electrophilic Substitution & Mechanism (i), (j) 7 3 Reactions of Monosubstituted Benzene 3.1 Electrophilic Substitution on the Benzene Ring (l), (m) 14 3.2 Inductive versus resonance effect of substituents 17 3.3 Reactions on Side–Chain of Benzene (k), (l), (m) 19 3.4 Identification test for alkylbenzene (k)(ii) 22 4 Environmental Consequences and Recycling (n), (o) 23 Annex A: Electron movement in mechanism Annex B: The Effects of Substituents on Orientation 27 28

2026 JPJC JC1 H2 Chemistry 2 Topic 11: Arenes 1 INTRODUCTION TO BENZENE • Arenes are aromatic hydrocarbons that contain at least 1 benzene ring . The simplest arene is benzene, C6H6. Displayed & structural formula of benzene: • The word ‘aromatic’ was originally applied to natural products with strong aroma. It was later found that these oils have compounds containing a benzene ring. • The term ‘ aromatic’ eventually became associated with the presence of the ring of delocalised  electrons, whether or not the compound had a pleasant aroma. • Due to this aromaticity property which is associated with extra stability of a molecule, benzene undergoes chemical reactions differently from alkenes, even though benzene is also unsaturated. 1.1 Proposed Structure of Benzene Benzene with molecular formula of C6H6, was discovered by chemist Faraday. Initially, the structure for benzene is a six-sided ring with three C=C double bonds as proposed by German chemist August Kekulé. Or Fig 1: Proposed incorrect Kekulé structure of benzene,cyclohexa−1,3,5−triene • But this is NOT the structure of benzene due to the evidence as discussed in the following sections. (a) Reactivity: Benzene does not undergo electrophilic addition • Benzene does not undergo electrophilic addition reactions as alkenes do. If benzene contained 3 C=C double bonds, benzene is predicted to undergo addition reactions readily like alkenes. But, benzene does not. Reagent Benzene Cyclohexene Br2(aq) no reaction immediate decolourisation  electrophilic addition occurred Table 1: Cyclohexene (alkene) undergoes electrophilic addition but not benzene. • Instead of addition reactions, benzene undergoes substitution reactions. (See Section 2.3 for more details) http://tinyurl.com/a renes1 Video on Introduction to Benzene Hydrocarbons Aliphatic Aromatic Alkanes, Alkenes, Alkynes Arenes C C C HC C H C H H H H Alkene: electrophilic addition Arene: electrophilic substitution

2026 JPJC JC1 H2 Chemistry 3 Topic 11: Arenes (b) EQUAL carbon-to-carbon bond length in benzene • X-ray crystallography data shows all the carbon-to-carbon bonds in benzene are of __________________ and _______________ between that of C−C bonds in alkanes and C=C bond in alkenes, i.e. partial double bond character in carbon- carbon bonds in benzene. bond type C−C (in alkanes) C C (in benzene) C=C (in alkenes) bond order 1 1.5 2 bond length / nm 0.154 0.139 0.134 Table 2: C-C bonds in benzene compared to that in alkanes and alkenes (c) Thermochemical data • Hhydrogenation of cyclohexene (with 1 C=C bond) is –120 kJ mol–1. • We would thus expect Δ Hhydrogenation of the proposed Kekulé structure cyclohexa−1,3,5−triene, having 3 C=C bonds, to be 3 times as exothermic at –360 kJ mol–1. • However, Hhydrogenation of benzene is found to be –208 kJ mol–1. This implies the actual structure of benzene is energetically _______________ (lower in energy) than the ‘cyclohexa−1,3,5−triene' structure proposed by Kekulé as shown in Fig 2. (Note: Hhydrogenation is the heat change, under constant pressure, when 1 mole of unsaturated compound was reacted with excess H 2 gas to become fully saturated.) (g) + H2(g) (g) + 2H2(g) (g) + 3H2(g) −120 −232 (~2x) (proposed structure of benzene) (predicted) −360 (~3x) (actual) −208 (< 2x) stabilisation by 152 kJ mol-1 energy / kJ mol-1 Benzene(g) + 3H2(g) Fig 2 : Energy level diagram for the Hhydrogenation of cyclohexenes and benzene.

2026 JPJC JC1 H2 Chemistry 4 Topic 11: Arenes 1.2 Actual Structure of Benzene • The evidences proved that Kekulé structure is incorrect. • The true structure of benzene is commonly represented by the symbol , where the circle represents the delocalised  electrons. • The 6  electrons are not localised in 3 C=C bonds but ________________________ ______________________. Hybridisation: • Each of the 6 C atoms is ________ hybridised so each C atom has three sp2 hybrid orbitals lying on the same plane and at 120 to each other. • Each C has an unhybridised 2p orbital perpendicular to the plane. Fig 3 : Hybridisation of C orbitals 2s 2px 2py 2pz hybridisation sp2 hybrid orbitals 2pz  bonding in benzene: benzene is planar Fig 4: Formation of  bonds in benzene through head−on overlap of orbitals. • Each C atom uses two ______ hybrid orbitals to form ________bonds with 2 other C atoms, by overlapping head-on with the ____hybrid orbitals of two adjacent C atoms. • Each C uses its remaining sp2 hybrid orbital to overlap head-on with 1 s orbital of H, forming 1  (sigma) bond with a H atom. • The geometry around each C atom is ________________ (i.e. the C−C−C and H−C−C bond angles are 120 ), thu