ASRJC 2024 H2 Chem Arenes

2024 ASRJC JC1 H2 Arenes 2024/AndersonSerangoonJC/Chemistry 1 Anderson Serangoon Junior College JC1 H2 Chemistry ARENES CONTENT 1 Introduction 2 Benzene 3 Reactions of Benzene 4 Reactivity of Mono-substituted Aromatic Rings 5 Methylbenzene 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 Bronsted-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. References 1. A-Level Chemistry by E.N. Ramsden 2. Understanding Chemistry for Advanced Level by Ted Lister and Janet Renshaw 3. Chemistry in Context by Graham Hill and John Holman

2024 ASRJC JC1 H2 Arenes 2024/AndersonSerangoonJC/Chemistry 2 1 Introduction Arenes, also known as aromatic hydrocarbons, are hydrocarbons which are derivatives of benzene, C6H6. The word ‘aromatic’ was initially associated with pleasant smell, but it was later accepted as structures that are stabilised by a ring of delocalised  electrons. The simplest arene is benzene itself. Benzene Naphthalene 2 Benzene 2.1 Properties • A colourless liquid at room temperature with a characteristic smell. • C:H ratio is 1:1. • Burns with a smoky and luminous flame due to its high carbon content. • Non-polar in nature hence is immiscible with water but mix with other hydrocarbons and other non-polar solvents. • Toxic and found to carcinogenic and may harm reproductive organs. 2.2 Structure of Benzene • The benzene is a flat, symmetrical molecule with the shape of a regular hexagon. All C-C-C bond angles are 120o. • Each carbon atom is bonded to a hydrogen atom. • Cloud of __________________________________ lying above and below the ring. Displayed formula Usually represented as (Full) • All the C –C bonds have the same length (intermediate between those expected for a C–C bond and a C=C bond) Note: the ring denotes delocalisation of electrons

2024 ASRJC JC1 H2 Arenes 2024/AndersonSerangoonJC/Chemistry 3 How is the structure formed? (Molecular Orbital Model) represents electron • Each carbon atom has three sp2 orbitals and one p orbital. • The three sp2 orbitals are arranged in trigonal planar shape giving a bond angle of 120o. • The p orbital is perpendicular to the plane of the ring and contains a single electron. • For the formation of σ bonds in benzene, o Each C atom uses two sp 2 hybrid orbitals to __________________________ with the sp2 hybrid orbitals of two adjacent C atoms to form two C–C σ bonds o Each C atom also uses the remaining sp 2 hybrid orbital to overlap head-on with the 1s orbitals of the H atoms to form a C–H σ bonds. • For the formation of π bonds in benzene, o Each unhybridised p orbital overlap equally well with both neighbouring p orbitals above and below the plane of ring, leading to a ________________________. Hence, benzene has two doughnut-shaped clouds of electrons, one above and one below the ring. • The six electrons in the  electron cloud are free to move throughout the system. So the se electrons are said to be delocalised. • This delocalisation gives benzene its extra _______________. Resonance in benzene Due to the delocalisation of  electrons, benzene is said to have resonance. The concept of resonance is explained in Appendix A1 on page 19 of your lecture notes. Though you do not need to describe what resonance is, you will need to identify molecules that can exhibit resonance as it affects chemical reactivity of compounds. What molecules can exhibit resonance? • Resonance exists in molecules with extensive  bonding. They have more than two consecutive atoms with unhydridised p orbitals orientated in the same direction. • A typical  bond such as in C=C in ethene is formed by the sideways overlap of the p orbitals of two carbon atoms. For molecules that show resonance, the  bonding can be across multiple consecutive atoms in the structure (e.g. across six C atoms in benzene). • Their p orbitals merge into one large overlapping region and electrons from these orbitals spread out over the entire overlapping region (i.e. they do not belong to any one specific atom). Hence, they are delocalised. • The electron density is spread out, rather than being concentrated between two atoms as in a typical C=C. This leads to stabilisation of the molecule and there are experimental evidences of such stabilisation as explained in Section 2.3.

2024 ASRJC JC1 H2 Arenes 2024/AndersonSerangoonJC/Chemistry 4 2.3 Experimental Evidence of Resonance in Benzene Benzene has three experimental evidence that shows resonance in benzene: 1. All carbon-carbon bonds in benzene are equal and intermediate in length between single and double carbon-carbon bonds. 2. The enthalpy change of hydrogenation of benzene is less exothermic than expected. 3. Benzene undergoes substitution reactions rather than addition reactions. 2.3.1 Carbon-carbon bond lengths • X-ray diffraction studies show that all the carbon -carbon bonds in benzene are ____________ and __________ in length. Compound Bond Bond length / nm cyclohexane CH4 CH4 0.154 cyclohexene CH2 CH2 0.133 benzene CH3 CH3 0.139 • Carbon-carbon bond in benzene have ____________________________________________. 2.3.2 Enthalpy Change of Hydrogenation of Benzene • The actual structure of benzene is ____________________________________________ than what is expected of hypothetical cyclohexa-1,3,5-triene structure . This can be shown using enthalpy changes of hydrogenation. • When hydrogen is added to cyclohexene, cyclohexane, C 6H12, is formed. The enthalpy change during this reaction is –120 kJ mol–1. In other words, when 1 mole of cyclohexene reacts, 120 kJ of heat energy is evolved (to saturate one C=C). + H2 • For both cyclohexa-1,4-diene and cylcohexa -1,3-diene, you would expect the enthalpy change of hydrogenation of both C=C to be –240 kJ mol –1 (twice th