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Orgo 9 - 9 Benzene and Its Derivatives(301-325 334-339...

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9. Benzene and Its Derivatives (301-325, 334-339) Structure of Benzene Benzene is a colourless liquid, with molecular formula C 6 H 6 , and is remarkably unreactive. It doesn’t undergo the addition, oxidation, and reduction reactions characteristic of alkenes. When benzene reacts, it does so by substitution in which a hydrogen atom is replaced by another atom of group of atoms. The term aromatic refers to the fact that benzene and its derivatives are highly unsaturated compounds that are unexpectedly stable toward reagents that react with alkenes. Arene describes aromatic hydrocarbons. Benzene is the parent arene. We call a group derived by the removal of an H from an arene an aryl group and give it the symbol Ar-. Kekule incorrectly believed that the double bonds of benzene rapidly shift back and forth. The carbon skeleton of benzene forms a regular hexagon with C-C-C and H-C-C bond angles of 120 degrees. Carbon uses sp 2 hybrid orbitals, each carbon forms sigma bonds to two adjacent carbons and one sigma bond to hydrogen. All carbon-carbon bonds in benzene are the same length. Each carbon also has a single unhybridized 2p orbital that contains one electron, these 6 2p orbitals lie perpendicular to the plane of the ring and overlap to form a continuous pi cloud encompassing all 6 carbons. The electron density of the pi system of a benzene ring lies in one torus above the plane and one torus below the plane. We represent benzene as a hybrid of two equivalent contributing structures often referred to as Kekule structures. Resonance energy is the difference in energy between resonance hybrid and its most stable hypothetical contributing structure. Compare the heats of hydrogenation of cyclohexene and benzene to estimate resonance energy. In the presence of a transition metal catalyst, hydrogen readily reduces cyclohexene to cyclohexane. Benzene is reduced only very lowly to cyclohexane under these conditions, it is reduced more rapidly when heated and under pressure. The catalytic reduction of an alkane is an exothermic reaction. The difference of 151kJ/mol between the expected value and the experimentally observed value is the resonance energy of benzene. Resonance energy of benzene and other aromatic hydrocarbons has a dramatic effect on their chemical reactivity. What Is Aromaticity?
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