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What are aromatic hydrocarbons?, • The aromatic hydrocarbons are also known as ‘arenes’., • Since most of them possess pleasant odour (Greek; aroma means, pleasant smelling), the class of compounds was named as ‘aromatic, compounds’., • The parent member of the family is benzene having the molecular, formula C6H6. it has hexagonal ring of six carbon atoms with three, double bond in alternate position., •, , Aromatic compounds containing benzene ring are known as, benzenoids and those not containing a benzene ring are known as, non-benzenoids.

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Examples of aromatic hydrocarbons

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Benzenoid Compound, In organic chemistry, benzenoids are a class of, chemical compounds with at least one benzene ring.

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Non-Benzenoid Compound, A compound which exhibits an aromatic behaviour, but does not contain any benzene nucleus

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CONDITIONS FOR AROMATICITY, , The main essential for Aromaticity are:, Delocalisation: the molecule should contain a cyclic cloud, of delocalized πelectron above and below the plane of the, molecule, , Planarity: for the delocalisation of π-electron the ring must, be planar to allow cyclic overlap of p-orbitals. Therefore, for a, molecule to be aromatic, the ring must be planar., , (4n+2)π electron: for Aromaticity, the π-electron could, , must contain a total of (4n+2)π electrons where n is an integer, equal to 0,1,2,3……..n . This is known as Huckel Rule.

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The Criteria for Aromaticity, Four structural criteria must be satisfied for a compound to be, aromatic., [1] A molecule must be cyclic., , To be aromatic, each p orbital must overlap with p orbitals on adjacent, atoms., 5

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[2], , A molecule must be planar., , All adjacent p orbitals must be aligned so that the  electron density, can be delocalized.

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[3], , A molecule must be completely conjugated., , Aromatic compounds must have a p orbital on every, atom., , 6

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[4], , A molecule must satisfy Hückel’s rule, and, contain a particular number of  electrons., , Hückel's rule:, , Benzene is aromatic and especially stable because it contains 6 , electrons. Cyclobutadiene is antiaromatic and especially unstable, because it contains 4  electrons.

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Considering aromaticity, a compound can be, classified in one of three ways:, 1. Aromatic—A cyclic, planar, completely conjugated compound with, 4n +2  electrons., , 2. Antiaromatic—A cyclic, planar, completely conjugated compound, with 4n  electrons., , 3. Not aromatic (nonaromatic)—A compound that lacks one (or more), of the following requirements for aromaticity: being cyclic, planar,, and completely conjugated., , 9

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, , , , All its C-C bonds are the same length: 139 pm —, between single (154 pm) and double (134 pm) bonds, , Electron density in all six C-C bonds is identical

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, , Structure is planar, hexagonal, , , , All C–C–C bond angles are 120°, , , , Each C is sp2-hybridized and has a p orbital perpendicular to the, plane of the six-membered ring

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Evidence in Support of Structure of Benzene, 1.Molecular Formula