Structure of Benzene

Elemental analysis and molecular-weight determination fix benzene's molecular formula as C6H6, indicating a highly unsaturated compound. A simple straight-chain (or open, non-cyclic) structure was ruled out because benzene does not behave like an alkene or alkyne -- it does not decolourise bromine in CCl4 or acidified KMnO4, and it does not react with water in the presence of acid -- pointing instead …

Two lines of experimental evidence establish that benzene is cyclic. (i) SUBSTITUTION: benzene reacts with bromine in the presence of AlCl3 to give only ONE monobromobenzene product (C6H6 + Br2 --AlCl3--> C6H5Br + HBr); getting only a single monosubstituted product means all six hydrogens in benzene must be identical, which is possible only if the six carbons form a ring, each bearing one hydrogen. (ii) ADDITION OF HYDROGEN: benzene adds exactly THREE moles of H2 over a Raney nickel catalyst to give cyclohexane (C6H6 + 3H2 --Raney Ni--> C6H12) -- confirming both the cyclic ( …

In 1865, August Kekulé proposed that benzene is a cyclic, planar ring of six carbons with ALTERNATING single and double bonds. This structure faced two objections. (i) It predicts TWO different ortho-disubstituted products for a compound like ortho-A2-benzene -- one where the two substituents flank a formal double bond, another where they flank a formal single bond -- but experimentally benzene gives only ONE ortho-disubstituted product. (ii) It cannot explain why benzene, despite having three formal double bonds, fails to give the addition reactions typical of alkenes. To address objection (ii), Kekulé proposed that benzene actually exists as a rapidly interconverting EQUILIBRIUM MIXTURE of the two …

Resonance is the phenomenon where two or more valid structures can be drawn for a substance that has identical atomic positions but different electron-pair placements; the molecule's real structure is a RESONANCE HYBRID of all such contributing structures, not any single one of them. For benzene, Kekulé's two alternating-bond structures (I and II) are the two contributing resonance structures, and the actual molecule (structure III) is their resonance hybrid -- so structures I and II exist only on paper, as a bookkeeping device, while the true structure of benzene is a single, intermediate hybrid that explains both the single type of ortho product and benzene's reluctance to undergo addition. Spectroscopic measurements confirm this picture directly: benzene is planar, and all six of its carbon-carbon bonds are of EQUAL length, 1.40 A -- a value that lies between a t …

The molecular-orbital picture gives the fullest description of benzene's structure. All six carbons are sp2 hybridised: the six sp2 orbitals overlap linearly with six hydrogen 1s orbitals to form six C-H sigma bonds, while the remaining sp2 orbitals on neighbouring carbons overlap with each other to form six C-C sigma bonds, all lying in one plane with 120-degree bond angles. Each carbon additionally has one unhybridised p orbital, perpendicular to this plane, holding one electron; the SIDEWAYS (lateral) overlap of these six parallel p orbitals forms a single, continuous, delocalised pi molecular orbital system spanning all six carbons (conventionally described as three pi bonds' worth of electron density, fully spread around the ring rather than fixed between specific carbon pairs). This full delocalisation makes the pi system unusually stable, which is why benzene undergoes SUBSTITUTION reactions in preference to the ADDITION reactions typical of ordinary alkenes/alkynes under normal conditions. Benzene and its homologues, as a class, are colourless liquids with a pleasant odour, lighter than water and insoluble in it, with vapours that are highly flammable, volatile and toxic. Three equivalent ways of representing benzene's structure are in common use: the expanded (Kekulé) form showing every atom and alternating double bonds explicitly; the shor …