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  Benzene Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Benzene

By organic chemists the adjective "aromatic" is used in a rather distinct way than it is generally applied. It has its origin in the observation that definite natural substances, like wintergreen leaves, cinnamon bark, anise seeds and vanilla beans, contained fragrant compounds having general but properties that can't expected. Cinnamon bark, for an instance, yielded a pleasant smelling compound, C9H8O is the formula, named as cinnamaldehyde. Due to the low hydrogen to carbon ratio in this and other aromatic compounds (note: the H:C ratio in an alkane is >2), chemists supposed their structural formulas would consist of a large number of double or triple bonds. Because double bonds are simply cleaved through oxidative reagents like ozone or potassium permanganate, and quickly add chlorine and bromine, to these aromatic compounds these reactions were applied. Unpredictably, products that come into existence to retain many of the double bonds were obtained and these compounds as compared with known alkenes and cycloalkenes, exhibited a high degree of chemical stability.On treatment with hot permanganate solution, cinnamaldehyde gave a crystalline, stable C7H6O2 compound that is known as benzoic acid. In benzoic acid the H:C ratio is <1, again suggesting the existence of various double bonds. Benzoic acid was eventually converted to the stable hydrocarbon benzene, C6H6, to common double bond transformations which also proved unreactive, as shown below in the diagram. In contrast, cyclohexene's reactions, a typical alkene, with these reagents are also displayed in the green box (in the diagram). Like experimental evidence for a wide assortment of compounds was acquired, those incorporating this exceptionally stable six-carbon core came to be called "aromatic".

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By addition of a catalyst and/or increasing the temperature if the benzene is forced to react. It goes through the substitution reactions rather than the addition reactions which are usual of alkenes. This further confirms previous indication that to chemical modification the six-carbon benzene core is unusually stable. Conceptual contradiction that is presented through a high degree of unsaturation (low H:C ratio) and high chemical stability for the benzene and related compounds stay an unsolved puzzle for so many years. The currently structure that is accepted of a regular-hexagonal, the planar ring of carbons was adopted, and exceptional thermodynamic and chemical stability of this system was attributed to resonance stabilization of a conjugated cyclic triene.

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For a stable compound here, two energetically and structurally equal electronic structures are written but no single structure provides a correct or even an adequate demonstration of the true molecule. In the benzene, six-membered ring is a perfect hexagon (all carbon-carbon bonds have an identical length of 1.40 Å). To show alternating bond lengths cyclohexatriene contributors expected, single bonds (1.54 Å) is being longer than double bonds (1.34 Å). Another representation for benzene (circle within a hexagon) gives emphasis to the pi-electron delocalization in this molecule, and has benefits of being a single diagram. In examples such as these, the electron delocalization illustrated by resonance enhances the stability of the molecules and compounds composed of this type of molecules often show exceptional stability and related properties.

Evidence intended for improved thermodynamic stability of the benzene was acquired from measurements of heat released when double bonds in a six-carbon ring are hydrogenated (hydrogen is added catalytically) to give cyclohexane like a common product. Cyclohexane appear as a low-energy reference point in the picture. Addition of the hydrogen to cyclohexene generates cyclohexane and releases heat amounting to 28.6 kcal per mole. To represent energy cost of introducing one double bond into a six-carbon ring, if we take this value, we would expect the cyclohexadiene to release the 57.2 kcal per mole on complete hydrogenation and the 1,3,5-cyclohexatriene to release 85.8 kcal per mole. These heats of the hydrogenation would reflect relative thermodynamic stability of the compounds. Practically, the 1,3-cyclohexadiene is little more stable than supposed by about 2 kcal, presumably due to conjugation of the double bonds. Though, The Benzene is an extraordinary 36 kcal/mole more stable than expected. As features of all aromatic compounds this type of stability improvement is now accepted.

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The benzene's molecular orbital description provides more usual and satisfying treatment of "aromaticity". This is very well known that the benzene has a planar hexagonal structure where all carbon atoms are sp2 hybridized and all carbon-carbon bonds are the same in length. As displayed in the below diagram, to generate six molecular orbitals, the remaining cyclic array of the six p-orbitals ( one on each carbon) overlap, three antibonding and three bonding. The signs of Plus and minus displayed in the diagram do not stand for electrostatic charge, but refer to the phase signs in the equations that explain these orbitals (in the picture the phases are also color coded). When phases correspond, the orbitals overlap to generate a common region of like phase with those orbitals having the greatest overlap (for example π1) being lowest in energy. Remaining carbon valence electrons then occupy these molecular orbitals in pairs, resulting in a fully occupied set of bonding molecular orbitals. It is this entirely filled set of the closed shell or bonding orbitals that gives benzene ring its thermodynamic and chemical stability, just as the filled valence shell octet confers stability on the inert gases.

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