Chemistry > Aromatic Compounds > 1.0 The Structure of Benzene
Aromatic Compounds
1.0 The Structure of Benzene
1.1 A Resonance Picture of Benzene
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
2.0 Electrophilic Aromatic Substitution Reactions
3.0 Nitration
4.0 Sulphonation
5.0 Halogenation
6.0 Friedel-Crafts Alkylation
7.0 Friedel-Crafts Acylation
8.0 Orientation and Reactivity in Electrophilic Aromatic Substitution
8.1 Donation of electrons into a benzene ring by resonance
8.2 Withdrawal of electrons from a benzene ring by resonance
9.0 Ortho / Para Ratio
9.1 Directive influence of the groups during substitutions in benzene ring
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
10.0 Reactions of Alkyl Benzenes
1.2 The Stability of Benzene
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
8.2 Withdrawal of electrons from a benzene ring by resonance
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
We have seen that benzene shows unusual behaviour by undergoing substitution reactions when, on the basis of its Kekulé structure, we should expect it to undergo addition. Benzene is unusual in another sense. It is more stable than the Kekulé structure suggests. To see how, consider the following thermochemical results.
Cyclohexene, a six-membered ring containing one double bond, can be hydrogenated easily to cyclohexane. When the $\Delta {H^0}$ for this reaction is measured it is found to be $-120kJ{\text{ }}mo{l^{-1}},$ very much like that of any similarly substituted alkene.
We would expect that hydrogenation of 1,3-cyclohexadiene would liberate roughly twice as much heat and thus have a $\Delta H^\circ $ equal to about $ - 240kJ{\text{ }}mo{l^{-1}},$ When this experiment is done, the result is $\Delta H^\circ {\text{ }} = - 232{\text{ }}kJ{\text{ }}mo{l^{-1}}.$ This result is quite close to what we calculated, and the difference can be explained by taking into account the fact that compounds containing conjugated double bonds are usually somewhat more stable than those that contain isolated double bonds.
In addition reaction with ${H_2}/Pt$ 1, 3 diene is definitely more reactive than monoene because of more unsaturation. But between two different dienes, conjugated and isolated, conjugate is more stable because of higher stabilisation.
If we extend this kind of thinking, and if benzene is simply 1,3,5-cyclohexatriene, we would predict benzene to liberate approximately $-360{\text{ }}kJ{\text{ }}mo{l^{-1}}\left( {3{\text{ }} \times {\text{ }}-120} \right)$ when it is hydrogenated. When the experiment is actually done the result is surprisingly different. The reaction is exothermic, but only by $ - 208{\text{ }}kJ{\text{ }}mo{l^{-1}}.$
When these results are represented as in figure, it becomes clear that benzene is much more stable than we calculated it to be. Indeed, it is more stable than the hypothetical
1,3,5-cyclohexatriene by $152{\text{ }}kJ{\text{ }}mo{l^{-1}}.$ This difference between the amount of heat actually released and that calculated on the basis of the Kekulé structure is now called the resonance energy of the compound.
