Hydrocarbons
15.0 Modern Concept
15.1 Aromaticity in Benzene and Related Systems
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
15.0 Modern Concept
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
Stability of Benzene (Resonance)
Benzene resists addition whereas it readily undergoes substitution reactions, like nitration, halogenation etc. This indicates that benzene is more stable than the hypothetical cyclohexatriene molecule. This has been proved by the fact that the enthalpies of hydrogenation and combustion of benzene are lower than expected. Enthalpy of hydrogenation is the change in enthalpy when one mole of an unsaturated compound is hydrogenated. It has been found experimentally that enthalpy of hydrogenation for disubstituted alkenes, R-CH=CH-R varies between 117-125 kJ $mo{l^{ - 1}}$. Accordingly, the values for cyclohexene and cyclohexa-1, 3-diene and hypothetical cyclohexa-1, 3, 5-triene were calculated compared with their experimental values.
While the experimental values of enthalpy of hydrogenation for cyclohexene is similar to the value expected, the variation in the case of cyclohexa-1, 3-diene is small and is due to delocalization. The expected value of enthalpy of hydrogenation of benzene is much higher than the corresponding calculated value for hypothetical cyclohexa-1, 3, 5-triene indicating that benzene does not have this type of structure.
X-ray studies show that it is planar molecule and that all six C?C bonds in benzene are of equal length (139 pm), intermediate between C?C single bond (154 pm) and C=C (134 pm). All six carbons are $s{p^2}$ hybridized and all bond angles are ${120^o}.$ Benzene is a hybrid of various resonating structures, the two Kekule structures A and B, being the main contributing forms.
