Stability of Aromatics

Early chemists noticed that benzene and some other aromatics were unusually stable and did not undergo typical reactions as other compounds with π bonds (namely alkene and alkynes).

For example benzene does not undergo electrophilic addition like cyclohexene.

Instead, benzene will undergo a substitution reaction in the presence of a catalyst.  We will learn more about this reaction is a coming chapter.

In addition benzene is not reduced (hydrogenated) with ordinary Pd/C as alkenes are.

However with a more powerful catalysts benzene can indeed be hydrogenated.  As with straight chain alkenes and polyenes we can assess the stability by looking at the heats of hydrogenation of various alkenes and aromatics (Table 1). 

The heat released upon hydrogenation of a mole of benzene (ΔH)  is -49.8 kcal (entry 5).  The negative value indicates the heat was released (exothermic).  Now when cyclohexene (entry 1) which contains only pi bond is hydrogenated, -28.6 kcal/mol of energy was released.  If each of benzenes pi bonds were similar to the pi bonds in cyclohexene, we would have expected 3 x -28.6 =  -85.8 kcal/mol.  This indicates the pi bonds in benzene are more stable than those in ordinary alkenes.  Likewise, the ΔH for 1,3,5-hexatriene is -79.4 kcal/mol versus -49.8 kcal/mol for benzene.  Again benzene is more stable that the hexatriene by about 30 kcal/mol.

1		-28.6 kcal/mol
2		-53.4
3		-53.8
4		-79.4
5		-49.8 kcal./mol