We have studied aromaticity in detail and are acquainted with various compounds that are aromatic. A variety of physical properties have been used as a criteria to determine presence or absence of aromatic character in a compound. Let us study these parameters , which can be studied experimentally.
We shall consider four types of data –
- Energy data – Thermochemical stability where we study the delocalization energy and heats of hydrogenation.
- Structural information – Analyzing bond lengths in molecules.
- Magnetic properties – Ring currents, NMR spectra ,magnetic suspectibility.
- Other parameters – Hardness/HOMO-LUMO gap.
Criteria for Aromaticity.
The different criteria for aromaticity are –
1] Energy data / Thermodynamic stability.
All aromatic compounds are thermodynamically stable.The thermodynamic stability can be studied by –
I)Delocalization energy OR
II)Heat of hydrogenation.
I)Delocalisation energy – The stabilization of aromatic compounds in terms of simple HMO theory is based on the delocalization energy. As studied in post 110, the total π-electron energy of a molecule is expressed in terms of α and β .We compare the energy of the actual molecule to a hypothetical version with no delocalization and see the difference.
The HMO energy for benzene = 6α+8β.
HMO energy for hypothetical cyclohexatriene (with no delocalization) = 6α+6β.
Difference = 2β⇒ Delocalisation energy.
However, this is NOT a measurable physical quantity as we are using a hypothetical molecule for comparison with the real one.
An alternative way is to compare stabilization energies of the aromatic compound with a linear conjugated polyene containing the same number of double bonds.
e.g. We can compare benzene with 1,3,5-hexatriene.
Benzene is more stable than 1,3,5-Hexatriene by 20kcal/mol.
II)Heat of hydrogenation– Another way of finding out the thermodynamic stability of aromatic compounds is by studying the heats of hydrogenation(ΔH).
We have already studied how benzene is thermochemically stable using the heat of hydrogenation calculations, in post 107.
The heats of hydrogenation(ΔH) of cyclohexene,1,3-cyclohexadiene and benzene are experimentally found. If 28.6 kcal/mol energy is released after hydrogenating one double bond then, the energy released after hydrogenation of three double bonds in benzene must be 28.6 × 3 = 85.8 kcal/mol. However, as seen above the experimental value of ΔH for benzene is 49.8kcal/mol. So, benzene has very less energy than expected and thus it is very stable. It is stable by 85.8-49.8 = 36 kcal/mol.
Less energy means more stability.
In marked contrast to benzene, heat of hydrogenation of cyclooctatetraene to cyclooctane is -98 kcal/mol and that of cyclooctene is -23kcal/mol. The negative sign of ΔH indicates that the compound is less stable.cyclohexene,1,3-cyclohexadiene and benzene are experimentally found. If 28.6 kcal/mol energy is released after hydrogenating one double bond then, the energy released after hydrogenation of three double bonds in benzene must be 28.6 × 3 = 85.8 kcal/mol. However, as seen above the experimental value of ΔH for benzene is 49.8kcal/mol. So, benzene has very less energy than expected and thus it is very stable. It is stable by 85.8-49.8 = 36 kcal/mol.
If we calculate, -23 × 4 = -92 kcal / mol , which is nearly equal to -98kcal/mol. So, cyclooctatetraene exhibits no characteristic stabilisation when compared to hypothetical cyclic polyene i.e it is NOT AROMATIC.
2] Structural properties.
The bond length in benzene is 1.39 Å intermediate between single and double bonds . However, cyclobutadiene has a rectangular shape i.e all the bond lengths are not same. This contrasting behavior suggests that bond length alternation can be used as a criteria to determine aromaticity of compounds.
C-C bond length = 1.54Å
C=C bond length = 1.33Å
C=C bond length in benzene = 1.39Å ⇒ Intermediate bond length.
This indicates resonance between conjugated double bonds of benzene. The X- ray studies, IR spectroscopy data and electron-diffraction photographs of benzene vapour have clearly suggested all C-C bonds are symmetrical in nature and benzene is a perfectly hexagonal molecule.
This character of compounds, where carbon atoms are in a triagonal state, in a planar ring (without any ring strain), where there is uniformity in bond lengths , is a unique feature confirming aromaticity.
In the next post we will study some more parameters which are characteristic of aromatic compounds. Till then,
Be a perpetual student of life and keep learning…..
Good Day !