For benzene, the total -electron energy is E = 6 + 8 and for 1,3,5-hexatriene it

Question

For benzene, the total -electron energy is E = 6 + 8 and for 1,3,5-hexatriene it is E = 6 + 6.99.

(a) Using Huckel molecular orbital theory, compare the total -electron energy of 1,3,5-hexatriene to the energy of a molecule with 3 localized -bonds (i.e. 3 ethene molecules). What factor leads to the lower energy of 1,3,5-hexatriene as compared to the molecule with 3 localized -bonds? Explain.

(b) Explain why the total -electron energies of benzene and 1,3,5-hexatriene are different even though both have 6 carbons and 6 -electrons. Why does benzene have a lower energy? Explain.  

Explanation / Answer

the extent of delocalization causes less energetic system or more stabilized system.

In 1,3,5-hexatriene the three double bonds are in conjugation. Conjugated double bonds are localized in nature and results to have more -electron energy and eventually produces stable system as compared to the independent 3 localized systems (3 ethene molecules). One -bond causes 2( + ) diminishing in the system energy, 3 independent localized systems causes 6( + ) -bond energy and it is 0.99 less than the 1,3,5-hexatriene -bond energy.

In case of benzene, the delocalization is further more than compare to 1,3,5-hexatriene. According to Huckel (4n+2) e- cyclic planar systems causes more delocalization than compare to acyclic (4n+2) e- conjugated systems. Due to this aspect, benzene aquires more -electron energy than compare to 1,3,5-hexatriene. Hence, benzene is more stable than 1,3,5-hexatriene.   

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