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Kekulé Structures

Posted: Tue Jan 29, 2019 1:37 pm
by Camille Marangi 2E
So 4E.9 from 7th edition reads as follows:

Benzene is more stable and less reactive than would be predicted from its Kekulé structures. Use the data in Table 4E.3 to calculate the lowering in molar energy when resonance is allowed between the Kekulé structures of benzene.

Am I supposed to assume a kekulé stucture would just be benzene without resonance? Not sure what to make of this so any insight would greatly help. Thanks!

Re: Kekulé Structures

Posted: Tue Jan 29, 2019 10:26 pm
by Tam To 1B
For this problem, they are just asking you to compare the differences between the bond enthalpies of benzene.
In one structure, you have benzene with 3 C-C and 3 C=C bonds. Using the table, this would mean (3*348) + (3*612) = 2880 kJ.
However, benzene in its resonance structure has 6 C-C/C=C bonds (indicated with dots above a line) on the table). Using the table values, this would be 6*518 = 3108 kJ.
The difference between these values gives you 228 kJ.