Subject: 1,3,5-cyclohexatriene, REPLY

From: elewars <[email-masked]>
Date: Sun, 21 May 2000 17:59:23 -0400

2000 mAY 21

Hello,

The answer to your question (below), phrased as a general problem, is
not as quite simple as one may think. The conventional response would be
that cyclohexatrine structures are resonance structures (canonical
structures) contributing to the actual structure, a resonance hybrid,
and so can't have any real existence: they are just bond-stretching
vibrational extremes with benzene vibrating about a geometry of mean
(average) hexagonal symmetry (think of a Morse curve). the energy of the
purely hypothetical 1,3,5-cyclohexatriene has been evaluted in
connection with its use as areference molecule in assigning a resonance
energy to benzene (e.g. work by M. J. S. Dewar--treated in some org chem
textbooks). 

 So much for benzene, but for cyclobutadiene the alternating C-C/C=C
structures are real: the potential energy surface has two minima
separated by a transition state. Predicting the results for simple
polyene cases requires, usually, only knowing about aromaticity and
antiaromaticity and the Hueckel rule, but _generally_ distinguishing
between resonance (as in benzene) and valence tautomerism (as in
cyclobutadiene) requires actual calculation. The problem of "the thin
line" (quoting Paquette) between the two was discussedby L. Paquette in
J Am Chem Soc in a paper on homoaromaticity about 10-15 years ago.

E. Lewars
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-------- Original Message --------
Subject: CCL:1,3,5-cyclohexatriene
Date: Thu, 11 May 2000 09:21:17 +0400
From: Valentine Ananikov <[email-masked]>
Organization: IOC
To: CCL_post <[email-masked]>

   Dear List Members,

Does anybody have an information about predicted structure and 
stability for 1,3,5-cyclohexatriene?

I mean a six-membered ring with alternating single and double
C-C bonds, but without aromatic pi-electrons delocalization due
to non-planar conformation (i.e. non-planar benzene isomer).
Would such structure exist in principle?

Thanks!  Valentin.

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