Subject: 2 Questions re TLC

From: [email-masked] (Hr Dr. S. Shapiro)
Date: Mon, 1 Jul 1996 17:21:25 +0200 (MEST)

Dear Colleagues;

     I have recently performed TLC (stationary phase = silica gel) of some
simple o-, m-, and p-monoalkyl- and o-, m-, and p-monoalkylphenols, and have
obtained results that are a bit puzzling.  Perhaps someone can answer one or
both of these questions:

     (i) In the case of alkoxyphenols I find that the o-isomers have much higher
Rf values than the corresponding m- or p-isomers.  This is to be expected if
an intramolecular hydrogen bond exists between the hydroxy H-atom and the
alkoxy -O- atom in the ortho position.  However, the same thing happens with,
e.g., cresols and t-butylphenols.  Since molecular mechanics studies indicate
that the hydroxy H-atom is oriented trans to the ortho substituent in o-cresol
or o-tert-butylphenol, why should the ortho isomers of monoalkylphenols con-
sistently have higher Rf values than their meta or para counterparts?

    (ii) For virtually all of the monoalkylphenol and monoalkoxyphenol compounds
I have examined the Rf values for the meta and para isomers are very close to
each other, but the meta derivative almost always has a _very slightly higher_
Rf value (usually < 0.05 Rf units) than the corresponding para derivative.  Can
anyone suggest a reason for this very slight but apparently real difference in
Rf value between meta and para isomers?

     Of course, any literature references bearng on these two questions are
extremely welcome.

     Thanks in advance to all responders.

Sincerely,

S. Shapiro
ZH
[email-masked]