Subject: 2 questions re TLC results

From: "Hr Dr. S. Shapiro" <[email-masked]>
Date: Mon, 1 Jul 1996 16:06:53 +0100 (MET)

Dear Colleagues;

     I have recently been performing TLC (stationary phase, silica gel) of
some simple o-, m-, and p-monoalkyl- and -monoalkoxyphenols, and have obtain-
ed results which I have difficulty rationalising.  Perhaps someone can
answer one or both of these questions for me:

     (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 forms between the hydroxy H-atom
and the alkoxy -O- atom in the ortho position.  However, the same thing ap-
pears to be true for , 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 and o-t-butylphenol, why should the ortho iso-
mers of monoalkylphenols consistently have higher Rf values than their meta
or para counterparts?

    (ii) For virtually all of the monoalkylphenol and monoalkoxyphenol com-
pounds that 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 ap-
parently real difference in Rf value between meta and para isomers?

     Of course, any literatures references bearing on these two questions are
extremely welcome.

     Thanks in advance to all responders.

Sincerely,

S. Shapiro
H
[email-masked]