Micellar and Solvent Effects on the Geometrical Isomerism of Hydroxamic Acids and Their Anions
journal contribution
posted on 1997-11-26, 00:00 authored by Andrei Blasko, Clifford A. Bunton, Nicholas D. GillittHydroxamic acids and their anions exhibit geometrical isomerism
due to amide-like resonance and an
increase in the C−N bond order which can be monitored by
<sup>1</sup>H-NMR spectroscopy.
<i>N</i>-Phenylpropiono-
and dodecanohydroxamic acids, <b>1a</b> and <b>2a</b>,
respectively, exist as <i>E</i>-isomers in
DMSO-<i>d</i><sub>6</sub> and CD<sub>3</sub>OD
but
<b>1a</b> is an <i>E</i>−<i>Z</i> mixture in
D<sub>2</sub>O. The corresponding hydroxamate ions, <b>1b</b>
and <b>2b</b>, also exist as <i>E</i>-isomers
in DMSO-<i>d</i><sub>6</sub>, but <i>Z</i>-isomers form on
addition of D<sub>2</sub>O and <i>Z</i>-<b>2b</b> is dominant
with χ<sub>D</sub><sub><sub>2</sub></sub><sub>O</sub> ≥ 0.9.
In CD<sub>3</sub>OD
<i>E</i>- and <i>Z</i>-isomers coexist, but
<i>Z</i>-<b>2b</b> becomes dominant on addition of
D<sub>2</sub>O. The <i>E</i>:<i>Z</i> ratio of
<b>1b</b> is very
similar in cetyltrimethylammonium bromide (CTABr) and in
D<sub>2</sub>O, but only <i>Z</i>-<b>2b</b> is detected in
cationic
CTABr micelles and in anionic, zwitterionic, and nonionic micelles.
Formation of <i>Z</i>-<b>2b</b> in micelles
and
water-rich mixed solvents is favored by hydrophobic interactions
between phenyl and <i>n</i>-alkyl groups,
despite the proximity of anionoid oxygens. In micelles of sodium
dodecyl sulfate the <i>N</i>-phenyl residue of
<b>2b</b> is in a more aqueous region of the micelle−water
interface than in micelles of the other surfactants.
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