benefit from the fluorine gauche effect. On the other hand, the
extended zig-zag conformation 5a of (±)-threo 5 has the vicinal
fluorine atoms gauche to each other (Fig. 3). As a consequence
it was anticipated that the threo stereoisomer 5 would be the
conformationally more stable system of the two.
the Langmuir isotherm for the erythro isomer 4 (Fig. 4a) is
extremely expanded indicating a significant level of conforma-
tional disorder prior to attainment of a condensed monolayer
with a limiting area per molecule (i.e. extrapolated to zero
surface pressure) of ca. 0.20 ± 0.1 nm2 molecule21 13
. However,
the isotherm for the threo isomer 5 (Fig. 4b) is similar to that for
stearic acid (data not shown) with a limiting surface area per
molecule also of ca. 0.20 ± 0.1 nm2 molecule21. Clearly in 5 the
presence of the fluorine atoms does not significantly perturb the
isotherm relative to the hydrocarbon model. Surprisingly the
different behaviour of 4 and 5 arises as a consequence of a
single stereochemical inversion of one C–F bond. In 4 the
fluorine gauche effect is competing with the classical anti-zig-
zag preference of the R groups leading to considerable
conformational disorder. It is well known from rotational
energy profiles of butane that the anti conformer is ~ 0.6 kcal
mol21 more stable than the gauche conformer in solution.14
However in this case the fluorine gauche effect contributes upto
0.9 kcal mol21 stabilisation to conformer 4b and thus the
opposing effects result in conformers 4a and 4b becoming
closer in energy and thus more equally populated. The increased
chain disorder in 4 accounts for the lower melting point and the
expanded isotherm shown in Fig 4a.
Fig. 3 Staggered rotamers of (±)-erythro and (±)-threo 9,10-difluorostearic
acids 4 and 5.
Indeed a competition was anticipated between the anti-zig-
zag conformer of 4a and the gauche–gauche conformer 4b. If
the fluorine atoms achieve a gauche relationship this will
necessarily result in chain disorder as the carbon chains (R1 and
R2) must adopt a gauche relationship. It was not clear whether
the gauche effect would be sufficient to over-ride the classical
anti zig-zag preference in these stearic acids. In order to test this
each of the difluorostearic acids 4 and 5 was deposited from a
solution in chloroform (conc. approx. 0.5 g l21) onto the surface
of ultrapure water (pH 5.8 ± 0.2, temperature 20 ± 2 °C) in a
Langmuir trough (Molecular Photonics LB700) and surface
pressures (mN m21) versus area per molecule (nm2 mole-
cule21) measured. The compression rate was about 1 3 1022
nm2 molecule21 s21. Surface pressure versus area analysis of
selectively fluorinated stearic acids has already been used as a
sensitive method to assess their conformational mobility on a
water subphase.11,12 The resultant Langmuir isotherms of
(±)-erythro 4 and (±)-threo 5 are shown in Fig. 4. The shape of
In conclusion the study illustrates that the fluorine gauche
effect is of a significant magnitude that it can influence the
conformational stability of extended hydrocarbon chains, a
property which could be used to design mobility into hydro-
carbon chains e.g. in membrane models and liquid crystalline
materials.
We thank EPSRC and CNRS for the award of a collaborative
CERC-3 research grant.
Notes and references
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13 M. C. Petty, Langmuir-Blodgett Films, Cambridge University Press,
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Fig. 4 Langmuir isotherms of (±)-erythro-4 (a) and (±)-threo-5 (b) on a
water subphase showing condensed pressure versus area curves at 20 °C.
14 E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds,
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