6
40
R. A. MORE O’FERRALL AND D. O’BRIEN
5
acetate and hydroxylamine buffers may, however, have
arisen from this source.
kinetic measurements were initiated by injection of
20 ml of stock solution into 2 ml of aqueous perchloric
acid or chloroacetic acid buffer contained in the thermo-
statted cell compartment of a spectrophotometer. Rate
constants for hydrolysis of the oxime were monitored
from the limiting rate of increase in absorbance of the p-
methoxybenzaldehyde product at 285 nm (ꢀmax). Rate
constants for isomerization were usually measured start-
ing with the E-isomer because in the acid concentration
range studied the Z-isomer (and usually the E-isomer)
was fully protonated and the protonated Z-isomer was the
most stable species. Because the spectra for the isomers-
were similar, it was important that measurements of rates
of isomerization should maximize the change in absor-
bance by starting with the less stable form.
Despite the complexity of the above analysis, we
believe that the present paper clarifies the influence of
E–Z isomerization on the hydrolysis (and formation) of
oximes and offers a straightforward guide to analysing
when and how the existence of isomers may be taken into
account in the interpretation of existing or new kinetic
and equilibrium measurements.
EXPERIMENTAL
Spectrophotometric measurements made use of Phillips
PU8600, Perkin-Elmer Hitachi 124 and Genesys 2PC
UV–visible spectrophotometers. Measurements of pH
were made with a Metrohm 744 pH meter and NMR
spectra were recorded on a Jeol JNM-GX270 instrument.
The (E)- and (Z)-p-methoxybenzaldehyde oximes were
prepared as described previously and were recrystallized
Equilibration of (E)- and (Z)-oximes was carried out
over 8 days in CDCl . The relative concentrations of the
two isomers was determined from the intensities of the
3
—
CH—N proton peaks as 8:1 in favour of the E-isomer. It
was assumed that this equilibrium was not significantly
affected by changing the solvent from chloroform to water.
ꢃ
from petroleum spirit (b.p. 40–60 C) to give white,
needle-like crystals. They showed the characteristically
different chemical shifts for the CH hydrogen of the
oxime group, ꢂ 8.13 for the E-isomer and ꢂ 7.26 for the
Z-isomer.
REFERENCES
Values of the pK s for the two isomers were based on
a
1
. Jencks WP. Prog. Phys. Org. Chem. 1964; 2: 63–128; Jencks WP.
Catalysis in Chemistry and Enzymology. McGraw-Hill: New
York, 1969.
. Reiman JE, Jencks WP. J. Am. Chem. Soc. 1966; 86: 3973–3982;
Jencks WP. J. Am. Chem. Soc. 1959; 81: 475–481.
. Rosenberg S, Silver SM, Sayer JM, Jencks WP. J. Am. Chem. Soc.
1974; 96: 7986–7998.
UV absorbance measurements at ꢀmax ¼ 305 nm for the
E-isomer and ꢀmax ¼ 302 nm for the Z-isomer. The ex-
tinction coefficients at these wavelengths were found to
2
3
4
4
be the same, with " ¼ 1.6 ꢁ 10 . Measured absorbances at
the indicated concentrations of perchloric acid for the E-
isomer were as follows: 0.075, 0.061 M; 0.10, 0.122 M;
. Sayer JM, Pinsky B, Schonbrunn A, Washtien W. J. Am. Chem.
Soc. 1974; 96: 7998–8009; Sayer JM, Conlon P. J. Am. Chem. Soc.
0
1
2
.335, 0.451 M; 0.50, 0.722 M; 0.64, 0.902 M; 0.855,
.13 M; 0.850, 1.22 M; 0.965, 1.32 M; 1.09, 1.50 M; 1.52,
.23 M; 1.71, 2.63 M; 1.78, 3.01 M;1.87, 3.38 M. Corre-
sponding absorbances and acid concentrations for the Z-
isomer were as follows: 0.250, 0.01 M; 0.715, 0.059 M;
.725, 0.075 M; 0.875, 0.104 M; 1.34, 0.232 M; 1.51,
.293 M; 1.44, 0.301 M; 1.54, 0.383 M; 1.68, 0.439 M;
.78, 0.754 M; 1.85, 0.928 M. For the E-isomer the pKa
1
980; 102: 3592–3600.
5. More O’Ferrall RA, O’Brien DM, Murphy DG. Can. J. Chem.
000; 78: 1594–1612.
6
7
2
. Karabatsos GJ, Taller RA. Tetrahedron 1968; 24: 3347–3360.
. Finch P, Merchant ZM. J. Chem. Soc., Perkin Trans 2 1982; 199–
203.
. Vogel AI. Practical Organic Chemistry. Longman: London, 1961.
. Calzadilla M, Malpica A, Cordova T. J. Phys. Org. Chem. 1999;
8
9
0
0
1
1
10. Holloway CE, Vuik CPJ. Tetrahedron Lett. 1979; 12: 1017–1020.
1. Cocivera M, Fyfe CA, Effio A, Vaish SP, Chen HE. J. Am. Chem.
Soc. 1976; 98: 1573–1578.
12. Satterthwait AC, Jencks WP. J. Am. Chem. Soc. 1974; 96: 7045–
7052.
3. Cox RA, Yates K. Can. J. Chem. 1981; 59: 2116–2124; Cox RA.
Adv. Phys. Org. Chem. 2000; 34: 1–66.
2: 708–712.
1
was determined from a plot of calculated pK values
a
þ
(
K ¼ [H ](A ꢀ A)/(A ꢀ A ), where A is the measured
a
a
b
þ
absorbance at the appropriate value of [H ] and A ¼ 2.0
a
1
and A ¼ 0.22 are limiting absorbances at high and low
b
ꢀ4
acidities for a substrate concentration of 1.3 ꢁ 10 M)
against the solvent acidity parameter X as shown in Fig. 1.
1
4. Bagno A, Scorrano G, More O’Ferrall RA. Rev. Chem. Intermed.
1987; 7: 313–352.
1
1
1
5. Fastrez J. J. Am. Chem. Soc. 1977; 99: 7004–7013.
6. Guthrie JP. J. Am. Chem. Soc. 2000; 122: 5529–5538.
7. Wei-Mei C, Kallen RG. J. Am. Chem. Soc. 1978; 100: 6119–6124.
The pK ¼ ꢀ0.55 in water corresponds to the intercept of
a
the plot at X ¼ 0; the slope of the plot m* ¼ 1.14. For the
Z-isomer the pK was determined in the same way with
a
18. Shariff MR, Zalewski RI. Bull. Acad. Pol. Sci. 1982; 29: 385–391.
1
9. Richard JP, Rothenburg MEE, Lin S-S, O’Donoghue AC, Toteva
MM, Tsuji Y, Williams KB. Adv. Phys. Org. Chem. 2000; 34: 67–
A ¼ 2.04 and A ¼ 0.035, but in the dilute acid concen-
a
b
tration range studied (0.01–0.93 M) departures from the
aqueous value of the pK ( ¼ 0.80) were small.
1
15; MacCormac AC, McDonnell CM, More O’Ferrall RA,
O’Donoghue AC, Rao SN. J. Am. Chem. Soc. 2002; 124: 8575–
583.
0. Koehler K, Sandstrom W, Cordes EW. J. Am. Chem. Soc. 1964;
0: 2413–2419.
21. Gregory BJ, Moodie RB. J. Chem. Soc. B 1970; 862–866.
a
8
Kinetic measurements were carried out by conven-
2
ꢀ2
tional methods. Stock solutions ꢈ10 M in (Z)- or (E)-
8
oximes were prepared in acetonitrile or methanol and
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 631–640