Acid–Base Chemistry
FULL PAPER
physical origin of this behavior is similar. Of course the pos-
sible relationship between the gas- and solution-phase mech-
anisms of rearrangement monocationic protonated pivalal-
dehyde is a moot point at this time.
Acknowledgement
This work was supported by DGES Projects PB2000–1497, PB2003–
0
5897, and PB2000–0245. We thank Dr. Josep M. Oliva (Bristol Universi-
ty, UK) for useful discussions. We are grateful to CTI (CSIC) for allow-
ing extensive use of their computing facilities.
[
[
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Figure 4. 0 K energies profile for the isomerization of the oxygen pro-
+
tonated form of isobutyraldehyde (4H ) to yield the oxygen protonated
+
ꢀ1
form of butanone (5H ). All values (in kJmol ) where obtained at the
[
[
MP2/6–31G(d)//HF/6–31G(d) level of theory.
cess would bring about an increase in the strain of the
system, when evolving to yield protonated 4-homoadaman-
tanone. For the sake of consistency with the G2(MP2) calcu-
lation, harmonic vibrational frequencies were obtained at
the HF/6–31G(d) level and corrected with the 0.8929 factor.
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[12a]
It has recently been shown experimentally by Olah
Gomperts, P. Mꢇller, M. V. Roux, J. Org. Chem. 2002, 67, 1057–
1060.
and co-workers that in moderate to strongly acidic solutions,
[
6] For recent applications of this methodology, see, for example: a) J.-
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compound 1 reversibly protonates to yield unrearranged
+
[12b]
1
3
H . More precisely, at an H
H
value of ꢀ7.7, the yield of
0
+
is only 17%, while already half of the pivaIaldehyde is
already monoprotonated. The optimal acidity for isomeriza-
tion is ꢀ10.9, close to the superacid limit (H =ꢀ12). The
0
1
998, 120, 13224–13229; d) M. Lamsabhi, M. Alcamꢁ, O. Mꢂ, W.
Bouab, M. Esseffar, J.-L. M. Abboud, M. YꢀÇez, J. Phys. Chem. A
000, 104, 5122–5130.
mechanism for the high-acidity isomerization of 1 in so-
[
12a,14,15]
lution as suggested by these authors„
involves high
2
2
+
2+
+
energy dications 6 and 7 . They finally lead to 3H .
[
[
[
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We have shown in this paper that in the gas phase, the
system overcomes the activation barrier for isomerization,
most likely thanks to the energy released in the formation
of the encounter complex between neutral 1(g) and a pro-
[
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1
trate acidic solutions to protonate a family of weak reference bases
(L. P. Hammet, A. J. Deyrup, J. Am. Chem. Soc. 1932, 54, 2721–
0
1561; b) H was developed as a measure of the tendency of concen-
+
tonated base BH (g). In solution, this mechanism no longer
2
739). It becomes identical to pH in dilute aqueous solutions of
operates. We have recently reported a similar situation in
acids and changes from 7.0 in pure water to ꢀ12.5 in pure sulfuric
acid. For a comprehensive review of the topic, see, for example:
R. A. Cox, Adv. Phys. Org. Chem. 2000, 35, 1–66.
[5a]
the case of the protonation of cubylamine. In water, pro-
tonation takes place at the nitrogen atom, whereas in the
gas phase, it takes place on the cubane framework. The
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Chem. Eur. J. 2005, 11, 1826 – 1832
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