Angewandte
Chemie
[5] H. Mayr, M. Patz, Angew. Chem. 1994, 106, 990 – 1010; Angew.
decreases. If we neglect ion-pair recombinations and the fact
that the combination rates refer to 208C whereas the
ionization rates refer to 258C, the pseudo-first-order rate
constants depicted in Figure 4 are directly comparable. Since
the rate constants for ionization and trapping by the solvent
are identical at the point of intersection, conventional SN1
reactions (slow ionization, fast solvent trapping) are found on
the right of the intersections, whereas reactions with inverse
rate profiles (fast ionization, slow solvent trapping) are found
on the left. With the approximations k208C ꢂ k258C, Ef ꢂ ꢀ E
and s, sf ꢂ 1, Equations (2) and (3) can be combined to yield a
rough estimate for the point of intersection at E = (Nf ꢀ N)/2.
It is clear from this formula as well as from Figure 4 that
accumulation of carbocationic intermediates must be
expected in numerous solvolysis reactions (even with moder-
ately stabilized carbocations) if solvents of low nucleophilicity
(N) and systems with high nucleofugality (Nf) are employed.
Figure 4 shows, for example, that alkyl bromide solvolysis
reactions in 90% aqueous acetone will proceed with accu-
mulation of the intermediate carbocations if E < ꢀ 2. In
solvents of lower nucleophilicity,[16,17] this border is shifted
towards less-stabilized carbocations. Accordingly, the 4,4’-
dimethoxy-substituted benzhydryl cation 1 was observed by
UV/Vis spectroscopy during the trifluoroethanolysis of the
benzhydryl chloride 1-Cl.[4] More solvent nucleophilicity and
nucleofugality parameters are required for the general
prediction of the borderline between the two mechanistic
alternatives.
Chem. Int. Ed. Engl. 1994, 33, 938 – 957;
[6] a) H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker,
B. Kempf, R. Loos, A. R. Ofial, G. Remennikov, H. Schimmel, J.
Am. Chem. Soc. 2001, 123, 9500 – 9512; b) H. Mayr, B. Kempf,
A. R. Ofial, Acc. Chem. Res. 2003, 36, 66 – 77.
[7] a) B. Kempf, N. Hampel, A. R. Ofial, H. Mayr, Chem. Eur. J.
2003, 9, 2209 – 2218; b) T. Bug, M. Hartnagel, C. Schlierf, H.
Mayr, Chem. Eur. J. 2003, 9, 4068 – 4076.
[8] a) R. Lucius, R. Loos, H. Mayr, Angew. Chem. 2002, 114, 97 –
102; Angew. Chem. Int. Ed. 2002, 41, 91 – 95; b) S. Minegishi, H.
Mayr, J. Am. Chem. Soc. 2003, 125, 286 – 295; c) T. Bug, H. Mayr,
J. Am. Chem. Soc. 2003, 125, 12980 – 12986; d) R. Loos, S.
Kobayashi, H. Mayr, J. Am. Chem. Soc. 2003, 125, 14126 –
14132.
[9] H. Mayr, G. Lang, A. R. Ofial, J. Am. Chem. Soc. 2002, 124,
4076 – 4083.
[10] Since most rate constants for electrophile–nucleophile combi-
nations are reported for a temperature of 208C, whereas those
for solvolysis reactions usually refer to 258C, it was necessary to
use different temperatures for Equations (2) and (3).
[11] Because sf as well as Nf are nucleofuge-specific parameters, one
might ask the question as to why Equation (3) is used instead of
the mathematically equivalent expression logk = N0f + sfEf (Nf0 =
sfNf), in analogy to most common linear free-energy relation-
ships.[12] As we have repeatedly discussed for Equation (2), it is
this special term (with N as the negative intercept on the abscissa
(E axis)) that renders nucleophilicity parameters N that are of
immediate practical use.[5,13] Whereas the intersections of the
correlation lines with the abscissa (logk = 0) are always within or
close to the experimental range, intersections with the ordinate
(E or Ef = 0) will often be far outside the experimental range.
When the intercepts on the ordinate N’ or N0f are considered,
even qualitative comparisons of compounds with large differ-
ences in reactivity are only possible in combination with the
corresponding slope parameters. In contrast, nucleophilicity N
and nucleofugality Nf (the intercepts on the abscissa) can always
be qualitatively discussed without consideration of the slopes s
or sf.
[12] A. Williams, Free Energy Relationships in Organic and Bio-
organic Chemistry, The Royal Society of Chemistry, Cambridge,
2003, and references therein.
[13] a) H. Mayr, M. Patz, M. F. Gotta, A. R. Ofial, Pure Appl. Chem.
1998, 70, 1993 – 2000; b) H. Mayr, O. Kuhn, M. F. Gotta, M. Patz,
J. Phys. Org. Chem. 1998, 11, 642 – 654.
[14] The parameters Ef, Nf, and sf were calculated by minimizing
ꢀD2 = ꢀ(logk1 ꢀ logk1calcd)2 = ꢀ(logk1 ꢀ sf (Nf + Ef))2 with the
program What'sBest! 4.0 Commercial, Lindo Systems Inc.
[15] a) C. D. Ritchie, Acc. Chem. Res. 1972, 5, 348 – 354; b) C. D.
Ritchie, Can. J. Chem. 1986, 64, 2239 – 2250.
[16] S. Minegishi, S. Kobayashi, H. Mayr, J. Am. Chem. Soc., in press.
[17] a) D. N. Kevill in Advances in Quantitative Structure–Property
Relationships, Vol. 1 (Ed.: M. Charton), JAI, Greenwich, 1996,
pp. 81 – 115; b) R. A. McClelland, Tetrahedron 1996, 52, 6823 –
6858.
Received: December 5, 2003 [Z53468]
Keywords: carbocations · kinetics · linear free energy
.
relationships · nucleophilic substitution · reaction mechanisms ·
solvent effects
[1] R. D. Guthrie, W. P. Jencks, Acc. Chem. Res. 1989, 22, 343 – 349;
R. D. Guthrie, W. P. Jencks, Acc. Chem. Res. 1990, 23, 270.
[2] a) A. Streitwieser, Jr., Solvolytic Displacement Reactions,
McGraw-Hill, New York, 1962; b )Carbonium Ions, Vol. 1–5
(Eds.: G. A. Olah, P. von R. Schleyer), Interscience, New York,
1968–1976; c) P. Vogel, Carbocation Chemistry, Elsevier,
Amsterdam, 1985; d) X. Creary, Advances in Carbocation
Chemistry, Vol. 1, JAI, Greenwich, 1989; e) J. M. Coxon, Advan-
ces in Carbocation Chemistry, Vol. 2, JAI, Greenwich, 1995;
f) D. J. Raber, J. M. Harris, P. von R. Schleyer in Ions and Ion
Pairs in Organic Reactions, Vol. 2 (Ed.: M. Szwarc), Wiley, New
York, 1974, pp. 247 – 374.
[3] E. Gelles, E. D. Hughes, C. K. Ingold, J. Chem. Soc. 1954, 2918 –
2929.
[4] H. Mayr, S. Minegishi, Angew. Chem. 2002, 114, 4674 – 4676;
Angew. Chem. Int. Ed. 2002, 41, 4493 – 4495.
Angew. Chem. Int. Ed. 2004, 43, 2302 –2305
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2305