Property: DG (acid) for the following gas phase reaction:
HOCZ3 = (-)OCZ3 + H(+)
References: At the bottom.
Related Links:
Property: DG (acid) for the following gas phase reaction:
HOSiZ3 = (-)OSiZ3 + H(+) (Analysis)
Summary:
DG (acid) = (393±2) - (0.18±0.02)q - (5±1)Ear - (4.02±0.07)pp
n = 34 s = 1.402 r2 = 0.990
%cd = %q = 16 %Ear = 24 %pp = 60
outliers - none
General Comments: The involvement of Ear in the following analysis rests on a single datum namely that for HOCH2Ph. Accordingly, its contribution must be viewed as suspect.
Data:
|
HOCZ3 |
DG(acid) (kcal/mol) |
cd |
q |
Ear |
pp |
Ref |
|
CH3OH |
379.2 |
17.00 |
87 |
0 |
0.0 |
1 |
|
CH3CH2OH |
376.1 |
14.20 |
97 |
0 |
0.0 |
1 |
|
FCH2CH2OH |
371.0 |
16.90 |
99 |
0 |
1.2 |
1 |
|
F2CHCH2OH |
367.0 |
19.60 |
101 |
0 |
2.5 |
1 |
|
CH3CH2CH2CH2OH |
373.7 |
13.10 |
103 |
0 |
0.0 |
1 |
|
CH3CH2CH2OH |
374.7 |
13.40 |
103 |
0 |
0.0 |
1 |
|
F3CCH2OH |
356.8 |
22.30 |
103 |
0 |
3.7 |
2 |
|
F3CCH2OH |
364.4 |
22.3 |
103 |
0 |
3.7 |
1 |
|
F3COH |
323.0 |
33.00 |
104 |
0 |
13.2 |
3 |
|
PhCH2OH |
369.6 |
15.80 |
106 |
1 |
0.0 |
1 |
|
Me2CHOH |
374.1 |
11.40 |
107 |
0 |
0.0 |
1 |
|
(i-Pr)CH2COH |
373.4 |
12.50 |
111 |
0 |
0.0 |
1 |
|
EtMeCHOH |
372.9 |
10.60 |
112 |
0 |
0.0 |
1 |
|
(FCH2)2CHOH |
366.0 |
16.80 |
112 |
0 |
1.2 |
1 |
|
(F3C)MeCHOH |
356.4 |
19.50 |
114 |
0 |
3.7 |
2 |
|
Et2CHOH |
371.2 |
9.90 |
117 |
0 |
0.0 |
1 |
|
Me3COH |
373.3 |
8.55 |
118 |
0 |
0.0 |
1 |
|
(t-Bu)CH2OH |
371.8 |
11.30 |
119 |
0 |
0.0 |
1 |
|
(F3C)2HCOH |
339.7 |
27.70 |
120 |
0 |
7.5 |
2 |
|
(i-Pr)MeCHOH |
372.0 |
9.7 |
122 |
0 |
0 |
1 |
|
PrMe2COH |
371.4 |
7.50 |
123 |
0 |
0.0 |
1 |
|
EtMe2COH |
372.0 |
7.80 |
123 |
0 |
0.0 |
1 |
|
BuMe2COH |
370.9 |
7.45 |
124 |
0 |
0.0 |
1 |
|
(F3C)Me2COH |
356.2 |
16.70 |
124 |
0 |
3.7 |
2 |
|
(i-Pr)(Et)CHOH |
370.8 |
8.90 |
126 |
0 |
0.0 |
1 |
|
Et2MeCOH |
371.2 |
7.05 |
127 |
0 |
0.0 |
1 |
|
(t-Bu)Me(H)COH |
370.7 |
8.50 |
129 |
0 |
0.0 |
1 |
|
(F3C)2MeCOH |
342.0 |
24.90 |
130 |
0 |
7.5 |
2 |
|
Et3COH |
370.3 |
6.30 |
132 |
0 |
0.0 |
1 |
|
(t-Bu)EtCHOH |
369.6 |
7.80 |
134 |
0 |
0.0 |
1 |
|
(i-Pr)2CHOH |
370.2 |
8.00 |
136 |
0 |
0.0 |
1 |
|
(F3C)3COH* |
329.0 |
33.00 |
136 |
0 |
11.2 |
1 |
|
(F3C)3COH |
324.0 |
33.00 |
136 |
0 |
11.2 |
4 |
|
(t-Bu)(i-Pr)CHOH |
368.5 |
6.80 |
143 |
0 |
0.0 |
1 |
|
(t-Bu)2CHOH |
367.3 |
5.70 |
150 |
0 |
0.0 |
1 |
|
MeOCH2CH2OH |
372.5 |
1 |
||||
|
PhOH |
372.8 |
1 |
||||
|
(i-Pr)CH2CH2OH |
372.5 |
1 |
||||
|
C6H13OH |
372.2 |
1 |
||||
|
i-PrC3H6OH |
372.1 |
1 |
||||
|
C7H15OH |
371.6 |
1 |
||||
|
t-BuCH2CH2OH |
371.6 |
1 |
||||
|
C8H17OH |
371.1 |
1 |
||||
|
C9H19OH |
370.6 |
1 |
*Datum was not used because of the large error associated with it.
Graphical Analysis:
Interpretation of Graphs:
• Plot of data versus cd for C(p-XC6H4)3(Graph A): NA
• Slope of the CR3 line (Graph A): The primary, secondary and tertiary alcohols form three closely spaced and approximately parallel lines. The closeness of the lines suggests that this is not a p effect. Therefore, this appears to be a steric effect. As we will see there is a very significant p effect for alcohols bearing F and CF3 groups, therefore it appears that there is no p acidity connected with the C-H bonds. Accordingly, we assigned a value of 0 to pp for H groups.
• The point of intersection of the 2 lines in graph A: NA
• The plot of DG (acid) versus 'i' for CZ3-iXi (Graph B and C): The sets of ligands form good to excellent lines. One of the values for HOCH2CF3 might be an outlier.
• Outliers: None.
• Steric threshold: None that is obvious.
Statistical Analysis:
We began the analysis using all the data and a four parameter fit. The resulting regression equation is
DG (acid) = 394 - 0.028cd - 0.182q - 4.92Ear - 3.96 pp
|
Predictor |
Coef |
Stdev |
t-ratio |
p |
|
Constant |
394.277 |
4.526 |
87.11 |
0.000 |
|
cd |
-0.0277 |
0.1284 |
-0.22 |
0.831 |
|
q |
-0.18226 |
0.02852 |
-6.39 |
0.000 |
|
Ear |
-4.919 |
1.522 |
-3.23 |
0.003 |
|
pp |
-3.9643 |
0.2710 |
-14.63 |
0.000 |
s = 1.425 r2 = 0.990 r2 (adj) = 0.989
We dropped cd as a parameter because of its small 't-ratio' and large 'p-value'. The resulting and final regression equation is
DG (acid) = 393 - 0.177q - 5.01Ear - 4.02pp
|
Predictor |
Coef |
Stdev |
t-ratio |
p |
|
Constant |
393.409 |
2.042 |
192.66 |
0.000 |
|
cd |
||||
|
q |
-0.17739 |
0.01713 |
-10.36 |
0.000 |
|
Ear |
-5.006 |
1.444 |
-3.47 |
0.002 |
|
pp |
-4.02055 |
0.07333 |
-54.83 |
0.000 |
s = 1.402 r2 = 0.990 r2 (adj) = 0.989
%cd = %q = 16 %Ear = 24 %pp = 60
Stereoelectronic Profiles:
Discussion:
An important feature of the gas phase acidity of alcohols is the increase in acidity on going from methanol to tertiary alcohols such as t-BuOH. This reversal of the solution acidity has been attributed to the stabilization of the gas phase alkanoate ion through polarization of the alkyl groups. The QALE analysis qualitatively supports this thesis in that we see that the acidity increases as the size of CZ3 increases (larger q). However, we note that the p acidity of CZ3 dominates the analysis when the whole data set is considered. It is note worthy that we see no cd effect. In fact, when we included cd in the analysis, we found that its contribution was small and statistically insignificant.
We compare the QALE analysis of Z3COH with that of Z3SiOH (see below). We note that the analyses are very different. The Z3COH analysis shows a large dependence on pp and no dependence on cd. It is the other way around for Z3SiOH. Both show virtually the same dependence on q (polarizability?). Thus, based on the QALE model, we conclude that the differences in the trends in acidity observed for Z3COH and Z3SiOH are attributable not to polarizability differences but rather to changes in other electronic properties.
Property: DG (acid) for the following reaction:
HOSiZ3 = (-)OSiZ3 + H(+) (gas phase)
DGoacid = (390±3) - (1.03±0.03)cd - (0.23±0.02)q
%cd = 68 %q = 32 %Ear = 0 %pp = 0 r2 = 0.988
We do not believe that the differences in the Z3COH and Z3SiOH analysis are due to inappropriate stereoelectronic parameters because the of the large number of points, the absence of outliers, and the very high quality of the regression analyses.
References
1. Gasteiger, J. and Hutchings, M.G.: Quantitative Models of Gas Phase Proton Transfer Reactions involving Alcohols, Ethers, and their Thio Analogs. Correlation Analyses Based On Residual Electronegativity and Effective Polarizability. J. Am. Chem. Soc. 106, 6489 (1984) and references therein.
2. Caldwell, G., McMahon, T.B., Kebarle, P., Bartmess, J.E. and Kiplinger, J.P.: Methyl Substituent Effects in the Gas Phase Acidities of Halo Substituted Oxygen Acids. A Realignment with Substituent Effects in Solution. J. Am. Chem. Soc. 107, 80 (1985)
3. Huey, L.G., Dunlay, E.J. and Howard C. J.: Gas Phase Acidity of CF3OH. J. Phys. Chem. 100, 6504 (1996)
4. Koppel, I.A., Taft, R.W., Anvia, F., Zhu, S.Z., Hu, L.Q., Sung, K.S., DesMarteau, D.D., Yagupolskii, L.M., Yagupolskii, Y.L., Ignatev, N.V., Kondratenko, N.V., Volkonskii, A.U., Vlasov, V.M., Notario, R. and Maria, P.C.: The Gas Phase Acidities of Very Strong Neutral Bronsted Acids. J. Am. Chem. Soc. 116, 3047 (1994).