Measurements and computations of acidity and basicity of strong and superstrong acids (superacids) and bases in organic solvents is among the core research topics at the UT Chair of Analytical Chemistry. In a recent works (Chem. Sci. 2017, 8, 6964-6973.,    J. Phys. Org. Chem. 2013, 26, 162-170.,    J. Phys. Chem. A 2015, 119, 735–743.,    J. Phys. Chem. A 2016, 120, 3663–3669.) the behavior of a number of acids – ranging from weak to strong and superstrong acids (superacids) was examined in three solvents (water, acetonitrile, 1,2-dichloroethane) and in the gas phase. Acidities (pKa values) of a number of different acids including the well-known superacids trifluoromethanesulfonic (triflic) acid, bis-trifylimide (Tf2NH), etc as well as the carborane superacids (closo-carborane superacids), but also weaker acids (HCl, acetic acid, phenol) etc are examined in the above mentioned solvents. pKa of superacids are not easy to find in literature. Trends of acidity changes on moving from water to the gas phase depnding on the on the nature of the acidity centre and the molecular structure are analyzed. The acidity orders are different in water, MeCN, DCE and the gas phase. In some cases – notably, the hydrohalogenic acids HCl, HBr and HI – the differences are dramatic. These three acids are among the strongest known acids in water but have modest acidity in the gas phase. In contrast, 9‑C6F5‑Octafluorofluorene, a weak acid in water (approximately of the strength of phenol) is quite strong acid in the gas phase, beating any of the hydrohalogenic acids.

It is demonstrated that the decisive factor for behavior of the acids when transferring between different media is the extent of charge delocalization in the anion and that the recently introduced WAPS parameter in spite of its simplicity enables interpretation of the trends in the majority of cases. The acidity data together with references to specific publications are collected in the Table below.

Table of pKa valuesa of acids in different solvents.

 

 

 

 

 

 

Acid

H2O

MeCN

DCEb

GP

GP

 

pKa

pKa

pKa

pKa

GA

 

 

 

 

 

 

Fluoradene

11.1

23.90

12.5

238

324.9

Para-Toluenesulfonamide, 4-CH3-C6H4-SO2-NH2

10.21

26.87

15.6

245

334.0

9‑C6F5‑Octafluorofluorene

10.1

18.88

9.0c

220

300.6

Phenol

10.00

29.14

19.6

251

342.3

(C6F5)2CHCN

9.5

21.10

10.3

229

312.4

C6F5CH(CN)COOEt

5.89

17.75

7.5c

230

313.5

2,4-Dinitrophenol, 2,4-(NO2)2-C6H3OH

4.09

16.66

4.7

226

308.6

(CF3)3COH

5.40

20.55

9.2

238

324.0

Acetic acid, CH3COOH

4.76

23.51

15.5

250

341.1

(4-CF3-C6F4)2CHCN

4.4

16.13

6.0

221

302.1

4-NO2-C6H4-CH(CN)2

2.3

11.61

0.3

220

299.5

Saccharin

1.80

14.57

5.5

232

315.9

Picric acid, 2,4,6-Trinitrophenol

0.40

11.00

0.0

219

299.0

(4-NO2-C6H4-SO2)2NH

<-1

8.19

-3.9c

213

291.1

(CF3SO2)3C6H2OH

-2.0

4.48

-6.6c

208

284.2

(CF3SO2)2NH

<-2

0.3d

-11.9c

210

286.9f

(C2F5SO2)2NH

<-2

-0.10

-12.4c

208

283.7

Cyanoform, (CN)3CH

-5.1

5.1

-6.4c

216

294.8

Triflicid, trifluoromethanesulfonic acid, CF3SO2OH

-14.7g

0.70

-11.3c

215

292.7g

Hydrochloric acid, hydrogen chloride, HCl

-5.9g

10.30

0.2c

241

328.1g

Hydrobromic acid, hydrogen bromide, HBr

-8.8g

5.5

-4.4c

233

318.3g

Sulfuric acid, H2SO4

-9e

8.7d

-2.2c

221

301.2f

Pentacyanopropene

-9.02

-2.80

-15.3c

196

267.2

Hydroiodic acid, hydrogen iodide, HI

-9.5g

2.8

-7.3c

227

309.3g

CB11H12H, unsubstituted closo-Carborane superacid

-25h

-17h

-25f

195

266.5f

B12H12H2, unsubstituted closo-Dodecaborate superacid

-17h

-9h

-17f

196

267.5f

CB11F12H, perfluoro-closo-Carborane superacid

-47h

-39h

-47f

156

212.8f

B12F12H2, perfluoro-closo-Dodecaborate superacid

-45h

-37h

-45f

156

213.4f

 

 

 

 

 

 

a Values from http://dx.doi.org/10.1002/poc.2946 unless noted otherwise. There are numerous comments and details to these values. Please see the original articles for details and comments. GA values are given in kcal mol-1. pKa values in the gas phase are approximate and have been obtained from the GA values by dividing with 1.364 kcal mol-1. b Ion pair pKa values relative to picric acid in 1,2-dichloroethane. c Values from https://doi.org/10.1039/c7sc01424d. d Values from http://dx.doi.org/10.1021/jo101409p. e Value from E. V. Anslyn, D. A. Dougherty, Modern Physical Organic Chemistry, University Science Books, Sausalito, 2006. f Values from http://dx.doi.org/10.1021/jp506485x. DCE values have been recalculated from absolute to relative, in order to be comparable with the rest. g Values from http://dx.doi.org/10.1021/acs.jpca.6b02253. h Crude estimates from DCE data by considering that bulk water is by 53 kcal mol-1 more basic than bulk DCE and bulk acetonitrile is by 42 kcal mol-1 more basic than bulk DCE.

 

(This post was initially posted on Feb 2, 2013. On Jan 15, 2017 a number values in the table have been replaced by more reliable values from more recent publications. In addition, some compounds were added to the in order to give a fuller picture. The changes concern only the table. The image has not been changed and should be viewed as illustrative)

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