. Although many authors have argued about the solvent deuterium isotope ef- fect in terms of the different zero-point energies of acids with a proton and a deuteron

N O T I Z E N 5 7 1

NOTIZEN

Does the Solvent Deuterium Isotope Effect Depend upon the Acid Strength?

H I T O S H I O H T A K I a n d M A S U N O B U M A E D A Department of Electrochemistry, Tokyo Institute of Techno-

logy, O-okayama, Meguro, Tokyo, Japan

(Z. Naturforsch. 27 b, 571—572 [1972] ; received February 21, 1972)

There have been discussions how the solvent deute- rium isotope effect on the acid dissociation (zlpK) de- pends upon the acid strength (pK). In some cases changes in zIpK of acids have been observed with the increase of pK values

, but in other cases no ap- preciable increase in zlpK was reported for acids with a wide variety of strength

-

. Although many authors have argued about the solvent deuterium isotope ef- fect in terms of the different zero-point energies of acids with a proton and a deuteron

, measurements of infrared spectra of protio and deutero acids showed no regularity in variation of frequencies of the acids with their pK values

. Some workers have sug- gested that different solute-solvent interactions of an

ApK

0.6

0.5 0.4 0.6

0.5 0.4 0.6

0.5 0.4

0.6

0.5 0.4

o

•

o

" TO

•o

(a)

9- 6' O

" " • f i r

— ^ ,9 <D °

(b)

rfO "O

'2)9 P _{, .o •}

5

0

eQ)

ft

• • • •

o

• J r - b -

5

o

(d)

•J—

8 pK(in H20)

10

Requests for reprints should be sent to Dr. H. OHTAKI, Department of Electrochemistry, Tokyo Institute of Tech- nology, O-okayama, Meguro, Tokyo, Japan.

acid in light and heavy waters contribute to the solvent deuterium isotope effect

'

-

, but no satisfactory interpretation has been given to these phenomena.

The aim of the present paper is to propose an answer to a question whether or not the solvent deute- rium isotope effect depends upon the acid strength.

The solvent deuterium isotope effect (^IpK) is de- fined as follows:

zlpK = pK (in D

0) — pK (in H

0). (1) The pK (in H

0) value may be described in terms of the pK value of the substrate [pK

(in H

0 ) ] on the basis of the linear free energy relationship as follows:

pK (in H

0) = pK

(in H

0) + ' o (2) where represents H a m m e t t ' s reaction para- meter of the derivative in light water and o the sub- stituent parameter. Obviously a similar relationship is obtained for the acid in heavy water:

pK (in D

0) = pK

(in D

0) +

' o . (3)

Fig. 1. Solvent deuterium isotope effects (ZlpK) for acids Broken lines show average values of ZlpK, and arrows the range of standard deviations, (a) Malonic acid derivatives, (b) Acetic acid derivatives except malonic acid derivatives, (c) Phenol derivatives, (d) Anilinium ion derivatives. Num- bers in figures correspond to substances in the following (a prime at a shoulder of a number denotes a pK2 value of the

substance).

6 Dibutylmalonic acid 21

7 Phenylmalonic acid 21

8 Methyl (1-methylbutyl)- malonic acid 20

9 Isopropyl malonic acid 21

10 Malonic acid 21

11 Ethylmalonic acid 21

(a) 1 Ethylphenolmalonic acid 20

2 Ethylisopropylmalonic acid 20

3 Diisopropylmalonic acid 21

4 Diethylmalonic acid 20

5 Ethylisopentylmalonic acid 20

(b) 1 Maleic acid 22

2 Rac-a,a-di-t-butyl- succinic acid 23

3 Cyanoacetic acid 1

4 Fluoroacetic acid 1

5 Chloroacetic acid 24

(c) 1 4-Chloro-2,6-dinitro- phenol1

2 2,6-Dinitrophenol1

3 3,4-Dinitrophenol 28

4 2,5-Dinitrophenol 28

5 3,5-Dinitrophenol (18 + 2 °C) 29

(d) 1 4-Chlor-2-nitroanili- nium ion (22 ± 2 °C) 5

2 o-Nitroanilinium ion 27

3 p-Nitroanilinium ion (22 ± 2 °C) 5

6 Bromoacetic acid 1

7 Citric acid 25

8 Fumaric acid 26

9 Iodoacetic acid 27

10 Succine acid 27

11 Phenylacetic acid 1

6 o-Nitrophenol 30

7 p-Nitrophenol 24

8 4-Bromophenol 28

9 3-Methoxyphenol 28

10 Phenol2 8

11 4-Methoxyphenol 28

4 2,4-Dichloroanilinium ion (22 ± 2 °C) 5

5 m-Nitroanilinium ion 27

6 Anilinium ion 22

Filled symbols denote ortho-derivatives.

572

Insertion of Eqs. (2) and (3) into (1) leads to J p K = pK0 (in D20 ) — pK0 (in H , 0 ) + (op — O H ) • o

= J p K0 + ( G D- f f n ) -o. (4)

The reaction parameter is characteristic of the deriva- tive in question and is inversely proportional to the dielectric constant of the solvent at a given tempera- ture 18' 19. Since heavy water has the same dielectric constant as that of light water (£d20 = 7 8. 2 5 and £H20

= 78.54 at 25 ° C ) , we can put {?D = £>H- Thus, the fol- lowing simple realtion is derived:

zfpK = zlpK0. (5)

Eq. (5) shows that J p K is independent of pK values of acids with similar structure for which LFER holds.

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841 [1965].

In Fig. 1, plots of J p K vs. pK (in H20 ) at 25°C are shown for acids of four different types. In each case no systematic trend in J p K with pK was observed.

For these acids the following values of J p K were estimated: for malonic acid homologues J p K = 0.48 ± 0.06, for acetic acid derivatives 0.49 ± 0 . 0 6 , for phe- nols 0.55 ± 0.06 and for substituted anilinium ions 0.56 + 0.03 (uncertainties represent standard devia- tions) . The independent J p K on pK is thus substan- tiated.

Although one would say that J p K seems to in- crease slightly with increasing pK in the case of phe- nols, one should notice that most of the smaller values of ZlpK are given rise to ortho-derivatives.

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