982
Cabani et al.
Volume Changes in the Proton Ionization of Amines in Water. 1. Morpholines and Piperazines S. Cabani," V. Moilica, Istituto di Chimica Fisica, Universits di fisa, Via Risorgimento 35, 56100 fisa, Italy
L. Lepori, and S. T. Lobo' Laboratorio di Chimica Quantistica ed Energetica Molecolare del C.N.R., Via Risorgimento 35, 56 100 fisa, Italy (Received November 19, 1976) Publication costs assisted by Consiglio Nazionale delle Ricerche, Rome
Apparent molar volumes, a",at various concentrations in water at 25 OC of some cyclic bifunctional amines [morpholine,4-methylmorpholine, piperazine, 1-methylpiperazine, 1,4-dimethylpiperazine, l,4-diazabicyclo[2.2.2]octane (triethylenediamine)]and their mono- and dihydrochlorides have been determined. The volume changes 4V10 and AVzO, involved in the first and second proton ionizations from the protonated amines, have been calculated from the limiting partial molar volumes VZo. Furthermore, the volumes of ionization for the bifunctional cyclic amines have been compared with those for the monofunctional amines and the relationship between entropies and volumes of ionization has been examined.
Introduction Volume changes, 4Vo, in the proton ionization process of amines in aqueous solution have become the subject of considerable attention because such data can provide information on solute-solvent However compared with enthalpies and entropies of ionizationg*ld much less is known about AVO. This led us to commence studies on the infuence of substituents on 4 V 0 of some polyfunctional organic compounds containing nitrogen, varying in the number and type of radicals attached to the nitrogen atom. In this work, we report data for certain secondary and tertiary bifunctional cyclic amines, for which we have already reported data for transfer properties from the gaseous state to dilute aqueous solution'' and heat capacity changes for proton ionization.12J3 The volume changes, AVO, for proton ionization have been calculated from the limiting partial mala! volumes (V20) of the species involved in the reaction. Vzo values have been obtained from density measurementsof aqueous solutions of the amine, of the monohydrochloride, and of the dihydrochloride as a function of concentration. Experimental Section Materials. The amines examined [morpholine, 4methylmorpholine, piperazine, 1-methylpiperazine, 1,4dimethylpiperazine, and 1,4-diazabicyclo[2.2.2]octane (triethylenediamine)] were available commercially. All liquids have been purified by fractional distillation at atmospheric pressure after prolonged refluxing on metallic sodium. Piperazine (mp 110 OC) was purified by zone melting; triethylenediamine was sublimed twice under vacuum. The purity of these compounds, as examined by GLC, was found to be 199.5%. Stock solutions of the mono- and dihydrochlorides of the amines were prepared, immediately prior to measurements, by addition of a stoichiometric quantity of amine to a standardized HC1 solution. In the case of piperazine dihydrochloride, a commercial sample was used also. It was purified by recrystallization from ethanol and water (mp 320 "C). No appreciable difference was observed between the results obtained from the commercial solid sample and the prepared stock salt solution. The The Journal of Physical Chemistry, Vol. 8 1 , No. IO, 1977
water used in all the experiments was first deionized and then distilled from an alkaline KMn04 solution. Apparatus and Measurements. A differential hydrostatic balance, capable of a precision of 1ppm, was used for the density measurements. The details of the method have been described earlier.14 The solutions to be studied were prepared by successive additions of a weighed quantity of amine (B), or of stock solution of monohydrochloride (BeHCl),or of dihydrochloride (B-2HC1)of the amine to a known quantity of water. In each case, the observed apparent molar volume (@gbsd) was calculated from
M
QVobsd = - -
(5
-
1)-1000
do
C
where M is the formula weight of the solute, c its molar concentration, and do and d a r e the density of water and of solution, respectively. Treatment of Data From the avobsd values obtained by using eq 1, the apparent molar volume a#) of the species I (I = B, B.HC1, B.2HC1) has been calculated by applying corrections for hydrolysis (a), for acidic dissociation (b and c), and for disproportionation (d). B t H,O P BH'
+ OH-
BH' t B + H+ B H Z Z +BH+ t t H+ BH'F? '/*B + '/zBHz2+
The values of aV(I) are related to @Vobsd(I) by the following general equation: @"(I)= Qvob"(I) - O A V
(2)
where a is the degree of dissociation of species I, which is involved in processes a, b, c, or d, and AV is the corresponding reaction volume. In Figure 1are reported a values15afor processes a, b, and c vs. pK values for the correspondingprocesses at the initial concentration co = 0.01 M. Examination of Figure 1 shows that for the hydrochlorides of the monoamines
Q83
Volume Changes in the Proton Ionization of Amines
I
0.5
0.4
?3
0
1 1
L ,
1
1
1
1
I
2
3
1/2
,
I
I
4
( PK, -PK,)
Figure 2. A correlation of a vs. 'Iz(pK, - pKp). The pKrange, covered by the amines examined, is indicated.
0
L J
2
l
I
4
,
6
!
l
I
8
l
10
PK
Figure 1. A correlation of a vs. pKfor cIo = 0.01 mol L-'. The pK range for processes a, b, and c for the amines studied here is indicated.
studied, the degree of dissociation is always negligible (a < 0.002). Consequently, @,(B.HCl) is practically equal to avobsdand can be calculated directly from eq 1. For aqueous solutions of amines or diamine dihydrochlorides, however, a is nonnegligible and, hence, corrections must be a lied to obtain values for B and B.2HC1. A plot of agrvs. pK for process d is shown in Figure 2. Although the degree of disproportionation is small (a< 0.02) for the monohydrochlorides of the diamines examined here, a correction has been applied in all cases. The procedure to be followed for hydrolysis, Le., for process a, involving @.,(B) = {@vOb"(B) - a[Q,(BH'OH-) - V0(H20)I)/(1- a ) (3) has already been de~cribed.'~Vo(HzO)is the molar volume of liquid water. For process c, which must be considered in the case of solutions containing the diamine and hydrochloric acid in the ratio 1:2, the corrected values have been calculated by Qv(B*2HC1)= {@,0bSd(B.2HC1)- CY [Qv(B-HC1) + Qv(HC1)1}/(~-a ) (4) For process d, the corrected 9"values are given by @.,(B*HCl)= {@.,Obsd(B*HC1) - a/2[QV(B) + @.,(B.BHCl)])/(l - a ) (5) From eq 3-5, it is evident that the calculation of a, of a diamine (B) or of its salts (B.HC1 or B.2HC1) entails a knowledge of CP, of the other species present in solution. Experimental values for B.HC1 are used as initial values in an iterative procedure, which involves eq 3-5, successively. Three iterations were adequate to obtain convergent values of @"(I)(I = B, B.HC1, B.2HCl). The values of the equilibrium constants, needed for the calculation of a, were obtained from the thermodynamic equilibrium constants reported in the literature and from
the molal activity coefficients calculated from the Debye-Huckel limiting law. The accuracy of the above corrections depends on both the pK uncertainty and the magnitude of the correction itself, the latter being a function of AV as well as of a (see eq 2), and consequently of concentration and pK values. For the substances examined here, the magnitude of the corrections does not exceed 6 mL mol-' in any case, with an error which has been estimated to be always lower than 0.08 mL mol-'. The corrected 9,values, obtained via eq 3-5, were fitted to an equation of the type @,. =
V20 + Svc'i2 t hc
(6)
which is the Redlich-Meyer equation for electrolytes,'6 ~ ' ~1:l and 1:2 where S, = 1.868 and 9.706 mL L'" r n ~ l - for electrolytes, respectively, at 25 "C. In the case of amines, S, = 0. V2" is the partial molar volume at infinite dilution and h is an empirical constant determined from experimental results. c is the actual molar concentration of the solute related to the stoichiometric concentration co by c = (1 - a)cO. As an example, in Figure 3 are reported 9,values as a function of concentration for triethylenediamine for the free base, monohydrochloride, and dihydrochloride, with and without the corrections considered above.
Results and Discussion An a l t e r n a t i ~ e ~ ~to ' ~the - ' ~ hydrolysis correction applied above is to carry out density measurements of the amine in alkaline solutions, in order to suppress hydrolysis. The apparent molar volume of solute is then calculated directly from eq 1, where do is the density of the alkaline solut i ~ n ; ~ , ~a 'relationship ,'~ based on Young's rulelg has also been used. Both of these procedures, which are equivalent, do not take into account the terms arising from the deviation of the system from the above rule, i.e., terms that may not be negligible and that may not be predictable a priori. In Table I are compared U2"values of some nitrogen containing compoun_ds in water and in aqueous alkaline solutions. That the V20 values in water are greater than those obtained from measurement in aqueous alkaline solutions is clearly evident. Sometimes the data of the same authors differ by as much as 0.8 mL mol-'. In the specific case of cyclic bifunctional amines, the difference between V2" obtained from measurement in water and in aqueous alkaline solutions appears to be The Journal of Physical Chemistry, Vol. 81, No. 10, 1977
9a4
Cabani et ai.
x"
TABLE I: Comparison between the Values of for Nitrogen Bases Obtained from Measurements in Water or in Aqueous Alkaline Solutions at 25 "C
A
-
VZ",
Substance Piperidine
1-Methylpiperidine Methylamine Dimethylamine Trimethylamine Triethylamine
mL mol-'
Medium
Ref
92.13a 91.97 91.8 91.65 91-10 110.54 109.91 41.68 41.15 40.0 59.80 59.0 58.6 78.8 78.6 77.9 120.9 119.7
Water 0.1 NKOH Water 0.1 NNaOH 0.1 N KOH Water 0.1 N K O H Water 0.025NKOH 0.025 N KOH Water 0.025NKOH 0.025 NKOH Water 0.025NKOH 0.025NKOH Water 0.025 N KOH
14 14 18 18 17 14 17 20 20 5 20 20 5 20 20 5 20 5
1
0.2
0.4
,
1
1
1
0.6
0.8
1.o
0.6
0.8
1.0
a The value of 92.53 mL mol-' reported earlier (ref 14) is incorrect due to a typographical error,
negligible, as is seen from the close agreement of our results with those of others (cf. Table 11). By using the values of Yz"reported in Tables I1 and 111, and the value of Vzo(HC1)= 17.83 mL mol-' at 25 "C,'the volume changes in proton ionization have been calculated as follows: AVI" = V2"(B)
+ V2"(HC1)- V,"(B*HCl)
A V2" = V2"( B.HC1)
+ V,"(HC1) - V2"( B.BHC1)
1
I
0.2
0.4
C
123
0.1
0.2
0.3
0.4
c , mol L" Flgure 3. Apparent molar volumes of (A) triethylenediamine, (B) triethylenediamine monohydrochloride, (C) triethylenedlamlne dlhydrochloride in aqueous solution at 25 "C,uncorrected (0)and corrected (0)for hydrolysis, dlsproportionation, and acidic dissociation, respectively. For the monohydrochloride, corrected and uncorrected data are practically identical.
-
= V," t hc, of Bifunctional Cyclic Amines in Water at 25 "Ca
No. of expt
Concn range, M
Morpholine
23
0.03-1.03
4-Methylmorpholine Piperazine
12 14
0.02-1.08 0.03-0.72
1-Methylpiperazine 1,4-Dimethylpiperazine Triethylenediamine
18 21 25
0.03-0.91 0.03-0.84 0.02-0.95 0.03-0.38
Substance
1161
(7) (8)
The results are summarized in Table IV together with the values of the thermodynamic functions pK, AH",and AS" for proton ionization. To facilitate comparison, data for related compounds have been included in the table. The results are interpreted in terms of (1)introduction of a methyl group on the nitrogen atom of a cyclic amine and (2) introduction of a polar center in a cyclic monofunctional amine. Introduction of a Methyl Group on the Nitrogen Atom. All secondary amines considered in Table IV have an almost constant value for AS1"= -8.5 f 1.5 cal mol-' deg-', which is considerably less negative than that for the corresponding tertiary N-methylated amines (AS1' = -15.4 f 2 cal mol-' deg-'). This substantial difference in entropy on going from the secondary to the tertiary amine is accompanied by a decrease in basicity of the order of one pK unit and a decrease in AH1" 'v 3 kcal mol-'. AV'", however, remains practically unaffected on the. introduction of a methyl group on the nitrogen atom of a cyclic secondary amine. This trend is similar to that shown by TABLE 11: Apparent Molar Volumes,
f
a The reported uncertainties are standard deviations. ments in 0.011 N NaOH solutions.
The Journal of Physlcal Chemlstty, Vol. 81, No. 10, 1977
Vzo,mL mol-' 82.56 i 0.03 82.54b 82.7 101.28 i 0.03 83.53 f. 0.03 83.31b 102.30 i 0.03 121.00 i- 0.02 105.93 i 0.02 105.7 f 0.lc
h, mL L
-0.54
i
0.05
- 0.478
-1.22 i -0.93 i - 1.01 -1.95 i -2.91 i -1.80 i -1.5
0.06 0.07 0.05 0.05 0.05
From measurements in 0.1 N NaOH solutions.
Ref This work 18 21 This work This work 18 This work This work This work 19 From measure-
985
Volume Changes in the Proton Ionization of Amines
-
TABLE 111: Apparent Molar Volumes, a, = V," + Svc'/zt hc, of Bifunctional Cyclic Amine Salts in Water at 25 "C" Substance MorDholine hydrochloride 4-Methylmorpholine hydrochloride Piperazine hydrochloride 1-Methylpiperazine hydrochloride 1,4-Dimethylpiperazine 'hydrochloride Triethylenediamine hydrochloride Piperazine dihydrochloride 1-MethvlDiDerazine dihyd;&loride 1,4-Dimethylpiperazine dihvdrochloride Trietiylenediamine dihydrochloride
-
No.of expt
Concnrange, M
18
0.01-0.79
92.99
i
0.04
-0.17
i
0.09
This work
17
0.01-0.67
111.71
i
0.06
-0.53
t
0.19
This work
12
0.01-0.27
95.12
i
0.06
-0.30
* 0.44
This work
10
0.02-0.51
113.97
i
0.07
-0.86
This work
12
0.01-0.43
132.61 ?: 0.05
-1.15
* 0.25 * 0.20
15
116.97 i 0.06 117.03 t 0.06 99.26 r 0.11
-0.90 -0.9 -4.05
* 0.20
15
0.01-0.61 0.03-0.29 0.02-0.22
This work 19 This work
12
0.01-0.41
117.48
* 0.07
-3.63
This work
13
0.01-0.40
135.76 i 0.09
-3.57
* 0.40 * 0.45
14
0.01-0.33
118.55 i 0.14
-4.01
t
0.75
This work
V,", mL mol"
h , mL L mol-,
i
Ref
This work
0.96
This work
The S, values are 1.868 and 9.706 mL L 1 / ,m ~ l - for ~ 'mono~ and dihydrochloride salts, respectively.16 The reported uncertainties are standard deviations. TABLE IV: Thermodynamic Parameters for Ionization of Selected Cyclic Amines in Water at 25 a Ca
BH,'+ + BH' BH'
+
Substance
pK,
AH,', kcal mol-'
Morpholine 4-Methylmorpholine Piperazine 1-Methylpiperazine 1,4-Dimethylpiperazine Trie thylenediamine Pyrrolidine 1-Methylpyrrolidine Piperidine 1-Methylpiperidine
8.49 7.73 9.73 9.10 8.23 8.82 11.31 10.46 11.12 10.08
9.33 6.56 10.25 8.40 6.66 7.21 13.03 9.05 12.76 9.44
B t H+ AS,", cal mol-' deg-'
AV,", mL mol-'
- 7.6 - 13.6
7.4 7.4 6.2 6.2 6.2 6.8 3.9 4.5 3.2 2.9
- 10.1 - 13.5
- 15.4 - 16.2
-8.0
- 17.5 - 8.1 - 14.4
t
H'
pK,
AH,", kcal mol-'
AS,", cal mol-' deg-'
AV,", mL mol-'
5.33 4.94 4.18 2.95
7.43 4.00 3.96 3.00
0.5 -9.2 -5.8 -3.4
13.7 14.3 14.7 16.3
a pK, AH",and A S " values were taken from ref 22 (morpholine, piperazine), ref 23 (4-methylmorpholine in 0.5 M KNO,; a value of pK, = 7.41 at p = 0 was reported in ref 24), ref 25 (1-methylpiperazine, 1,4-dimethylpiperazine in 0.1 N KNO,), ref 26 (pyrrolidine, piperidine, first ionization of triethylenediamine), ref 27 (second ionization of triethylenediamine at p = O . l ) , ref 28 (1-methylpyrrolidine, 1-methylpiperidine). For pyrrolidine, 1-methylpyrrolidine,piperidine, l-methylpiperidine the AV,"values were taken from ref 29.
the aliphatic methylamine series.20 The observed variation in A s l o on increasing the number of methyl groups on the nitrogen atom (primary secondary t e r t i a r ~ ) , ~according ' to the model proposed by Trotman-Di~kenson,~~ is due to the different number of molecules of hydration attached to the charged nitrogen center. Since water bound to a charged center has presumably a different volume than that of bulk water, a variation in AV10 would be expected on going from the secondary to the tertiary N-methylated amines. In order to rationalize the lack of variation in AVlo, the following compensation phenomenon is proposed: the difference in the contribution to V20 of the methyl group in a neutral molecule and in the cation acid is equal to the difference in the contribution of a hydrogen atom attached to a neutral nitrogen atom and one bound to a charged nitrogen center. An indirect confirmation of this hypothesis is furnished by the practically identical estimated values of the above differences. On the basis of intrinsic volumes reported by Edward,32the difference in volume of a hydrogen atom on N and N+ centers, V20(HN)- V2'(H~+)= 4.4 mL is predominantly attributed to the fact that the volume of the bulk water is larger than the volume of the water
-
-
attached to a charged nitrogen center. On the contrary, the volume difference of a methyl group on N and N+ centers, vzo(MeN)- V20(Me~+) = 4.5 mL mo1-1,34is due mainly to intrinsic effects, although effects associated with changes in the arrangement of water molecules around the methyl group might also be taken into c o n ~ i d e r a t i o n . ~ ~ ~ ~ ~ ~ Unfortunately, the lack of data on the intrinsic volume of a methyl group bound to a charged nitrogen atom does not enable us to estimate the difference in volume of water molecules in the neighborhood of a methyl group, according to whether the nitrogen is charged or not. Comparison between Cyclic Monofunctional and Cyclic Bifunctional Amines. The data in Table IV show that the introduction of a second polar group in a cyclic amine causes an increase in AVlo. The greater the decrease in basicity caused by the presence of the hydrophilic group, the larger the increase in AVlo (cf. pK1 and AVlo, in the series piperidine piperazine morpholine and 1methylpiperidine 1-methylpiperazine 4-methylmorpholine). AHl0 shows a regular decrease which parallels the increase in AVlo. However, A s l o is almost independent of the nature of the second group introduced. Some indirect evidence for the larger value of AVI0 in bifunctional amines, when compared with monofunctional
--
- -
The Journal of Physical Chemistty, Vol. 81, No. IO. 1977
980
Cabani et al.
v,',
u,O
TABLE V: Differences, 8 of Limiting Partial Molar Volumes for Pairs of Molecules Differing in One Heteroatoma Process
X
V,",
fi
0.4 1.6 1.0 1.0 2.1
NH,+ FH3
C) -(11 r /N\
-
'73
/N\
i,J
NCH, NHCH;
18.8 20.3 19.7 19.7 20.9
CH, 0 NH NCH,
18.4 18.7 18.8 18.7
CH, 0
-
mL mol-'
CH,
u,*
TABLE VI: Comparison between and Values of Bifunctional Cyclic Amines and Their Mono- and Dihydrochlorides in Water at 25 "C
Substance 1,4-Dioxane Morpholine Morpholine. HCl 4-Methylmorpholine 4-Methylmorpholine.HC1 Piperazine Piperazine. HCl Piperazines2HCl 1-Methylpiperazine 1-Methylpiperazine.HC1 1-Methylpiperazine.2HCl 1,4-Dimethylpiperazine 1,4-Dimethylpiperazine.HCl
a 7,' values for tetrahydropyran, piperidine, l-methylpiperidine, and 1,4-dioxane have been taken from ref 14.
amines, can be obtained by considering the volume change, bV20, for the hypothetical process of the type:
V,", mL mol-'
80.96 82.56 92.99 101.28 111.71 83.53 95.12 99.26 102.30 113.97 117.48 121.00 132.61 1,4-Dimethylpiperazine2HCl 135.76
-
v;,a
mL mol-'
V," - -
v;, mL
mol-'
81.0 0.0 81.4 1.2 96.0 -3.0 99.8 1.5 114.7 -3.0 81.8 1.8 96.4 -1.3 111.1 -11.8 100.2 2.1 114.8 -0.9 129.8 -12.3 118.6 2.4 133.5 -0.9 148.5 - 12.7
a Values b t a i n e d using eq 10. Cyclohexane has been assigned a V,"value of 102.5 mL mol-' obtained as an average of (a) 103.1 mL mol-', estimated from grolp contributions reported by Hdiland (ref 40) and from V," (cyclohexanol) = 103.54 mL mol-' (ref 201, and ( b ) a value of 101.8 mL mol-', estimated from V 2 e (1,4-cyclohexanediol) = 1 t 5 . 3 mL mo1-I (ref 41) and V (cyclohexanol). The V," values for tetrahydropyran, 1,4-dioxane, piperidine, and 1-methylpiperidine have been taken from ref 1 4 and those of piperidine.HC1 and l-methylpiperidine.HC1 from ref 29. ,O
(9)
in which the Y group, e.g., ethereal 0, is substituted by a Z group, e.g., NH; the X group in position 4 being CHz, 0, NH, NCH3, NH2+, or NHCH3+. SV," is the formal difference of the contributions of Z and Y to Vzo,and the changes in 6Vzo on varying group X may be attributed to the different interactions between the groups in positions 1 and 4. In Table V are reported values of 6 V20 for process 9 cited above. The substitution of an ethereal oxygen by a nitrogen center causes a greater change in SVzOin bifunctional compounds than in monofunctional compounds (X = CH2). This confirms earlier observations, based on other thermodynamic properties"~'~of neutral and charged species, that there are interactions between the polar centers in positions 1and 4. These interactions produce a smaller effect on volumetric changes in the neutral molecule than in the charged species. In order to confirm that the 1,4 interaction is responsible for the volume change, a comparison can be made between the experimental partial molar velum!, Vzo,for the cyclic bifunctional amine with the values, Vz*, calculated from the following equation:
to monofunctional molecules, is due to the interaction between the centers in 1,4 positions, in both the neutral molecule and the cation acid. It is not possible, however, to establish without a doubt if the deviations from the additivity rule, caused by the interaction between the polar centers, is greater in_the n_eutral molecule or the cation acid. For example, V20 - Vz* for morpholine is less than that for its corresponding hydrochloride, whereas for 1,4-dimethylpiperazine and its monohydrochloride the opposite is true. This uncertainty may beattributed to the ambiguity in the estimated value of V20 for cyclohexane. Notwithstanding this uncertainty, the l_argeteviations from the additivity rule for the dications (Vzo- V2* -12 mL mol-') indicate that it is the strong interactions between the positively charged nitrogen centers which lead to the unusually high AV20 values for the proton ionization process represented by eq 8. The values of AVZo will be discussed in greater detail in the following article.
Acknowledgment. This work has been supported through financial assistance from the Consiglio Nazionale delle Ricerche (C.N.R.).
References and Notes for which it is necessary to have a value for V20 of cyclohexane and for the cyclic hexatomic monofunctional molecules. The values for Vz* obtained from eq 10 are partial molar volumes of bifunctional molecules, in which it is hypothesized that there are no interactions between polar groups X and Y. From the data in Table VI, it is obvious that the experimental partial molar volumes differ, in all cases, from the partial molar volumes calculated using the hypothejis based on group additivity (eq 10). The experimental V20 values fo_r the neutral molecules is larger than the calculated V,* values, whereas the contrary holds true for the cation acids. This implies that the increase in AVlo for proton ionization in bifunctional molecules, with respect The Journal of Physical Chemistry, Vol. 81, No. IO, 1977
(1) Present address: Department of Chemistry, University of Calgary,
Calgary, Alberta, Canada. Kauzmann, A. Bodanszky, and J. Rasper, J . Am. Chem. SOC.,
(2) . . W.
84, 1777 (1962). (3) L. 0. Hepler, J. Phys. Chem., 69, 965 (1965). (4) . , B. E. Conway, R. E. Verrall, and J. E. Desnoyers, Z . Phys. Chem., 230, 157 (1965). (5) R. E. Verrall and B. E. Conway, J. Phys. Chem., 70, 3961 (1966). (6) B. E. Conway and L. H. Lallbert6, "Hydrogen Bonded Solvent Systems", A. K. Covlngton and P. Jones, Ed., Taylor and Francis, London, 1968, p 139. (7) E. J. King, J . Phys. Chem., 73, 1220 (1969). (8) F. J. Millero, Chem. Rev., 71, 147 (1971). (9) J. W. Larson and L. G. Hepler, "Solute-Solvent Interactions", J. P. Coetzee and C. D. Ritchi, Ed., MarceCDekker, New York, N.Y., 1969. (10) L. 0. Hepler and E. M. Wooley, "Water: A ComprehensiveTreatise", Vol. 111, F. Franks, Ed., Plenum Press, New York, N.Y., 1973. (11) S. Cabani, G. Conti, D. Giannessi, and L. Leporl, J . Chem. Soc.,
987
Volume Changes in the Proton Ionization of Amines Faraday Trans. 1 , 71, 1154 (1975). (12) S. Cabani, G. Conti, P. Gianni, and E. Matteoli, Oazzetta, 106, 541 (1976). (13) S. Cabani, G. Conti, and E. Matteoii, J. sdution Chem., 5, 125 (1978). (14) S. Cabani, G. Conti, and L. Lepori, J. fhys. Chem., 76, 1338 (1972). (15) (a) a's for processes a b and c are defined by the following rebtion: a = [-K+ (K+ 4 Kc$'/']/2c0,where Kand coare the dissociation constant and the initial concentration of 8, BH,' or BHZ2+,respectively. in processes a, b, or c. (b) The value of a for process d is calculated from: a = 2(Kl/K,)"2/[1 i2(Kl/K2)1/2],where K1 and K, are the dissociation constants for proton ionization processes b and c. In derhrimg a,processes b and c for the products of disproportionatii have been assumed to be negligible. This is a valid assumption for the solutions of the monohydrochlorides of the diamines here examined, because the difference between cBand c B H F does not exceed 1 ppm. The pKfor process d is related to pK, and pK, by PK = 'Iz(PK1 - P K d (16) 0. Redlich and D. M. Meyer, Chem. Rev., 64, 221 (1964). (17) L. H. Lalibert6 and B. E. Conwav. J. f h y s . Chem., 74. 4116 (1970). (18j 0. Kiyohara, 0. Perron, and J.-E. Desnoyers, Can. J . Chefh., 53, 2591 (1975). (19) W. Y. Wen, N. Takeguchi, and D. P. Wilson, J. Solutlon Chem., 3, 103 (1974). (20) S. Cabani, G. Conti, and L. Lepori, J. fhys. Chem., 78, 1030 (1974). (21) M. V. Kaulgud and K. J. Patil, J. fhys. Chem., 80, 138 (1976). (22) H. B. Hetzer, R. A. Robinson, and R. G. Bates, J. fhys. Chem.. 72, 208 1 (1968). (23) M. J. Blais, 0. Enea, and G. Berthon, Thermochim. Acta, 12, 25 (1975). (24) H. K. Hall, J . Am. Chem. Soc., 60, 63 (1958). (25) G. Berthon, 0. Enea, and K. Houngbossa, Thermochim. Acta, 9, 379 (1974). (26) F. M. Jones and E. M. Arnett, "Progress in Physical Organic Chemistry", Vol. 11, A. Streltwieser and R. W. Taft, Ed., Wiley, New York, N.Y., 1974. (27) J. W. Larson, G. L. Bertrand, and L. G. Hepler, J. Chem. Eng. Data, 11, 595 (1966). (28) S. Cabani, G. Conti, and L. Lepori, Trans. Faraday Soc., 67, 1933 (1971). (29) S. Cabani, G. Conti, L. Lepori, and G. Leva, J . fhys. Chem., 76, 1343 (1972). (30) D. H. Everett and W. F. K. WynneJones, R o c . R. SOC. London, Ser. A , 177, 499 (1940). (31) A. F. Trotman-Dickenson, J. Chem. Soc., 1293 (1949).
(32) J. T. Edward, J . Chem. Educ., 47, 261 (1970). (33) The volume difference, 4.4 ml-md-', between a hydrcgen atom bound to a neutral nitrogen atom, V2"(HN), and one bound to a charged nttrioge?center, v2"(yl+),has e n evaluated by: v2"(H,J--P[(HNt) 13[Vz"(NH3) - V(N)] - ' I 4 [Vc(NHc)+- V(N')], where Vzl (NH3) = 24.85 mL mol-' (ref 20) and V,"(NH? ) = 12.46 mL mol- . The partial molar volume of the ammonium ion was obtained by adding the conventional partial molar volume of the N H c ion reported by Millero' and the value of v2"(H+) = -5.4 mL mol-' reported by R. Zana and E. Yeager (J. Phys. C k m . , 71, 521 (1967)). The intrinsic volumes of a neutral nitrogen atom, V(N) = 4.3 mL mol-', and a charged nitrogen center, V(N+) = 2.8 mL mol-', were taken from Edward.32 If the above volume dlfference, 4.4 mL mol-', which hcludes intrinsic and sdvent co&b&on s, is combined with the itminsic volumes of a hydrogen atom on N, V(") = 3.1 mL mol-', and on N', V(HN+) = 1.4 mL mol-', a volume decrease of 2.7 mL mol-' is evaluated for Ute change bulk water water bound to a >NH+ group. (34) he value of 4.5 n i W' for the volume difference of a methyl group
-
(35) (36)
(37) (38) (39) (40) (41)
Ion, as well as from the intrinsic volumes of a carbon atom, V(C) 3.4 mL mol;', a_nd of a charged nitrosen center? V,"(Me,+) Pzo(MeN+)= /,[ Vo(CMe4)- V(C)] - /,[ Pz0(NMe2)- V(N ) ] . R. Zana and E. Yeager, J . fhys. Chem., 71, 4241 (1967). Value estimated on the basis of determined by C. Jolicoeur and 0. Laaolw (Can.J. Chem., 54,624, (1976)) for pentaefyiMtol(101.81 mL mol-'), l,l,l-trlhydroxymethylethane (102.3 mL mol-'), 2,2dimethyl-1 Spropanediol(lO2.34 mL mol-'), and neopentanol (101.87 mL moi-'). W. Y. Wen and S. Saito, J . fhys. Chem., 68, 2639 (1964). F. J. Miller0 and W. DrostHansen, J . fhys. Chem., 72, 1758 (1968). 8. E. Conway, R. E. Verrall, and J. E. Desnoyers, Trans. Faraday SOC.,62, 2738 (1966). H. HBland and E. Vikingstad, Acta Chem. Scand., Ser. A , 30, 182 (1978). S. Cabani, L. Lepori, and E. Matteoli, Chim. Ind. (Mllano), 58, 221 (1978).
v2"
Volume Changes in the Proton Ionization of Amines in Water. 2. Amino Alcohols, Amino Ethers, and Diamines S. Cabani," V. Mollica, Istituto di Chimica Fisica, Universiti di Pisa, Via Risorgimento 35, 56 100 Pisa, Italy
L. Lepori, and S. 1.Lobo' Laboratorio di Chimica Quantistica ed Energetica Molecolare del C.N.R., Via Risorgimento 35, 56 100 Pisa, Italy (Received November 19, 1876) Publication costs assisted by Conslglio Nazionale delle Ricerche, Rome
From density measurements (25 "C) of aqueous solutions at various concentrations of solute, apparent molar volumes of some open-chain bifunctional amines, with the amine group being primary, secondary, and tertiary, have been determined. The compounds studied were 2-aminoethanol, 3-amino-1-propanol, 2-methoxyethylamine, 3-methoxy-l-propylamine, ethylenediamine, 1,3-diaminopropane,2-(methylamino)ethanol,2-(ethylamino)ethanol, 2-(dimethylamino)ethanol,2-(diethylamino)ethanol,and their mono- and dihydrochlorides. AV," and AV," for the first and second steps of proton ionization have been calculated from the limiting partial molar volumes. AV," values of the above bifunctional amines are compared with the corresponding values for monofunctional amines. The difference in AV," caused by the introduction of the second hydrophilic center is discussed. The volumes and entropies of ionization of the amines and carboxylic acids are compared and the deviations of these functions from predictions based on simple electrostatic theories are considered. Introduction The earlier work' on the volumes of ionization of cyclic bifunctional amines is here extended to some selected open-chain bifunctional amines. The compounds (2aminoethanol, 3-amino-1-propanol, 2-methoxyethylamine,
3-methoxy-l-propylamine,ethylenediamine, 1,3-diaminopropane, 2-(meth~lamino)ethanol, 24ethylamino)ethanol, 2-(dimethylamino)ethanol, 2-(diethylamino)ethanol) have been chosen with a view to varying the amine from primary to secondary to tertiary, and to The Journal of Physical Chemistty, Voi. 81, No, 10, 1977
