Thermodynamic properties of 3-pentanol + diethylamine
mixtures
Citation for published version (APA):
Diemen, van, A. J. G., Houtepen, C. J. M., & Stein, H. N. (1976). Thermodynamic properties of 3-pentanol + diethylamine mixtures. Thermochimica Acta, 15(1), 55-61. https://doi.org/10.1016/0040-6031(76)80091-3
DOI:
10.1016/0040-6031(76)80091-3
Document status and date: Published: 01/01/1976 Document Version:
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lkrmochimica Act ,, IS (1976) 55-61
@ &evicr Scientific Publishing Company. Amsterdam - Printed in Belgium
THERMODYNAMIC PROPERTIES OF 3-PENTANOL + DIETHYLAMINE
MIXTURES
A. J. G. VAN DIEMEN. C. J. M. HOUTEPEN AND H. N. STEIN
Lobororory of GezeraI Chemistry, Technological Vtzicersity. Eitdocen (ZXe NetherIan&)
(Received 28 August 1975)
Values for excess functions (HE, YE, GE, TSE) are reported for 3-pentanol+
diethylamine mixtures at 298.15 K. The results indicate bonds per molecule Zpentanol in excess diethylamine, per molecule diethylamine in excess 3-per&no!.
INTFtODUCXION
formation of three hydrogen and of two hydrogen bonds
Systems containing alcohols and amines are known to exhibit strong hydrogen bond formation’-J, resulting in strongly negative WE and TSE values, and in GE values which are (in absolute sense) smaller and which may be either positive or negative’.
In the present investigation, values for excess thermodynamic functions have been obtained for a binary amine+alcohol system not previously studied, consisting of a secondary alcohol (3pentanol) and a secondary amine (diethylamine)
of
similar molecular size and shape. The latter factor increases the reliability of some assump- tions made in the molecular interpretation of the data.EXPERIMENTAL
3-Pentanol, Merck (“Zur Synthese”), was dried on molecular sieve (Union Carbide type 4A) and distilled through a packed column of 1.5 m length at atmospheric pressure. The boiling temperature at 760 Torr* was 388.95-389.15 K (cf. literature6 value: 388.45 K). Its vapour pressure, measured at 20 different temperatures ranging from 303.30 io 389.00 K could be described by the Antoine equation:
log,,(pflorr) = 6.7265- 1014_7/(T/K- 125.13)
The refractive index nn (293.15 K) was 1_41060 (cf. literature6 value: 1.41&l), the density (if;:::;: 815.40 kg rns3 (cf. literature6 value: 816 kg m- 3).
56
Diethylamine, Merck (“Zur Synthese”), was dried on molecular sieve (Union Carbide type 4A), and distilled through a packed 1.5 m Ien,$h column_ The boiling temperature at 760 Torr was 3X45-328-65 K (cf_ literature vaIues: 328.48 K’, 328.60 K’)_ The refractive index n,, (293.15 K) was: 1.38497 (cf. literature6 vaIue:
I-3864)_ The density df;;::$ was: 698.84 kg m-’ (cf. literature values: 699-O’, 701 _66, 698.93’).
n-Heptane, Merck (“Zur Synthese”).
2-propanol, the same material as empIoyed previousIyg.
Me fhods
l’%e compositions of liquid mixtures were determined by refractive index.
Excess moiar volumes were measured both by means of a pycnometer, and diIatometricaIIy ” . Both methods were checked by measuring VE for the system n-hexane + benzene ’ ’ * ’ * ; agreement within +O.OI - IOP6 m3 mol-’ was obtained.
Excess enthalpies were determined by measuring differential enthalpies of soIutions of the pure components in various mixtures by means of an LKB 8700-l precision microcalorimeter at 298-15 K_ Special care was taken to avoid a vapour space in the calorimeter vessel. The instrument was checked by measuring HE for
2-propanol f benzene mixtures; agreement with data reported by Mrazek and Van Ness” and by Brown et al. ’ 4 within & I5 J mol- ’ was obtained.
Isobaric liquid-vapour equilibrium was determined in a recirculation still after Raai et ai_ 1 ’ as described in a previous paperg_
In Fig_ I, YE YS. x, for the systems Zpropanol -f- diethylamine and 3-pentanol + diethylamine, is compared with VE for the systems methanoI+diethyIaminea and ethanol + diethylamine *_ With increasing chain-length of the alcohol, VE decreases at a given composition, indicating a decreasing tendency for hydrogen bond for- mation, which is ascribed to sterical screening of the aicohol group by -CH, and +Hs groups as compared with -H atoms in CH,OH. The small differences between mixtures containing 2-propanol and 3-pentanol, respectively, at equal moiar fraction sug-m that increasing the alcohol chain-Iength beyond that in 2-propanol does not Iead to an increased screening of the aIcoho1 group.
Figure 2 shows H “, GE and TSE values. GE was calculated from LG equilibrium at 760 Torr (Fig- 3) by means of the equations:
Inl;-(298-15 K; 760 Tom; x1) = Inl;:(T=,; 760 Tom; x,)-
-1
298.15 K (Wi(_Y,)-hi)/RT2dTT-l
GE = RTC XI Inf;;. i
0 02 0-U O-6 OB 1 Xl
Fig. l_ YE as function of x, at 298 K for the systems: -, methanol + dirthyIaminee; - - -,
ethanol i diethyIamines; i-. Z-propanol i diethylamine (1); 0, 3-pentanoI+ diethylamine (1).
Fig. Z HE. GE and TSE as function of _re, for the system Epentanol+diethyIamine (1) at 2998 K.
x. GE; f). TSE; 92. HE.
l-ABLE 1
HI--B, AND HE IN MfXFURES Di~HY~MINE(l~~3-P~ANOL~Z~ . XX w&-k If=-fiz HE (kl moz- ‘) (W mz-‘) (klmuz-1) 0.000 0201 0260 0.297 0.33 1 0.382 0.427 0.460 0.506 0.527 0.570 0.600 0.659 0.742 0.830 I.Gm - 10.93 -6.21 -4.95 - 4.67 -4.10 - 3.39 -2.79 -2.44 --t-89 - I.69 --1.28 - 0.98 - 0.70 -0.37 -0.12 - - - 0.40 -0.73 - - 1.0s --I.sG - 1.92 - 2.26 -2-81 - 289 - - 3.76 -4.17 - 4.97 - 5.87 - 8.35 - - 1.57 - 1.83 - -2.0s -2.22 - 2.29 - 2-34 - 2.35 - 2.26 - - 2.09 - I;88 - 1.55 - 1.10 - TABLE 2
ACTIVITY C0EFFKIEW.S OF DlETHYLAMfNE cf;) AND PEhiANOL-3 cf2) AT 760 Torr LIQUID-VAPOR EQUILIBRIUM CONDITIONS
0.023 0.310 O-992 388.26 0.032 0.502 0.995 387.71 0.052 0.566 f.003 386.11 0.062 0.683 1.GO6 384.41 0.112 0.703 I.022 381.16 0.155 0.741 0.98 I 377.51 0.193 0.792 0.953 374.06 0.212 0.798 0.944 372.16 0.238 0.819 0.939 370.21 0.274 0.839 0.942 367.26 0.373 0.843 0.963 360.16 O-418 0.852 0.947 357.36 0.462 0.868 0.923 354_46 0.507 0.924 0.922 350.81 0.568 0.944 0.867 346.66 0.647 0.945 0.746 343.26 0.716 6.969 0.703 339.61 0.798 0.997 0.558 335.71 0.849 1.011 0.419 333.56 0.907 1.G2I 0.257 321.26
59 For these calculations, the vapour pressure of diethyjrlamine at the LG equilibrium temperatures was calculated from Copp and Everett’s Antoine equation’ 6; deviations from ideal gas behaviour were taken into account for diethylamine (the component with a vapour pressure, at some LG equilibrium temperatures, exceeding 760 Torr) by Hougen et al’s method *’ _ H-(Xi)--lli, WAS obtained by interpolation between
experimental values (Table 1). Tb; activity coefficients are tabulated in Table 2_ In TabIe 3 data concerning the differential enthalpies of mixing at low pentanoi- 3 and low diethylamine concentrations respectively are given. From these data the differential enthdpies of mixing at infinite dilution were extrapolated (see Discussionj. TABLE 3
ENTHALPY CHANGE ON ADDING 3-PENTANOL TO n-HEPTAPU’E Range
of
finai x (3-penfanol)
No. of l xperimenfs Range of AH (kJ per mol3-penfad)
0.0106-0.0125 5 I 9SO-20.27
0.00264003 I 6 21.28-21.80
The excess functions are nearly symmetrically arranged around x =O.S, as observed for other binary alcoholfamine systems by Krichevtsov and Kornaro~‘~ and by Nakanishi et al. ‘_ As in similar systems s, HE-cZ”SE~GE<O at 298.15 K over the whole concentration range. The absolute values of HE and TSE in equimolar mixtures, however, are lower than in the cases methanoI+~ethyIa~ne’ and ethanol + diethylamine’ 6, indicating that hydrogen bond formation tendency decreases with increasing alcohol chain-length (cf. the VE values).
The nearly symmetrical HE, GE, TSE and YE curves suggest predominant formation of symmetrical, e.g_ l:l, complexes, as found in similar systems by Stevenson4 from spectroscopic evidence_ Lambert and Zeegers-Huyskens3 assume I:1 complexes in dilute solutions of alcohols and diethylamine in cyclohtxane, although NMR data indicate association of diethyiamine with polymolecules of alcoholslg.
It follows from the Hi--hi -.-alues (Table 1) that it is an oversimplification to think of 1: 1 complexes, at least at infinite dilution, both of diethylamine in 3-pentanol, and of 3-pentanol in diethy~ne- in order to see this, we consider the process of adding isothermally one mol of 3-pentanol to a large amount of diethylamine_ The resulting enthalpy change (Ha -h for 3_pentanol= - 8.35 kJ) may be considered to be composed mainly of 3 parts:
(a) -AH (self association) of 3-pentanol; i.e. the enthalpy change associated with changing OH . . . 0 contacts into non-hydrogen-bonding contacts (e.g., contacts
with aikyiic -CHz-groups). This quantity was taken to be 22.1 kJ mol- I, found on extrapolating AN values for adding small amounts of 3-pentanol to n-heptane
60
(Table 3) to infinite dilution, This vaIue is of the same order of magnitude as values reported for other alcohoIszWZ2_
(b) AH (network destrucrion) of the compound present in excess: some bonds between diethylamine moIecuIes will be broken, when hydrogen bonds with alcohol inoIecuIes are formed.
(c) AH (bond formation).
Other than hydrogen (e.g., dispersion force) bonds are thought to be equal between amine moIecuIes; alcohol moIecuIes; or amine and aIcoho1 molecules The approximation invoIved in this assumption is considered to be smaIi compared with the effects of changes in hydrogen bonding (see also ref. 231, especially since the components in the 3+entanoI+diethylamine mixtures have simiiar moIecuIar sizes and shapes. Then, on adding one mo1 of 3-pentanol to excess diethyiamine, AH [bond formation) -i-AH (network destruction) = -30.5 W. Since the enthaIpy change on formation of one hydrogen bond bebwen an alcohol and diethylamine is known to be - I I-0 kJ mol- ’ (ref. 3) and since AH (network destruction) is a positive quantity, it foIIows that per molecule of 3-pentanol more than one hydrogen bond has to be formed in order to account for the -30.5 kJ mentioned_ In fact, the structure of 3pentanoI is compatible with three hydrogen bonds being formed (see Fig_ 4a).
,T-”
R H
Fig. 4. Hydrogen bond formation possibilities for: (a) 3-pentanol in excess diethvlamine; (b) di- ethybmine in excess 3-pentanof.
A simiIar calculation for diethylamine in excess 3-pentanol, where H” -h = - 10.93 kJ mol- I and -AH (self association) is estimated to be 8 kJ mol- ’ by comparison with similar systems,z3’24 results in:
AhH (bond formation) + AH (network destruction) = - 19 kJ on adding one mo! of diethylamine to excess 3-pentanol. This indicates two hydrogen bonds being formed per diethyfamine moIecuIe, as indeed is compatible with the moiecular struc- ture (Fig 4b). These bonds may be directed both towalds the same aIcoho1 molecule; this situation might be described as a iri cumpIex_ However, when excess afcohof molecules are present, there is no need to assume it; even if it appears, the alcohol motes&e concerned will have the possibility to be ?inked with other alcohol molecules.
61
The authors gratefully acknowledge the assistance of C. I-L C. Fransen in
carrying out the VE measurements.
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