M-2617

J. Chem. Thermodynamics 1991, 23, 877-881

Excess enthalpies of (butan- l -o l or 2-methylpropan-2-ol + formamide or N-methyl formamide or N, N-dimethyl formamide)

LIISA P I K K A R A I N E N

Department of Chemistry, University of Oulu, SF-90570 Oulu, Finland

(Received 5 February 1991)

The excess molar enthalpies of (butan-l-ol or 2-methylpropan-2-ol + formamide or N-methylformamide or N,N-dimethylformamide) were measured with a flow microcalorimeter at the temperature 313.15 K. The excess enthalpies are negative for (2-methylpropan-2-ol + N-methylformamide) at low mole fractions of the amide .and positive for the other mixtures. The results are discussed in terms of molecular interactions in the mixtures, especially those involving N-H protons.

1. Introduction

As an extension of ou r work on the t h e r m o d y n a m i c p roper t i e s of b inary mixtures con ta in ing ca rboxamides as one c o m p o n e n t we recent ly measured the excess enthalpies for {methanol or e thanol or p r o p a n - l - o l o r 2 ,2,2- t r i f luoroethanol + fo rmamide (F) or N-me thy l fo rmamide ( N M F ) o r N , N - d i m e t h y l f o r m a m i d e (DMF)}.(1,2j The a im of the studies was to see how the presence of the N - H p ro tons in the molecule of the amide affects the excess m o l a r en tha lp ies of the mixtures. The interes t ing results ob ta ined earl ier for {2-methy lp ropan-2-o l + N-me thy lace t amide (NMA)} (3) induced us to cont inue with ( b u t a n - l - o l o r 2 -me tby lp ropan-2 -o l + one of the formamldes) . Here we r epor t the excess m o l a r enthalpies HEm for [ ( 1 - x ) { C H 3 ( C H z ) 2 C H z O H or (CH3)3COH)} + x { H C O N H z or H C O N H C H 3 or HCON(CH3)2} ] a t 313.15 K, the same t empera tu re as used earlier. The results are c o m p a r e d with those for {the same a lkano l + N M A or N ,N-d ime thy l ace t amide (DMA)}.(3' 4)

2. Experimental

The a lkano l s and the amides were the same as had been used ear l ier ) 1 47 The excess en tha lp ies were de te rmined with a flow mic roca lo r ime te r as descr ibed earlier. ~4)

0021-9614/91/090877+05 $02.00/0 © 1991 Academic Press Limited

878 L. P I K K A R A I N E N

TABLE 1. Excess molar enthalpies at the temperature 313.15 K

H~ H~ H~ H~ H~ H~ J" mol ~ x J. mo l - ~ x J- mo l - ~ x J. mol ~ x J. mol 1 x J ' mo l - J

0.1025 0.1459 0.1864

0.1033 0.1338 0.1710

0.0807 0.1053 0.1358

0.1058 0.1504 0.1918

0.0739 0.1066 0.1380

0.0833 0.1087 0.1981

(1 - x ) C H 3 ( C H z ) 2 C H 2 O H + x H C O N H 2

501 0.2343 828 0.4597 1022 0.5531 1018 0.7583 724 0.9032 369 623 0.3171 956 0.4810 1041 0.6335 944 0.8219 589 0.9197 310 736 0.3634 965 0.5363 1014 0.6978 864 0.8514 521 0.9586 171

(1 - x ) C H 3 ( C H 2 ) 2 C H 2 O H + x H C O N H C H 3

309 0.2385 615 0.3846 787 0.5382 817 0.7568 577 0.8853 315 380 0.2779 665 0.4382 821 0.6089 790 0.7944 505 0.9398 173 468 0.3646 767 0.4549 830 0.6791 694 0.8629 365

(1 - -x )CH3(CHz)zCH2OH+xHCON(CH3) 2

364 0.1926 697 0.3727 954 0.4703 959 0.6385 798 0.8274 451 350 0.2267 778 0.3886 964 0.5425 926 0.7033 700 0.8547 394 549 0.3225 922 0.4430 963 0.6120 827 0.7464 632 0.9224 220

(1 - x ) ( C H 3 ) 3 C O H + x H C O N H 2

43 0.2407 197 0.4898 453 0.6416 494 0.8269 344 0.9222 197 85 0.3247 304 0.5447 482 0.7051 469 0.8563 308

136 0.3715 337 0.5618 479 0.7608 410 0.9062 221

(1 -x ) (CH3)3COH + x H C O N H C H 3

- 2 0 9 0.1927 - 2 0 2 0.3021 - 4 6 0.5196 260 0.6867 344 0.8670 221 - 2 3 0 0.2449 - 1 4 6 0.3930 107 0.5469 296 0.7632 318 0.8889 191 - 2 4 7 0.2851 - 7 1 0.4469 179 0.6173 337 0.8001 289 0.9418 113

(1 - x ) ( C H 3 ) 3 C O H + x H C O N ( C H 3 ) 2

180 0.2330 501 0.4517 786 0.6204 730 0.7530 532 0.9249 188 231 0.3303 650 0.4790 794 0.6466 715 0.8323 398 423 0.3810 723 0.5512 778 0.7105 609 0.8590 336

3. Results and discussion

The excess molar enthalpies of the binary mixtures are reported in table 1, and are presented graphically as functions of the mole fraction x of the amide in figures 1 and 2. To aid comparison of the excess molar enthalpies of the mixtures of a formamide with those of an acetamide, the H~(x) curves for {butan-l-ol or 2-methylpropan-2-ol + NMA or DMA} are included in the figures.

The excess molar enthalpies were fitted to the equation:

HEm/(J.mol 1)= X(1--X) ~ Ai(1-2x)k i=0

(1)

The parameters A/of these equations together with the standard deviations of the fits are reported in table 2.

The carboxamides are relatively good proton acceptors and form O - H .-. O = C hydrogen bonds with alcohols/s) However, the excess enthalpies for (butan-l-ol + any of the amides studied) are positive. The results indicate that the prevailing contributions in these mixtures are due to the break-up of interactions between like

H~{(1-x)CH3(CH2)2CH2OH OR (CH3)3COH + x H C O R } ; R = NH2, NHCH3, OR N(CH3) 2 8 7 9

I , , I , , , J

900

~ 6 0 0

3OO

0 0 0.2 0.4 0.6 0.8

x

F I G U R E 1. Excess molar enthalpies H~(x) for {(1 -x)CH3(CH2)2CH20H + xHCOR}: O, R = NH2; ~ , R = NHCH3; O, R = N(CH3) 2. -, {(1-x)CH3(CH2)2CH2OH + xCH3COR}: 1, R = NHCH3; t3) 2, R = N(CH3)2} 4~

molecules, which in D M F are dipolar interactions, t6 8) in N M F and F are N - H . . - O = C hydrogen bonds, tv'9' 10) and in butan-l -ol are O - H - . " O hydrogen bonds, t11) The excess enthalpies for the different amides are relatively close to one another, which indicates that the opposing contributions due to the substitution of

t i i i i i i ~ i i

600 ~E

0 0.2 0.4 0.6 0.8 X

F I G U R E 2. Excess molar enthalpies It~(x) for {(1 -x ) (CH3)3COH + xHCOR}: D, R = NH2; I[], R = NHCH3; I , R = N(CH3) 2. - - - , {(1-x)(CH3)3COH + xCH3COR}: 1, R = NHCH3; (3) 2, R = N(CH3)2J 4J

880 L. P I K K A R A I N E N

TABLE 2. Parameters A i of equation (1) and the standard deviations s

A o A 1 A 2 A 3 A, s

(1 -- x)CH3(CH2)zCH2OH + x H C O N H 2 4121.0 338.10 370.66 724.66 1106.00 10.1 + x H C O N H C H 3 3323.5 184.51 -459.51 -26.21 417.11 6.0 + xHCON(CH3) 2 3777.2 1152.0 251.16 --183.68 6.1

(1 -- x)(CH3)3COH + x H C O N H 2 1825.3 1133.0 --494.65 -314.12 7.0 + x H C O N H C H 3 988.8 --2159.6 -1799.2 -1076.5 --377.18 4.6 +xHCON(CH3) 2 3175.3 - 7 6 . 4 -1501.9 -209.47 878.84 6.8

the N H protons for the C H 3 groups are in near balance. The excess enthalpies are slightly greater for DMF than for NMF, however, despite the somewhat better proton-accepting ability of the first. ~S) This suggests that the effect of N - H " . O hydrogen bonding between NMF and butan-l-ol outweighs the effect of break-up of the hydrogen-bonded chains of pure NMF. On the other hand, the excess molar enthalpies are slightly greater for F than for DMF and NMF. This might be due to the weaker proton-accepting ability of F than of NMF and DMF but also to the less effective association of the second NH proton to the oxygen atom of butan-l-ol than to the carbonyl group of F. In agreement with the proton-accepting abilities of the amides ~5) the excess molar enthalpies for a formamide are greater than those for the corresponding acetamide.

The excess molar enthalpies for the mixtures of 2-methylpropan-2-ol are smaller than those for the corresponding mixtures of butan-l-ol. The values are negative for NMF at low mole fractions of the amide and positive for F and DMF. The values increase in the order NMF < F < DMF and the differences between the amides are considerably greater than those for the mixtures of butan-l-ol. Accordingly, the presence of the N-H protons in the molecules of the amides give rise to greater net effects on the excess molar enthalpies for the mixtures of 2-methylpropan-2-ol than for the mixtures of butan-l-ol.

In earlier results for (an alkanol + NMA or DMA), 2:methylpropan-2-ol was found to display rather surprising features. ~3'4) As shown in figure 2, the excess enthalpy curve for (2-methylpropan-2-ol + NMA) has a relatively deep minimum at low mole fractions of the amide. Such a minimum is not present in the curves for the other (alkanol + NMA or DMA) mixtures. The phenomenon might be due to association equilibria involving the N - H proton or to special structural effects found only in (2-methylpropan-2-ol + NMA). The fact that a minimum, albeit less prominent, also occurs in the curve for (2-methylpropan-2-ol + NMF) suggest'~ that similar contributions are present in the mixtures of NMF. Since the oxygen atom of 2-methylpropan-2-ol is a better proton acceptor than the oxygen atom of butan-l-ol, a considerable part of the negative contributions in the mixtures of NMF and NMA is very likely due to formation of hydrogen bonds between the NH groups of the amides and the oxygen atom of 2-methylpropan-2-ol. The effect of this association outweighs the effect of break-up of the amide-amide self-association more effectively

HEm{(1 --x)CH3(CH2)2CH2OH OR (CH3)3COH + xHCOR}; R = NH 2, NHCH 3, OR N(CH3) 2 881

than does the assoc ia t ion of the N H p ro tons with the oxygen a tom of bu t an - l -o l . The presence of special s t ruc tura l effects in (2 -methy lp ropan-2-o l + N M F or N M A ) seems possible, as well, bu t the ques t ion canno t be answered on the basis of the present results. The excess enthalpies for the mixtures of F indicate tha t the subs t i tu t ion of a p r o t o n for the methyl g roup of N M F results in posi t ive net con t r ibu t ions to the excess enthalpies.

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