Computational Thermochemistry - ACS Publications - American

Chapter 3

Downloaded by STANFORD UNIV GREEN LIBR on September 30, 2012 | http://pubs.acs.org Publication Date: February 1, 1998 | doi: 10.1021/bk-1998-0677.ch003

Estimation of Enthalpies of Formation of Organic Compounds at Infinite Dilution in Water at 298.15 K A Pathway for Estimation of Enthalpies of Solution Eugene S. Domalski Physical and Chemical Properties Division, National Institute of Standards and Technology, Gaithersburg, M D 20899

A method has been developed for estimation o f enthalpies o f formation o f organic compounds at infinite dilution in water at 298.15 K . This method provides a pathway for estimation o f enthalpies o f solution at infinite dilution when enthalpies o f formation o f the compound in the gas, liquid, or solid phase are also available. The approach taken follows the well -established predictive techniques devised by S.W. Benson and coworkers (1-5). These techniques have had wide usage in group additivity calculations o f thermodynamic properties and reactions for organic compounds, primarily in the gas phase. In this chapter, the coverage o f organic compounds is limited to those compounds which contain the elements carbon, hydrogen, and oxygen. A tabular comparison is provided which shows the difference between experimental and estimated values for enthalpies o f solution at infinite dilution in water at 298.15 K.

Because of the recent extension to include the liquid and solid phases in group additivity schemes for the prediction o f thermodynamic properties o f organic compounds (6), it appeared reasonable to develop the capability o f these schemes further to accommodate prediction for infinitely dilute aqueous solutions. Other researchers as well have developed additivity schemes for predicting the thermodynamic properties o f organic compounds in dilute aqueous solution. Nichols et al. (7) derived a simple group additivity scheme for partial molar heat capacities o f non-electrolytes in infinitely dilute aqueous solutions to facilitate the interpretation o f results from thermodynamic studies on biochemical systems. Cabani et al. (8) developed group contribution parameters for thermodynamic properties o f hydration associated with the transfer o f non-ionic organic compounds from the gas phase to dilute aqueous solution.

This chapter not subject to U.S. copyright. Published 1998 American Chemical Society

In Computational Thermochemistry; Irikura, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1905.

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COMPUTATIONAL THERMOCHEMISTRY


The Estimation Approach From a search o f the literature, experimental data for enthalpies o f solution at 298.15 K at infinite dilution and experimental data for enthalpies o f formation at 298.15 K for organic compounds in the gas, liquid, or solid phase were summed to yield the corresponding enthalpies o f formation at 298.15 K for the infinitely dilute solute. From this collection o f data, group values were derived which permit the estimation o f the enthalpy o f formation for the solute at infinite dilution at 298.15 K . The difference between the enthalpy o f formation for a given compound in aqueous solution at infinite dilution and its enthalpy o f formation in the gas, liquid, or solid phase, both at 298.15 K , is the enthalpy of solution at infinite dilution; i.e., A H ° ( a q ) - [ A H ° ( g a s ) , A H°(liquid), o r A ^ ° ( s o l i d ) ] = A H°(aq). f

f

f

sol

Numerically, the enthalpy of solution for an organic compound in water is a small quantity in comparison to its enthalpy o f formation in aqueous solution or its enthalpy o f formation for a specific physical phase. Hence, extreme accuracy is needed for both enthalpies o f formation. Unfortunately, the number o f calorimetric data reported for enthalpies o f solution at infinite dilution in water are limited in comparison to the data available for enthalpies of formation o f gases, liquids, or solids which are usually derived from combustion or reaction calorimetry. Hence, the coverage provided here shows comparisons between experimental and estimated values o f the standard molar enthalpy o f solution at infinite dilution for only 17 hydrocarbons and 46 organic oxygen compounds. Upon occasion, in addition to calorimetric data, good quality data for the enthalpy o f solution o f a substance can be derived from a knowledge o f its solubility as a function o f temperature. However, calorimetric data are preferred because high accuracies and small uncertainties are more readily achievable using this technique. Development of Group Values In the notation for molecular groups developed by Benson and coworkers, C denotes an aliphatic carbon atom, C is a doubly-bonded carbon atom, C is a triply-bonded carbon atom, and C is a benzene-type carbon atom. The development of group values begins with aliphatic hydrocarbons in order to establish the group values C-(H) (C), C - ( H ) ( C ) , C -(H)(C) , and C - ( C ) upon which further development can be made for other families of compounds such as alcohols, ethers, aldehydes, ketones, acids, and esters. Because of the limited amount and uneven quality of data for enthalpies o f solution, no global least squares, least sums, or regression-type fit o f the data was carried out. The generation o f group values was in part manual and in part computer-assisted. Some computations o f average values or average deviations were performed using a desk-top calculator. Others were made using computer spread-sheet analysis. Group values generated for hydrocarbons were held fixed for the generation o f non-hydrocarbon values. The value derived for C-(H) (C) = -51.65 kJ/mol for hydrocarbons, was kept constant for a methyl group attached to any other element or functional group, i.e., C-(H) (C) = C - ( H ) ( 0 ) = C-(H) (CO). Experimental data for some compounds are used to form the basis for a missing group and group value. Some examples are: ethane which forms the basis for the C-(H) (C) group, 2-methylpropane which forms the basis for the C-(H)(C) , and methanol which forms the basis for the 0 - ( H ) ( C ) group. Consequently, differences between d

t

B

3

3

2

4

3

3

3

3

3

3


2

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DOMALSKI

Enthalpies of Formation of Organic Compounds

49

experimental and estimated A H ° ( a q ) values for such compounds will be zero. Table I offers an example o f how the estimation procedure is performed for 1-hexanol. Group values are summed to give A H ° ( a q ) = -383.90 kJ/mol. Data also provided are: the equation for the solution reaction, experimental values for A H ° ( a q ) and references, a selected value f o r A H ° ( a q ) , and the estimated 4oi H ° ( a q ) . The reader interested in additional detail concerning the second-order group-additivity method or definitions o f the group notation is referred to earlier publications (1-6). Table II provides the values for organic groups and group values so that the enthalpy o f formation of the infinitely dilute solute in water, A H ° ( a q ) , at 298.15 K can be estimated. Group values applicable to hydrocarbons and hydrocarbon moieties are listed under CH group while group values applicable to organic oxygen compounds and related moieties are listed under CHO group. Cyclic compounds require an additional group value for a ring strain correction (rsc) and are shown at the end of the table. sol

f

sol


sol

f

Estimation of Enthalpies of Solution for Hydrocarbons (CH) and Organic Oxygen Compounds (CHO) Table III provides a comparison of a selected experimental A H ° ( a q ) with the estimated A jH°(aq). In this comparison, each organic compound is identified by: (a) its name and group notation obtained from Table II, (b) the equation for the solution reaction specifying the physical phase o f the reactant, the experimental A H ° ( a q ) values for the reaction, the corresponding (references), and a selected value, S V , for A H ° ( a q ) . The last entry for each compound, (c), gives the estimated A H ° ( a q ) which is derived from the difference between A H°(aq) and A , H (gas), (liquid), or (solid). The corresponding (reference) for the latter value is also provided. sol

so

sol

sol

sol

0

f

Hydrocarbons The agreement between experimental and estimated enthalpies o f solution for the hydrocarbons listed in Table III appears to be quite satisfactory. Calorimetric data are available for both the gas and liquid phases for pentane, hexane, and cyclohexane. Differences between the selected A , H ° ( a q ) based on experiment and the estimated A ^f°(aq) are within the experimental uncertainities for these hydrocarbons except for liquid hexane which is -0.84 kJ/mol. Another significant difference appears to be that between experimental and estimated values for n-propylbenzene, amounting to 1.64 kJ/mol; otherwise, agreement is usually within ±0.20 kJ/mol. High quality data for both experimental enthalpies o f solution and enthalpies o f formation in the gas and liquid phases are essential if good agreement between selected and estimated A , H ° values is expected. S0

so

so

Alcohols Agreement between experimental and estimated enthalpies o f solution for aliphatic alcohols is reasonably good; the absolute value o f most differences is less than 1 kJ/mol; the average deviation is 0.60 kJ/mol.


50


Table I. Estimation of the Enthalpy of Aqueous Solution of 1-Hexanol at 298.15 K 1-Hexanol: C H - C H - C H - C H - C H , - C H - O H group AjH °(aq), kJ/mol groups group sum, kJ/mol 3

C-(H) (C) C-(H) (C) C-(H) (C)(0) 2

2


2

?

2

-51.65 -24.05 -41.95 -194.10

3

2

2

0-(C)(H)

-51.65 -96.20 -41.95 -194.10

1 4 1 1

-383.90

total sumCAfH^aq))

C H 0 ( l i q ) = C H 0 ( a q ) : -4.64±0.33 (9), -6.56±0.13 (10), -6.41±0.04 (11), -5.77±0.10 (12) Selected Value = -6.41 est*d A . J - T = ( A H ° ( a q ) , - 3 8 3 . 9 0 ) - (A H°(liq),-377.50 (13)) = -6.40 6

14

6

14

f

f

Table II. Group Values for Estimating Enthalpies of Formation in Water at Infinite Dilution at 298.15 K CH group CHO group Afl°(aq) Afi (aq) kJ/mol kJ/mol -51.65 C-(H) (C) -51.65 C-(H) (0) -41.95 C-(H) (C) -24.05 C-(H) (C)(0) -33.70 C-(H)(C) -3.42 C-(H)(C) (0) C-(C) -27.60 13.56 C-(C) (0) C -(H) -33.86 18.02 C-CH^CaXO) -31.46 C -(H)(C) 31.75 C-(H) (C )(0) C-(H )(C ) -51.65 -194.10 0-(H)(C) -284.90 C-(H) (C)(C ) -23.54 0-(H)(CO) -201.00 106.79 0-(CO)(CH ) C,-(H) C -(H)(C ) -194.00 8.50 0-(CO)(C) C -(C)(C ) -115.30 22.89 0-(C) -155.00 C-(H )(C ) -51.65 CO-(C) C-(H) (C)(C ) -149.10 -24.05 CO-(C)(0) cyclopropane (rsc) 102.15 -158.89 CO-(H)(C) cyclohexane (rsc) -11.95 -141.30 CO-(H)(0) -51.65 C-(H) (CO) -24.55 C-(H) (C)(CO) -2.60 C-(H)(C) (CO) 26.96 C-(C) (CO) -0.60 C -(0)(C ) 13.63 cyclopentanol (rsc) -8.35 cyclohexanol (rsc) e

3

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2

2

3

2

4

d

3

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B

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B

B

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B

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DOMALSKI

Enthalpies of Formation of Organic Compounds

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a,G)-Alkanediols The values for the molar enthalpies o f solution and the enthalpies o f formation o f the liquid and solid phases are not always well-behaved for a,G)-alkanediols. The measurements o f A H ( a q ) for C through C diols by Nichols et al. (49) are o f very high quality in contrast to the A H ° ( a q ) measurements o f Corkill et al. (47) which are not. A l s o , general agreement o f AfH° for liquid and solid a,G)-alkanediols is lacking; their measurement has been a challenge to the thermochemist. Lastly, it is likely that a correction term is needed for O H - O H interactions. The magnitude o f this correction term seems to be in the vicinity o f 5-15 kJ/mol, however, until general agreement among the AjH° values for a,G)-alkanediols in the liquid and solid phases is achieved as well as a set of high quality A H ° ( a q ) values, one cannot proceed much further with estimation and prediction procedures for this class o f compounds. In addition, one should also expect a diminution o f the correction term as the number o f carbon atoms increases for a,G)alkanediol homologies. However, this diminution is not apparent from an examination of the current data. o

S0l

2

5


sol

sol

Ethers, Aldehydes, and Ketones Data on the enthalpies of aqueous solution for ethers, aldehydes, and ketones are limited in that precise and accurate measurements have not been made for a sufficient number o f compounds in a homologous series so that good quality group values may be derived for key groups such as 0 - ( C ) , CO-(H)(C), and C O - ( C ) . Values for these and related groups have been derived, however, their values may have uncertainties which are significantly large, i.e., ± 1 - 4 kJ/mol. 2

2

Monocarboxylic Acids The experimental data for A H ° ( a q ) o f the aliphatic monocarboxylic acids at 298.15 K represent the solute in an unionized standard state. Agreement between experimental and estimated values for A , H ° o f these acids is reasonably good and is usually better than ±0.5 kJ/mol. For some monocarboxylic acids, we have listed two estimated A H ° ( a q ) values to show that reasonably close agreement between A H°(liq) data o f 1-3 kJ/mol can result in significant differences with the selected value ( S V ) derived from experimental values for A , H ° ( a q ) . For example, two values for AH°(liq) are listed with heptanoic acid: -611.41±0.71 (68) and -608.27±0.88 (70) kJ/mol. The latter A , H ( l i q ) offers better agreement when combined with the estimated A H ° ( a q ) value o f -606.40 kJ/mol to calculate an estimated A ^ ^ a q ) , (1.76 vs. 1.87 kJ/mol) than the former A ^ i ° ( l i q ) , (1.76 vs. 5.01 kJ/mol). soI

so

soI

f

S0

0

f

Esters In order to establish as small a difference as possible between the experimental and estimated A ^ ° ( a q ) for esters, separate values were derived for the A H ° ( a q ) group for methyl and other kinds o f esters. For methyl esters the 0 - ( C O ) ( C H ) group value is -201.00 kJ/mol while for other esters the 0 - ( C O ) ( C ) group value is -194.00 kJ/mol. s o

f

3



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Table III. Estimated Enthalpies of Solution for Hydrocarbons and C H O Compounds at Infinite Dilution at 298.15 K (all values are in kJ/mol) (a) compound name and group notation (b) equation for solution reaction, expt 7 A H ° values(references), selected value SV so}

(c) est'd A H xnl

°(aq) = (est'd Afl °(aq) from Table II) - (expt 7 Afi °(g/liq/s) (ref))

1(a) ethane (2xC-(H) (C)) 1(b) C.H^g) = C.H^aq): -19.52±0.12 (14), -19.30±0.12 (15), -19.50±0.10 (16): SV = -19.45 Downloaded by STANFORD UNIV GREEN LIBR on September 30, 2012 | http://pubs.acs.org Publication Date: February 1, 1998 | doi: 10.1021/bk-1998-0677.ch003

3

o

1(c) A H°(aq) = (A,H (aq),-103.30) - (A H°(g), -83.85 (17)) sol

= -19.45

f

2(a) propane (2xC-(H) (C))+(lxC-(H) (C) ) 3

2

2

2(b) C H (g) = C H (aq): A H°(aq): -23.27±0.26 (14), -22.90±0.08 (75), -22.61±0.06 (18), SV=-22.93 2(c) A H°(aq) = (A H°(aq),-127.35) - (A ^(g),-104.68(7 7)) = -22.67 3(a) n-butane (2xC-(H) (C))+(2xC-(H) (C) ) 3(b) C H (g) = C H (aq):-25.92±0.17 (7

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