ARTICLE pubs.acs.org/JPCA
Group Additivity Values for Estimating the Enthalpy of Formation of Organic Compounds: An Update and Reappraisal. 1. C, H, and O John L. Holmes and Christiane Aubry* Chemistry Department, University of Ottawa, Ottawa, Ontario, K1N 6N5 Canada
bS Supporting Information ABSTRACT: This study examines critically the present state and utility of the Benson additivity schemes for estimating the enthalpy of formation of organic compounds. Old and new group additivity values (GAV) for a wide variety of compounds containing C, H and O are described and are revised where appropriate. When new terms are proposed, or old ones significantly altered, the rationale for so doing is provided. Corrections for such items as cis-isomer effects, gauche interactions, ring strain energies, double-bond position, conjugation effects, steric hindrance in aromatic molecules, etc. are included and discussed. Also provided are the thermochemical consequences of functional group replacements, in which one group in a molecule is substituted by another, thus providing quick short cuts to estimating new ΔfH0 values. Results derived from the new additivity terms are consistent with those produced by computational chemistry methods in general use.
’ INTRODUCTION There is still a surprising dearth of thermochemical data for many simple organic compounds, particularly their standard enthalpies of formation, ΔfH0, essential for determining the energy balance (exo- or endothermicity) of chemical reactions and the strengths of chemical bonds. A major source of such data is the NIST database1 that contains ΔfH0 and/or ionization energies and proton affinities for some 8000 molecules. A short assessment of the status quo for thermochemical data appeared in 2007.2 Where the desired data are doubtful or missing (e.g., some of the data listed by NIST for a given compound are in very poor agreement and no advice is given as to the best value), one approach is to use an additivity scheme, such as that first fully formulated by Benson.3 An alternative is computational chemistry, and the present methods (e.g., G3, CBS-QB3, CBSAPNO)4 provide ΔfH0 values having satisfactory accuracy, often to within (1 kcal/mol ((4 kJ/mol) of a reliable result from experiment. However, for compounds that are very difficult to prepare, owing to their high instability or reactivity, computations are the best resource and a recent publication provides a good example.5 These results in turn may be used to provide new additivity terms and so it is still quick and convenient to use additivity or a related method (see below) to estimate the numerous missing ΔfH0 values of organic compounds and/or to assess the reliability of a published experimental or computed datum. Two related approaches are described here. The first is the use of additivity coefficients, group additivity values (GAVs), chiefly those initially defined by Benson3 and later revised by Cohen6 that we have critically updated and extended. When appropriate, a rationale is given to justify the revised GAV. It must be emphasized that GAV terms evolve with the steady improvement r 2011 American Chemical Society
of ΔfH0 values and so the results obtained from their use remain wholly dependent on the accuracy of the available experimental and/or computed data from which they were derived. To achieve the best possible GAV terms, the key ΔfH0 values must therefore be critically evaluated and carefully selected. The second approach uses the established effects of atom or group substitutions in a compound of known ΔfH0 and we describe examples of some shortcuts to determine or assess a new ΔfH0. This latter method is particularly useful because it can lead to an easy familiarity with the mainly constant effect(s) on ΔfH0 resulting from a simple chemical substitution(s) within a molecule. In turn, such familiarity also allows a quick, critical assessment of new thermochemical data. What additivity cannot easily provide for are quantitative corrections for steric interactions of indefinite magnitude, where, for example, multiple substitutions at adjacent or nearby atoms can seriously inhibit free rotations, or lead to internal hydrogen bonding, or result in unexpected ring strain effects. Also, the position of substituents in an aromatic ring affects the ΔfH0 in a nonadditive manner. Some of these limitations will be discussed below.
’ REVISED GROUP ADDITIVITY VALUES (GAVS) A variety of methods7 have been used to represent the connectivity of atoms in organic compounds in schemes for estimating ΔfH0 values, but on balance we prefer that devised by Benson3 as being unambiguous and simple to use, and so in the tables and discussions that follow, we will use that notation. Received: March 23, 2011 Revised: August 11, 2011 Published: September 01, 2011 10576
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The Journal of Physical Chemistry A To obtain a molecular enthalpy of formation using Benson’s scheme, each atom is identified by a term that shows its connectivities, and to which a group additivity value (GAV) is assigned. The notation used to describe each term is simple. The central atom is stated first, followed in parentheses by the atoms to which it is bonded. For example, in the case of 1-bromo-3propanol (BrCH2CH2CH2OH), a compound for which there is no published ΔfH0, the following four terms are needed: C (C)(H)2(Br), C(C)2(H)2, C(H)2(C)(O), and O(H)(C). Each of these terms already has an assigned GAV, and their sum gives the desired ΔfH0. Thus, C(C)2(H)2 = 20.9 kJ/mol, C(Br)(C)(H)2 = 23 kJ/mol, C(H)2(C)(O) = 33 kJ/ mol, O(H)(C) = 159 kJ/mol, and their sum, 236 kJ/mol, yields ΔfH0(BrCH2CH2CH2OH). Double bonded, triple bonded, and aromatic carbons are identified as Cd, Ct, and CB, respectively. All the GAV terms presented in the tables below are from the analysis of the available reliable enthalpies of formation from experiment or chemical computation for structurally related molecules. Since Benson’s initial GAVs, published over thirty years ago, the thermochemical literature has expanded considerably. Cohen undertook a major review of Benson’s GAV terms in 1996, but progress in computational chemistry during the past few years now allows ΔfH0 values to be determined with chemical accuracy and this, combined with recent experimental results, has significantly expanded the available data. In light of these advances we have reviewed, updated, and when possible, extended the earlier terms. The complete set of revised GAVs is shown in the tables that follow. When particular difficulties are to be addressed, an appropriate discussion or critique of the available data is provided. All the GAV energies are given in kJ/mol and for the most part we do not report any figures after the decimal point, there being no justification for so doing except when the available experimental data are conspicuously closely reproduced. This is rarely the case, apart from in some homologous series. In general, it is difficult to assess accurately the errors inherent in all of the GAV terms given in this work. The change in data presentation from kcal/mol to kJ/mol has resulted in a different mindset concerning the precision of enthalpy data. A range of (1 kcal/mol is currently the precision of many advanced chemical computational methods and such a spread would represent good agreement between theory and experimental results. Indeed, where experimental and computed values are close, we have averaged them. On this basis it is proposed that for the majority of organic compounds’ enthalpy of formation derived from GAV terms for which no individual error is given, an overall uncertainty of (4 kJ/mol is reasonable. However, when an individual error ((x) is shown in the tables, it has been estimated from consideration of the available data. This will result in an uncertainty of up to ( (4 + x) kJ/mol for the derived ΔfH0 value. The reliability of the GAVs and the ΔfH0 values obtained from the additivity scheme will thus reflect the accuracy of the available reference data. In this paper we deal only with compounds containing carbon, hydrogen and oxygen. A second article will include the elements C, H, N, O, S, and halogen.
1. CARBON AND HYDROGEN Hydrocarbons comprise a group of molecules having some of the most reliable thermochemical data, not least because of their long-standing industrial importance.
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Table 1.1. Group Additivity Values for Hydrocarbon Terms terms
GAV (kJ/mol)
terms
GAV (kJ/mol)
Saturated Hydrocarbons C(C)(H)3
42.0
C(C)3(H)
7
C(C)2(H)2
20.9
C(C)4
2
Unsaturated and Aromatic Hydrocarbons Cd(H)2
26.2
C(Cd)(H)3
42.0
Cd(C)(H)
35.8
C(Cd)(C)(H)2
20.1
Cd(C)2
42.6
C(Cd)2(H)2
18.0
Cd(Cd)(H)
28.4
C(Cd)(C)2(H)
7.0
Cd(Cd)(C) Cd(Cd)2
36.7 53 ( 3
C(Cd)2(C)(H) C(Cd)(C)3
7.0c 6(3
Cd(Ct)(H)
36
Cd(Ct)(C)
40
Cd(CB)(H)
28
Cd(CB)(C)
36
Cd(CB)2
50 ( 4a
Ct(H)
113
C(Ct)(C)(H)2
20
Ct(C) Ct(Cd)
115 126
C(Ct)(C)2(H) C(Ct)(C)3
6.5 5.5
42
Ct(Ct)
118
Ct(CB) + CB(Ct)
125 ( 4b
CB(H)
13.8
C(CB)(H)3
CB(C)
23.0
C(CB)(C)(H)2
20
CB(Cd)
27
C(CB)(C)2(H)
4
C(CB)(C)3
11 ( 2
CB(CB)d
21 ( 3
C(CB)2(H)2 C(CB)3(H)
20 4
Callene
141 ( 2
C(CB)4
31
C(Cd)(CB)(H)2
18
a
Refer to section 1c for discussion. b Note that these two terms are combined. c Reference 8. d Not to be used for fused rings larger than naphthalene (see discussion in section 1h).
Although many of the data presented in the following tables remain essentially unchanged from the original Benson formulation3 and/or Cohen’s results,6 they too have been reviewed and are included for completeness. All the revised and new terms are in italics. Where significant changes have occurred or ambiguities clarified, a discussion is provided in the text. Note that differences from previously established GAV terms, of less than (4 kJ/mol are ignored. This first section concerns the GAV terms used to obtain ΔfH0 values for saturated, unsaturated, and aromatic hydrocarbons. Also listed in Tables 1.2 and 1.3 are corrections to be applied for ring strain energies and steric effects between neighboring groups. Generalities concerning gauche effects and 1,5-interactions are also discussed. When substitution takes place at a ring position, any impact that it may have on the ring strain energy must also be considered and so, when appropriate, a discussion of such changes is presented. In addition, the effects of some simple group substitutions at a variety of hydrocarbon centers are described; an approach that provides a quick alternative route for estimating new ΔfH0 values. 1a. C(Cd)(C)3 Term. There is some uncertainty concerning the value for C(Cd)(C)3. The model compounds used to evaluate this term are 3,3-dimethylbut-1-ene (ΔfH0 = 60.5 ( 1.37), 10577
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Table 1.2. Ring Strain Corrections for Hydrocarbons ring straina molecule
ring straina
(kJ/mol)
molecule
(kJ/mol)
Cycloalkanes cyclopropane
118
cyclooctane
43
cyclobutane
111
cyclononane
56
cyclopentane
28
cyclodecane
55
cyclohexane
2
cycloundecane
51
cycloheptane
28
cyclododecane
21
Cycloalkenes cyclopropene cyclobutene
225 125
cycloheptene cyclooctene
22 25 (cis)
cyclopentene
25
cyclononene
41 (cis)
cyclohexene
5
64 (trans) 54 (trans)
Other Unsaturated Cyclic Hydrocarbons cyclobutadieneb
319
cyclopentadiene
24
cyclohexa-1,3-diene
cyclohepta-1,3-diene cyclooctatriene
27 72
cyclohexa-1,4-diene 2 cyclohepta-1,3,5-triene 22
methylenecyclopropanec
172
cycloocta-1,3,5-triene 38 cycloocta-1,3,6-triene
33
bicyclo[1.1.0]butane
280
spiropentane
270 ( 4
bicyclo[2.1.0]pentane
231
bicycloheptadiene
132
bicyclo[3.1.0]hexane
137
biphenylene
246
bicyclo[4.1.0]heptane
121
indan
20
bicyclo[2.2.1]heptane bicyclo[5.1.0]octane
68 124
bicyclo[6.1.0]nonane norbornene
18
130 90 ( 2
(bicyclo[2.2.0]hept-2-ene) a
Heat of formation values are from NIST1 or otherwise stated and a reasonable uncertainty for these data is (4 kJ/mol. b Computed ΔfH0(C4H4) = 432.99 kJ/mol.9 c Other methylene substituted cyclic species are separately discussed below.
4,4-dimethylpent-2-ene (ΔfH0(trans) = 88.8 ( 1 kJ/mol7), 2,2-dimethylhex-3-ene (ΔfH0(trans) = 107.7 ( 1 kJ/mol7) and 2,3,3-trimethylbut-1-ene (ΔfH0 = 85.5 ( 1 kJ/mol7). Using the established GAV terms for the other functions, the individual values obtained for C(Cd)(C)3 from these four molecules are 3.5, 7.6, 8.8, and 13.7 kJ/mol, respectively. Note, however, that 2,3,3-trimethylbut-1-ene will likely require a correction from a steric effect (see section 1e below) of unknown magnitude. Averaging the first three values results in the selected value, C(Cd)(C)3 = 6 ( 3 kJ/mol. The data in Table 1.1 also show that there is a slight positive change in the GAV when Cd is replaced by a CB, as in C(Cd)(C2)(H) = 7 kJ/mol and C(CB)(C2)(H) = 4 kJ/mol. This is also the case when this proposed value for C(Cd)(C)3 is compared with that of the corresponding aromatic term, C(CB)(C)3, namely, 6 ( 3 kJ/mol vs 11 ( 2 kJ/mol, and therefore is in keeping with the first result. C (CB)(C)3 is obtained from tert-butylbenzene (22.6 kJ/mol1) and is supported by the ΔfH0 values for 3- and 4-tert-butyltoluene (54 ( 2 and 57 ( 2 kJ/mol, respectively7).
1b. Cd(Ct)(H) and Cd(Ct)(C) Terms. Using the ΔfH0 values
for CH2dCHCtCH and cis- and trans-HCtCCHdCHCH31 results in a revised Cd(Ct)(H) = 36 ( 2 kJ/mol, whereas using the ΔfH0 value1 of HCtCC(CH3)dCH2 gives Cd (Ct)(C) = 40 kJ/mol. Using the revised values, the trend in going from Cd(Ct)(H) f Cd(Ct)(C) now parallels that for Cd (C)(H) f Cd(C)2. 1c. Cd(Cd)2 and Cd(CB)2 Terms. More recent thermochemical and computed data allow the Cd(Cd)2 term to be reevaluated. Using ΔfH0(CH2dC(CHCH2)C(CHCH2)dCH2) = 259 kJ/mol1 ΔfH0(CH3CHdC(CHCH2)2) = 159 kJ/mol1 and ΔfH0(CH2dC(CHCH2)2) = 188.5 kJ/mol (computed unpublished data, present authors), leads to a revised Cd(Cd)2 = 53 ( 3 kJ/mol. This new value leads to good agreement with the data for ΔfH0(fulvene) = 224 kJ/mol1 and ΔfH0(methyl fulvene) = 185 kJ/mol.1 No data are in the NIST collection to allow an estimated value for the term Cd(CB)2. Pedley et al.7 give 246 ( 4 kJ/mol for ΔfH0 1,1-diphenylethene, but this results in a low GAV term of only 28 kJ/mol. Considering the trend in the series, Cd(Cd)(H) f Cd(Cd)(C) f Cd(Cd)2 (28.4 f 36.7 f 53 kJ/mol), one can reasonably suggest that Cd(CB)2 = 53 ( 4 kJ/mol, to complete the sequence Cd(CB)(H) f Cd(CB)(C) (28 f 36 kJ/mol). Thus a revised ΔfH0 1,1diphenylethene = 271 ( 4 kJ/mol. 1d. Corrections for Ring Strain. Summation of the appropriate GAV terms for a cyclic molecule yields an enthalpy of formation that does not include any contribution from ring strain. The ring strain is generally destabilizing and its magnitude is deduced from the difference between the experimental (or computed) ΔfH0 value and that obtained from the GAV summation. Table 1.2 lists the ring strain values for the more common cyclic hydrocarbons. Note that there are more such data in Cohen’s paper,6 which we have chosen not to reproduce here, as many are of rather limited utility and most are based on a single determination. Note that NIST includes two ΔfH0 values for cyclopropane, 53.3 and 39.3 kJ/mol. A G3 level computation10 gives ΔfH0 = 56.7 ( 1 kJ/mol. The selected value of 55 kJ/mol results in the above quoted ring strain of 118 kJ/mol. Note also that although the listed ΔfH0 for spiropentane, C5H8 (two cyclopropane rings joined at a common carbon), 185.1 ( 0.75 kJ/mol, dates back to 1955,1 it is supported by a computed value of 188 ( 4 kJ/mol.11 The total ring strain for this molecule (GAV ΔfH0 = 83 kJ/mol) is thus estimated to be 270 kJ/mol, or 136 kJ/mol per ring, slightly more than in cyclopropane itself (118 kJ/mol). In general, the cis correction for the paired alkyl groups methyl, ethyl, propyl, isopropyl, and butyl is always small and additivity reproduces the great majority of the available data. There are, however, some anomalous items in the NIST collection. For example ΔfH0(trans-2,5-dimethyl-hex-3-ene) = 119.5 kJ/mol,1 whereas additivity gives 110 kJ/mol, indicating that the experimental result is incorrect. See Table 1.3 for steric effect corrections. 1e. Steric Effects, Gauche Effects, and 1,5-Interactions. Steric effects result from interactions that limit free rotation between nearby substituent groups. They are destabilizing and the magnitude of the correction depends on the proximity and the size of the substituents. Gauche effects are small and result from the interaction of adjacent groups. In Benson’s book,3 the proposed correction for each gauche effect in simple saturated 10578
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Table 1.3. Corrections for Cis Effects in Olefins adjacent groups cis correction
correction (kJ/mol)
Table 1.4. Effect of Replacing H by CH3 at Selected Carbon Atoms type of
4(2
cis (one tert-butyl)
17 ( 4
carbon atom
cis (two tert-butyl)
44 ( 4
CH3 CH2
hydrocarbons is ca. 3 kJ/mol, whereas a 1,5- interaction contributes ca. 6 kJ/mol. A survey of the available data for 53 isomeric alkanes from C5 up to C11 listed in the NIST WebBook,1 allows the following conclusions to be made, based on the number and type of substituents attached to the longest chain. For an alkane bearing a single methyl branch, there is no need for any gauche correction to a GAV-estimated ΔfH0 to within an uncertainty of (3 kJ/mol. 1. For dimethyl- substituted alkanes, 1,5-interactions can come into play for hexanes and larger species. The available data show that the magnitude of this effect is ca. 6 ( 2 kJ/mol per interaction. 2. Trimethyl substitution results in larger steric effects ranging from 10 to 19 kJ/mol. Note that 2,2,4-trimethylpentane (ΔfH0 = 224.1 ( 1.3 kJ/mol;1 GAV = 237 kJ/mol, steric correction = 13 kJ/mol) is the hydrocarbon analog of tert-butyl isopropyl ether, a molecule that requires no GAV correction for a steric effect (see also discussion below). 3. Tetramethyl substitution on adjacent or one-atomremoved positions results in a large correction of 32 ( 3 kJ/mol, but it is a little smaller for 2,2,3,3-tetramethylbutane (di-tert-butyl) ΔfH0 = 225.6 ( 2 kJ/mol (average of two values),1 GAV = 251 kJ/mol, steric correction = 25 ( 2 kJ/mol. Where two methylene groups separate the substituents as in 2,2,5,5-tetramethylhexane, the steric interaction is less severe and so the correction term is less, ca. 14 ( 3 kJ/mol. 4. The molecule 3-ethylpentane (ΔfH0 = 190 ( 1 kJ/mol (average of two values),1 GAV = 196 kJ/mol) has three 1,5-interactions but apparently requires a correction term of only ca. 6 kJ/mol. This is, however, not in keeping with the mixed methyl, ethyl, and diethyl analogs. The ΔfH0 values for 3-methyl-3-ethylpentane, 2-methyl-3-ethylpentane, and 3,3-diethylpentane are 215.0 ( 1.3,1 211.2 ( 1.3,1 and 232.3 ( 1.7 kJ/mol,7 respectively, and the corresponding GAV results are 230, 224, and 251 kJ/mol, respectively, requiring corrections of 15, 13, and 19 kJ/mol. Models of these three molecules show that they involve three, five, and six 1,5-interactions respectively. If the 1,5correction were set at 6 ( 2 kJ/mol, the GAV results for these three molecules would require additions of 18 ( 6, 30 ( 12, and 36 ( 12 kJ/mol, a hardly satisfactory outcome. It is of course possible that the experimental data are themselves in error. The problem could likely be solved by computation. 5. The C9 molecule, 2,2,4,4-tetramethylpentane (ΔfH0 = 241.5 ( 1.5 kJ/mol,1 GAV = 272 kJ/mol (correction = 31 kJ/mol), which contains nine 1,5-interactions, has a smaller correction than its oxy-analog, di-tert-butyl ether (43 kJ/mol). This is in keeping with a scaled model of the two molecules where the ether is clearly a more crowded species. See also the footnote in Table 2.1 below.
ΔΔfH0
type of
ΔΔfH0
(kJ/mol)
carbon atom
(kJ/mol)
21 28
CBH tCH
32 42
C(H)(dO)
52
CH
34
dCH2
32
dCH
35
6. Finally, some very large steric effects can be encountered among highly substituted alkanes such as 3,3,4,4-tetraethylhexane (ΔfH0 = 265.5 ( 2.6 kJ/mol,1 GAV = 376 kJ/ mol) for which the steric correction is 110 kJ/mol, 2,2,3,3,4,4,5,5-octamethylhexane (ΔfH0 = 248.3 ( 2.4 kJ/mol,1 GAV = 321 kJ/mol) where this correction is worth 170 kJ/mol and finally the remarkable tetra-tertbutylethane (ΔfH0 = 251 ( 4 kJ/mol,1 GAV = 516 kJ/ mol), requiring a huge steric correction of 265 kJ/mol. 1f. Methyl Substitution for H at Carbon. The effect of a methyl (or any alkyl) group substitution for H at a carbon atom is always stabilizing, but the magnitude of the enthalpy change depends upon the type of C-atom to which it bonds. These positional differences are large enough to deserve separate listings (Table 1.4). Their uncertainty is ca. (2 kJ/mol. It should also be noted that these values are independent of the other attachments to the selected carbon, such as C(H)(OR), C(dO)(OR), etc. 1g. Insertion of a Single Π-System in a Hydrocarbon Chain. The effect of this type of substitution also depends upon position (e.g., internal vs terminal), but the data can conveniently be grouped (Table 1.5). As shown in Table 1.5 and using the current GAV values, the change in ΔfH0 that results from expanding CH2CH2CH2 to CH2C(dCH2)CH2 in a saturated, acyclic hydrocarbon is 91 kJ/mol. The effect of changing CH2 into C(dCH2) in a cyclic hydrocarbon is shown in Table 1.6. The enthalpy change in the cyclic species is of the same magnitude and so, except for the three-membered ring, does not greatly affect the ring strain energy; i.e., ΔRS is small. The changes that are noted for the larger species may only reflect the uncertainties in the enthalpy of formation data and therefore may not be of physicochemical significance. Although the data in Table 1.6 show that the addition of an exocyclic methylene group does not greatly affect the ring strain, the following comments should be made: 1. NIST1 lists two substantially different ΔfH0 values for methylenecyclobutane, 106 and 121.5 ( 0.7 kJ/mol. The sum of the GAVs (8 kJ/mol) and including the ring strain of cyclobutane (112 kJ/mol) leads to Δ fH 0(methylenecyclobutane) = 120 kJ/mol, and so suggests that the lower 106 kJ/mol may be erroneous. ΔfH0(1,2dimethylenecyclobutane) = 204 kJ/mol;1 summation of the GAV terms plus the cyclobutane ring strain gives 210 kJ/mol. 2. Fulvene (C6H6, methylene-1,3-cyclopentadiene) deserves mention. A recent computation7 lowers the ΔfH0 to 214 kJ/mol, leaving the 1,3-cyclopentadiene GAV ring strain unchanged. 10579
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3. For 5-methylene-1,3-cyclohexadiene, NIST1 again lists two conflicting results, 150 ( 10 (an estimated value) and 196.6 kJ/mol. Using the established simple GAV terms gives a result of 166 kJ/mol, and with the addition of the ring strain for the unsubstituted species (17 kJ/mol) leads to ΔfH0(5-methylene-1,3-cyclohexadiene) = 183 kJ/mol, in poor agreement with the experimental value of 196.6 kJ/ mol and far from the estimate of 150 ( 10 kJ/mol. Note, however, that NIST states that the authors of the experimental report had some reservations about their data. When a second exocyclic methylene group is added, the GAV result for ΔfH0(5,6-dimethylene-1,3-cyclohexadiene) = 268 kJ/mol, requiring a RS value of 14 to agree with the 254 kJ/mol datum. 4. A similar problem arises for the 1,4-analogue. The GAVs give a ΔfH0 value of 190 kJ/mol, which when corrected for the (small) ring strain of the unsubstituted species, results in ΔfH0(3-methylene-1,4-cyclohexadiene) = 187 kJ/mol, Table 1.5. Effect of Replacing H by a Π-System at Carbon ΔΔfH0
insertion
((2 kJ/mol)
position internal
CH2CH2 to CH2CHdCH—CH2
74
terminal
CH2CH3 to CH2CH2—CHdCH2
84
internal at
CH2CH(R) to
73
branch internal at branch conjugateda
CH2—CHdCH—CH(R) CH2C(R,R0 ) to
78
CH2—CHdCH—C(R,R0 ) CHdCH to CHdCH—CHdCH
exo to chain CH2CH2CH2 to CH2C(dCH2)CH2 a
57 91
Note that the stabilization resulting from conjugation is ca. 17 kJ/mol.
in poor agreement with the NIST1 value, an estimated 150 ( 10 kJ/mol. The second methylene addition (3,6dimethylene-1,4-cyclohexadiene) leads to a GAV estimated value of 268 kJ/mol, far from the NIST listed estimate, 210 ( 20. Given the available data and our capacity to quantify changes in ring strain upon addition of the exocyclic methylene group, we can offer no simple solution to the above two examples, other than to suggest that the data listed in NIST may deserve revision. Other cyclic hydrocarbons worth mentioning are trimethylenecyclopropane, ΔfH0 = 396 ( 12 kJ/mol.1 Our GAVs give 136 kJ/mol, leaving a ring strain of 260 kJ/mol, 144 kJ/mol greater than for cyclopropane itself. The addition of one exocyclic methylene resulted in an extra strain energy of 55 kJ/mol (Table 1.6) and so for the trimethylene species the strain has indeed increased approximately 3-fold. NIST1 also lists a heat of formation value for 1,2-dimethylenecyclobutane of 204 kJ/mol, in agreement with the value of 209 kJ/mol, obtained from the summation of GAVs plus the cyclobutane ring strain energy. There is also a value for dimethylenecyclobutene of 336 kJ/mol (NIST).1 The GAV result plus the cyclobutene ring strain gives only 273 kJ/mol, and so an additional ring strain of 60 kJ/mol appears to be required or possibly the experimental value should be revised. 1h. Aromatic Substitution. Recent data have allowed some aromatic GAV terms to be reconsidered (Table 1.7). Also, ortho-, meta-, and para-positional effects on the enthalpy of formation of isomers have been revisited. For meta- and parasubstituted benzenes, interaction effects might be expected to be minimal but for ortho substitutions, the interaction could be appreciable, or at least be similar to the cis effect in an olefin. Table 1.7 shows that the ortho effects are not negligible; see ΔΔfH. When the adjacent groups are highly polar and/or bulky, more significant effects may ensue. These are discussed later; see section 2i.
Table 1.6. Exocyclic Methylene Effects (dCH2) on the Ring Strain Energy (RS) cyclic compound
ΔfH0 a (kJ/mol)
RS (kJ/mol)
ΔfH0(dCH2)a (kJ/mol)
RS (kJ/mol)
ΔRS (kJ/mol)
Saturated Rings cyclopropane
53.30 ( 0.59
116
201 ( 2
174
58
methylenecyclopropane cyclobutane
201 ( 2 28.4 ( 0.610
174 112
396 ( 12b 106
190 see discussion above
16
cyclopentane
77 ( 2i
28
10.2
23
5
methylenecyclopentane
10.2
23
94.1d
14
9
121.5 ( 0.71
cyclohexane
124 ( 1i
1
25.2 ( 3.810
10
methylenecyclohexane
25.2 ( 3.87
10 ( 4
54.4e
4
10 ( 4
64.4f
7
cycloheptane
118.1 ( 1.010
28
27g
28
0
42
16
14 ( 4 3 ( 4
Unsaturated Rings 1,3-cyclopentadiene
136 ( 3
26 ( 3
224h
1,3-cyclohexadiene
104.6 ( 0.6
16
150 ( 10
5-methylene-1,3- cyclohexadiene
150 ( 10
1,4-cyclohexadiene
105 ( 5
3-methylene-1,4-cyclohexadiene
150 ( 10
c
196.6
see discussion above
220 ( 20
196.6
254 2
150 ( 10
see discussion above
210 ( 20
a
Values are from NIST or as otherwise stated.1 b Trimethylenecyclopropane. c Average of two values. d 1,3-Dimethylenecyclopentane. e 1,3Dimethylenecyclohexane. f 1,4-Dimethylenecyclohexane. g Estimated by adding 91 kJ/mol to ΔfH0 for (cycloheptane). h Fulvene (5-methylene-1,3cyclopentadiene). i Average of three values. 10580
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Table 1.7. Alkyl Substitutions on Benzene and Positional Effects on ΔfH0 ΔfH0 a molecule
(kJ/mol)
GAV ΔfH0
ΔΔfH
((4 kJ/mol)
(kJ/mol)
Table 2.1. Carbon Centered Group Additivity Values for Molecules Containing Carbon, Hydrogen, and Oxygen terms
GAV (kJ/mol)
terms
C(H)3(O)
42
C(H)3(CO)
GAV (kJ/mol) 42a
toluene 1,2-dimethylbenzene
50 ( 1 19.0 ( 1.1
50 17
0 2
C(H)2(O)(C) C(H)2(O)(Cd)
33 29
C(H)2(CO)(C) C(H)2(CO)(Cd)
22 16
1,3-dimethylbenzene
17.2 ( 0.75
17
0
C(H)2(O)(CB)
26
C(H)2(CO)(CB)
11
1,4-dimethylbenzene
17.9 ( 1.0
17
1
C(H)2(O)(Ct)
23
C(H)2(CO)(Ct)
9
1,2,4-trimethylbenzene
13.9 ( 1.1
16
2
C(H)(O)(C)2
26
C(H)(CO)(C)2
4
1,3,5-trimethylbenzene
15.9 ( 1.37
16
0
C(H)(O)(Cd)2
21
C(H)(CO)(Cd)2
12
b
1,2,3-trimethylbenzene
9.6 ( 1.3
16
6
C(O)(C)3
21 ( 5
C(CO)(C)3
1,2,4,5-tetramethylbenzene
46 ( 1b
48
2
C(H)2(O)2
67 ( 2
C(H)2(CO)2
10
1,2,3,5-tetramethylbenzene 1,2,3,4-tetramethylbenzene
45 ( 2b 39 ( 3b
48 48
3 9
C(H)(O)2(C) C(O)2(C)2
65 ( 2 48
C(H)2(O)(CO)
29
ethylbenzene
29.8 ( 0.84c
30
0
C(O)3(H)
1-ethyl-2-methylbenzene
1.2 ( 1.2
3
4
C(O)3(C)
105
1-ethyl-3-methylbenzene
1.9 ( 1.2
3
1
C(O)4
162
9
108
3
0
Cd(O)(H)
36
Cd(CO)(H)
21
23
0
Cd(O)(C)
43
Cd(CO)(C)
31
129
2
Cd(O)(Cd)
37
Cd(CO)(Cd)
38
178 72 ( 3
2 0
Cd(O)(Ct) CB(O)
37 25
CB(CO)
18
34 ( 10
0
Ct(O)
180
Ct(CO)
120
1-ethyl-4-methylbenzene
3.3 ( 1.5
tert-butylbenzene
22.7 ( 1.412
1,4-di-tert-butylbenzene
12712
biphenyl 4-tert-butylbiphenyl
180 ( 3 72.213
4,40 di-tert-butylbiphenyl
33.613
1,2-diphenylbenzene
282.8 ( 3.2
273
10
1,3-diphenylbenzene
280 ( 9
273
7
1,4-diphenylbenzene
282 ( 3d
273
9
1,2,3-triphenylbenzene
377 ( 514
368
11
1,3,5-triphenylbenzene
367 ( 5
368
1
Cd(O)(CO) a
49
Assuming C—(H)3(CO) t C—(H)3(C).
a
(226 ( 10 and 205 ( 5 kJ/mol1). Additivity, which requires a value for the CBCB GAV gives 222 ( 8 kJ/mol using the GAV derived from biphenyls and naphthalenes of 21 ( 3 kJ/mol.
For small groups, such as adjacent methyls, the ortho effect is indeed small, of the order of 2 kJ/mol and so therefore lies within the error limits of the experimental or computed results. If one of the groups is slightly larger, e.g., an ethyl group, the ortho effect remains small, as it does with simple cis-olefins. Note that few data are available for bulky groups such as tert-butyl or phenyl. The 4- and 4,40 -methyl substituted biphenyls have ΔfH0 = 138 ( 3 and 111 ( 4 kJ/mol,1 respectively. The latter is well reproduced by additivity (ΔfH0(GAV) = 114 ( 6 kJ/mol), whereas the former is less satisfactory (ΔfH0(GAV) = 147 ( 6 kJ/mol). 2-Methylbiphenyl is more positive, ΔfH0 = 153 ( 1 kJ/mol,1 in keeping with some steric hindrance. GAV’s satisfactorily reproduce the heats of formation of 1,2,3,4-tetrahydronaphthalene and 1,2,3,4-tetrahydrophenanthrene. The heats of formation are 28 ( 3 and 92 ( 1 kJ/mol,1 respectively, and GAVs give 26 ( 6 and 91 ( 6 kJ/mol, respectively, when the small ring strain for cyclohexene is included. When one wishes to examine the relative thermodynamic stabilities of isomeric fused multiring aromatics, the Clar effect15,16 must be considered. For example, there are four equivalent ways in which one can draw the isomers of anthracene using conventional doubles bonds around the rings. For its isomer phenanthrene, there are five such structures. The available thermochemical data show a difference of about 20 kJ/mol, between them, phenanthrene being the more stable species
2. CARBON, HYDROGEN, AND OXYGEN Tables 2.1, 2.2, and 2.3 give our current GAV terms for molecules containing C, H, and O. Individual cases are discussed thereafter. 2a. C(C)3(O) Term: A Bulky Constituent Problem. The Benson GAV value for the C(C)3(O) term, 28 kJ/mol, was derived from the heat of formation of tert-butyl alcohol, the simplest molecule containing the t-BuO moiety. However, this GAV persistently underestimates the ΔfH0 values for structurally closely similar molecules. To decide whether this term should be revised, and likewise to propose a “best” enthalpy of formation for tert-butyl alcohol, the values obtained for the C(C)3(O) terms derived from the ΔfH0 of a variety of molecules are shown in Table 2.4. These data strongly suggest that the two most negative experimental ΔfH0 values for tert-butyl alcohol are likely too low and that the computed value is better. One way to test the reliability of the experimental heat of formation of tert-butyl alcohol is to check the effect of methyl substitution at an alkanol hydroxyl group. For the tertiary alkanols, few data are available, but the change in ΔfH0 from CH3CH2C(CH3)2OH to CH3CH2C(CH3)2OCH3 is 23 kJ/mol (ΔfH0 values are 329.3 and 306.5 ( 1.1 kJ/mol, respectively).1 For the same substitution in tert-butyl alcohol, ΔΔfH0 is 29 kJ/mol, from the average of the experimental values,1 but 25 kJ/mol when only the computed value is used,17 indicating that this latter value is more consistent with the other thermochemical data. For the simpler molecules, CH3OH to CH3OCH3, ΔΔfH0 = 19 kJ/mol; CH3CH2OH to CH3CH2OCH3, ΔΔfH0 = 18 kJ/mol;
14
Values are from NIST1 or as otherwise stated. b Average of three value;, uncertainty represents the spread of the reported data. c Note that NIST1 also lists 49.0 ( 4.0 and 69.3 kJ/mol, both of which are inconsistent thermochemical data and should be ignored. d Average of two values; the uncertainty represents the spread of the reported data.
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Table 2.2. Oxygen Centered Group Additivity Values for Molecules Containing Carbon, Hydrogen, and Oxygen terms
GAV (kJ/mol)
terms
Table 2.4. C(C)3(O) GAV Term
GAV (kJ/mol)
O(H)2
242
O(H)(O)
69
O(H)(C) O(H)(Cd)
159 190 ( 5
O(C)(O)
24
O(H)(CB)
190 ( 5
O(H)(CO)
243
O(H)(Ct)
201 ( 55
O(C)(CO) O(O)(CO)
molecule
ΔfH0 a
C(C)3(O)
(kJ/mol)
(kJ/mol)
312.6, 313 ( 1.5,
(CH3)3COH
27 ( 3
309.7 308 ( 417
23 ( 4
(CH3)3COCH3
285, 283 ( 1, 282
16 ( 3
180
(CH3)3COCH2CH3
318; 314 ( 2
16 ( 3
80
(CH3)3COC(O)CH3
516 ( 1
21 ( 1
306 ( 1 357 ( 1
21 ( 1 23 ( 1
O(C)2a
99
O(Cd)(CO)
187
O(C)(Cd)
124 ( 5
O(CB)(CO)
182 ( 5
CH3CH2C(CH3)2OCH3 (CH3)2CHC(CH3)2OH
O(C)(CB)
124 ( 5
O(CO)2
202 ( 7
CH3CH2C(OH)(CH3)CH2CH3
348 ( 118
21 ( 1
O(C)(Ct) O(Ct)(Cd)
141 150 ( 5
CH3CH2C(CH3)2OH
329.3, 331 ( 1
25 ( 2
O(Cd)2
137
O(CB)2
136 ( 5
21 ( 5
average a
Values listed are from NIST or otherwise stated.1
a
Note that whereas there is no steric correction required for tert-butyl methyl ether (ΔfH0 = 283 ( 3 kJ/mol (average of three values),1 GAV = 287 kJ/mol), tert-butyl ethyl ether (ΔfH0 = 317 ( 2 kJ (average of two values),1 GAV = 320 kJ/mol), or tert-butyl isopropyl ether (ΔfH0 = 358 ( 3 kJ/mol,1 GAV = 355 kJ/mol), there is a significant correction for di-tert-butyl ether (ΔfH0 = 362 ( 2 kJ/mol (average of three values),1 GAV = 405 kJ/mol) of 43 kJ/mol. This latter feature is discussed below in section 2g.
Table 2.3. Carbonyl Centered Group Additivity Values for Molecules Containing Carbon, Hydrogen, and Oxygen terms
terms
CO(H)2
109
CO(H)(O)
134
123
CO(C)(O)
147
CO(H)(Cd)
113
CO(Cd)(O)
134
CO(H)(Ct)
107
CO(Ct)(O)
115
CO(H)(CB)
122
CO(CB)(O)
143
CO(C)2
133
CO(H)(CO)
106
CO(C)(Cd) CO(C)(CB)
123 130
CO(C)(CO) CO(CB)(CO)
121 110
CO(C)(Ct)
113
CO(O)2
129
CO(Cd)2
113
CO(O)(CO)
123
CO(CB)2
118 [COd]Cd(H)2a
78
40
experimental ketone
ΔfH
0a
(kJ/mol) ((4 kJ/mol) (kJ/mol)
CH3C(O)C(CH3)3
291 ( 1
292
CH3CH2C(O)C(CH3)3
313.8 ( 1.4
314
(CH3)2CHC(O)C(CH3)3
338.3 ( 1.2
338
(CH3)2CHC(O)CH(CH3)2 311.3 ( 1.1 (CH3)3CC(O)CH2CH(CH3)2 393.3 ( 2.319 a
GAV ΔfH0 steric effect
309 398
5
Values are from NIST or otherwise stated.1
GAV (kJ/mol)
CO(H)(C)
CO(Callene)
a
GAV (kJ/mol)
Table 2.5. Experimental and GAV ΔfH0 Data for Bulky Ketones
[COd]Cd(C)(H)
58
[COd]Cd(C)2
47
[COd]Cd(Cd)
50
[COd] corresponds to the ketene moiety dCdO.
(CH3)2CHOH to (CH3)2CHOCH3, ΔΔfH0 = 19 kJ/mol; and CH3(CH2)3OH to CH3(CH2)3OCH3, ΔΔfH0 = 19 kJ/mol. The small differences may arise from insufficient consideration of small steric effects or some uncertainty in the data. However, for the same molecules in which an OCH3 group is replaced by OCH2CH3, the ΔΔfH0 is constant at 33 ( 1 kJ/mol. This observation also holds for (CH 3 )3 COCH 3 and (CH 3 )3 COCH 2 CH 3 , implying that the ΔΔ f H 0 (33 kJ/mol) value is likely correct and, consequently, that the more negative Δ f H values for tert-butyl alcohol are too negative.
Diethyl ether deserves a brief mention. According to the above, the change in ΔfH0 from diethyl ether to methyl ethyl ether is expected to be ca. 33 kJ/mol, from 252 to 219 kJ/mol. This is in keeping with the NIST value for methyl ethyl ether of 216.4 ( 0.67 kJ/mol.1 Note that NIST also lists a 1957 value of 244 kJ/mol for diethyl ether. This latter value is inconsistent and should be discarded. Using the revised value for the C(C)3(O) term, 21 ( 5, the GAV ΔfH0 for (CH3)3COC(CH3)3, is 392 ( 10 kJ/mol. This is markedly more negative than the experimental value, 362 ( 2 kJ/mol1 and so the contribution from the steric effect is ca. 30 ( 10 kJ/mol. For the analogous hydrocarbon, 2,2,4,4tetramethylpentane, the 1,5-repulsion is of closely similar magnitude, ca. 31 kJ/mol. However, note (see Table 2.2) the steric effect appears not to be significant in the smaller tertbutylOR ethers. For (CH3)2CHOC(CH3)3, where tert-butyl is replaced by a sec-propyl group, the difference between the experimental and GAV ΔfH0 values is negligible, indicating no significant steric effect (358 ( 31 and 355 ( 4 kJ/mol, respectively). The formate ester HC(O)OC(CH3)3 is also well reproduced by additivity, the experimental and GAV values being respectively 457 ( 51 and 461 ( 5 kJ/mol. 2b. C(C)3(CO) Term: Another Bulky Constituent Problem. The heat of formation of di-tert-butylketone is not well reproduced by GAV terms, which give a ΔfH0 = 367 kJ/mol, to be compared with 346 ( 1 kJ/mol,1 a difference of 21 kJ/mol, presumably arising from steric interactions. However, for the simpler ketones, the experimental and additivity data are close (Table 2.5), supporting the GAV used for C(C)3(CO). Note that 10582
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Table 2.6. C(H)2(O)2 and C(C)(H)(O)2 Terms ΔfH0 a molecule CH3OCH2OCH3
C(H)2(O)2 C(C)(H)(O)2
(kJ/mol)
(kJ/mol)
348.2 ( 0.79,
Table 2.8. Revised Terms for Aromatic Carbons Bonded to Oxygen Benson3
(kJ/mol) terms
68
352.820 CH2(OCH2CH3)2
414 ( 1
b
average CH3CH(OCH3)2
66
O(Cd)(H) O(Cd)(C)
67 ( 2
CB(O)
389.7 ( 084
66
CH3CH2CH2CH(OCH3)2426 ( 2 CH3CH(OCH2CH3)2 average
60c
453.6 ( 3.1
64 65 ( 2
a Values listed are from NIST or otherwise stated.1 b Average of two values. c Not included in the average; see text for discussion.
Table 2.7. Ring Strain Energy in Cyclic Species Containing the OCH2O Moiety no. of atoms molecule
a
in ring
ΔfH0 a (kJ/mol)
GAV ΔfH0
RS
((4 kJ/mol) (kJ/mol)
1,3-dioxolane
5
301.7 ( 2.2
331
30
2-methyl-1,3-dioxolane
5
350 ( 3
371
21
1,3-dioxane 2-methyl-1,3-dioxane
6 6
345 ( 2 398 ( 3
352 392
7 6
4-methyl-1,3-dioxane
6
378 ( 1.5
387
9
2,4-dimethyl-1,3-dioxane
6
427 ( 3b
427
0
5,5-dimethyl-1,3-dioxane
6
421 ( 3
414
6
1,4-dioxane
6
317 ( 2
330
13
Average values from NIST or as otherwise stated.1 b cis-Methyl groups.
the data indicate a small but significant steric effect (ca. 5 kJ/mol) for 2,2,5-trimethyl-3-hexanone, in keeping with the similar correction for 2,2,5-trimethylhexane (ca. 5 kJ/mol). 2c. OCH2O Problem: Establishing C(H)2(O)2 and C (C)(H)(O)2. The use of the GAVs listed in Tables 2.1 and 2.2, results in ΔfH0(CH3OCH2OCH3) = 350 kJ/mol, in agreement with both the experimental1 and computed values.20 For the ethyl analogue CH2(OCH2CH3)2, ignoring any anomeric effect, the derived C(H)2(O)2 term is 66 kJ/mol (Table 2.6). For the branched species, the GAV value obtained from 1,1dimethoxybutane (60 kJ/mol) is somewhat lower than that from the other two molecules. Note that this latter molecule’s ΔfH0 appears to be slightly too positive, as the insertion of two methylene groups in CH3CH(OCH3)2 should result in a value of 431.5 kJ/mol, some 6 kJ/mol lower. However, a model of this species shows that three 1,5-interactions are present, thus in keeping with the more positive ΔfH0. From the data in Table 2.7, methyl substitution in 1,3dioxolane appears to reduce the ring strain by some 9 kJ/mol. However, the observed stabilization effect due to methyl substitution in 1,3-dioxolane is unusually large, the ΔΔfH0 being 48 ( 5 kJ/mol. In cyclopentane itself, the hydrocarbon analogue of 1,3-dioxolane, replacing H by a methyl group appears to be neutral with respect to ring strain and results in a ΔΔfH0 of 30 kJ/mol. Note that in the case of the 6-membered rings, cyclohexane and 1,3-dioxane, the effect of methyl substitution on the enthalpy of formation of the latter depends upon the position at which substitution takes place. At the 2-position, i.e., the carbon between the two oxygens, the ΔΔfH0 = 53 kJ/mol,
a
(kJ/mol) 159 128 a
Cohen6
proposed value
(kJ/mol)
((5 kJ/mol)
206.3 127.6
190 124
3.8
25
O(CB)(H)
159a
3.8
161.1
190
O(CB)(C)
96
90.4
124
Assigned as d O—(H)(C) by Benson.
whereas at the 4-position, the ΔΔfH0 is the same as in cyclohexane, 32 kJ/mol. For both 1,3-dioxolane and 1,3-dioxane, the small changes in ring strain may reflect differences in structures, i.e., that the cyclopentane ring is not puckered in the same way as in 1,3-dioxolane, or are a consequence of the uncertainty in the data. Similarly, the ring strain for the two dimethyl analogues of 1,3-dioxane may arise from discrepancies in the data. 2d. GAVs for O(Cd)(H), O(Cd)(C), CB(O), O(CB)(H), and O(C)(CB). These terms are chemically closely related, and those containing CB are usually paired to determine the enthalpy of formation of C6H5OR molecules. Because our revised values differ appreciably from those proposed by Benson and/or Cohen (Table 2.8), the above five GAV terms merit some discussion. Benson’s approach to establishing the GAVs containing both CB and O was to assign a value to O(CB)(H) equal to that for O(C)(H), so deriving CB(O) using the well established enthalpy of formation of phenol. This implied that the nature of the carbon atom bonded to oxygen had no thermochemical effect, irrespective of whether it was saturated, unsaturated or aromatic, i.e., O(C)(H) t O—(Cd)(H) t O—(CB)(H) t O —(Ct)(H) = 159 kJ/mol. However, recent reliable computed ΔfH0 values for molecules containing the O(Cd)(H) term5 have shown that the above assigned value is too positive and should be replaced by 190 ( 4 kJ/mol. This more negative value is moreover in keeping with the trend observed when C is replaced by Cd in the related pair O(C)2 f O(Cd)(C), i.e., 99 f 124 kJ/mol. We therefore propose, on the basis of the chemical similarity of the two unsaturated carbons, that O—(Cd)(H) t O—(CB)(H) = 190 ( 4 kJ/mol. Combining the latter term with ΔfH0 phenol (96 ( 1 kJ/mol)1 leads to CB(O) = 25 ( 5 kJ/mol. Having established CB(O), we can therefore re-evaluate the O(CB)(C) term using reliable ΔfH0 data for benzyl ethers. The difference between ΔfH0 for anisole, C6H5OCH3, and phenetole, C6H5OCH2CH3, can with confidence be placed as 32 ( 1 kJ/mol, from the well authenticated data for these alkyl ethers. For anisole, NIST1 gives ΔfH0 = 68, 77, and 71 kJ/mol (and a very old 1941 datum, not included here) leading to an average ΔfH0(C6H5OCH3) = 72 ( 5 kJ/mol and so phenetole could be expected to have a ΔfH0 = 104 ( 5 kJ/mol. The NIST data for phenetole are 102, 108, and 125 kJ/mol.1 Averaging the first two values gives ΔfH0(C6H5OCH2CH3) = 105 ( 3 kJ/mol, in very good agreement with the estimate and so supports the consistency of the experimental data. This also implies that the 125 kJ/mol for phenetole should be discarded from NIST. Using the enthalpy of formation of these two benzyl ethers leads to O(CB)(C) = 124 kJ/mol. This latter value is also supported by recent data for substituted chloroanisoles where using the average ΔfH0 for m- and p-chloroanisole (104 ( 10583
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Table 2.9. Effect of Replacing Methylene by Oxygen on the Ring Strain Energy (RS) cyclic compound cyclopropane
ΔfH0(ring)a (kJ/mol)
RS (kJ/mol)
53
ΔfH0(Oring)a (kJ/mol)
118
RS (kJ/mol)
ΔRS (kJ/mol)
52.6
112
6
80.6
105
6
23
5
56.710 cyclobutane
28
cyclopentane
77 ( 2
28
cyclohexane
124 ( 2
2
cyclopentadiene a
111
c
184 ( 0.7 222 ( 3b 31 ( 4,b 3522
24
136
6
8
25
49
Values are from NIST or otherwise stated,1 b Average of three values. c Average of two values.
Table 2.10. Ring Strain (RS) Energies in Cyclic Ketones compound
ΔfH0 a (kJ/mol)
GAV ΔfH0 ((4 kJ/mol)
RS (kJ/mol) b
RS in cycloalkane (kJ/mol)
ΔRS (kJ/mol)
cyclopropanone
17 ( 1, 6
177
183
118
67
cyclobutanone
96 ( 5,c 9124
198
102, 107
111
10, 5
cyclopentanone
195 ( 3d
219
24
28
cyclohexanone
228 ( 3e
240
12
2
cycloheptanone
247 ( 2c
261
14
28f
14
cyclooctanone
272.2 ( 1.8
282
10
43f
33
cyclononanone
279.7 ( 1.7
302
22
56f
33
cyclodecanone
305.1 ( 1.9
323
18
55f
37
23
4 10
Values are from NIST or otherwise stated.1 b Calculated using ΔfH0 = 6 kJ/mol (see also section 2d). c Average of two values. d Average of three values. e Average of four values. f Experimental values used to calculated the RS are taken from ref 7. a
Table 2.11. Ring Strain (RS) in Epoxy-Cycloalkanes ΔfH compound
a
GAV ΔfH
0a
(kJ/mol)
0
((4 kJ/mol)
RS (kJ/mol)
oxirane
52.6
165
112
1,2-epoxycyclopentane
97.1 ( 7.0
214
117b
1,2-epoxycyclohexane
124 ( 2c
235
111b
1,2-epoxycycloheptane
152.3 ( 3.1
256
103b
1,2-epoxycyclooctane
165.1 ( 2.4
276
111b
1b
Values are from NIST or otherwise stated. Represents the sum of the ring strain from the three-membered epoxide fused to the cycloalkane ring. For the epoxide part, the basic ring strain is 112 kJ/mol, from ethylene oxide itself. c Average of two values.
2 kJ/mol)21 results in O(C)(CB) = 124 ( 2 kJ/mol, in agreement with the above. These revised values show a trend for the series O(C)(H) f O(Cd)(H) f O(CB)(H), which is consistent with that observed in the analogous series, O(C)2 f O(Cd)(C) f O(CB)(C), i.e., 159 f 190 f 190 kJ/mol and 99 f 124 f 124 kJ/mol, respectively, each with a conservative uncertainty of (5 kJ/mol. 2e. Effect on Ring Strain Energy of Replacing CH2 by an Oxygen Atom in Cyclic Alkanes. The data in Table 2.9 show that going from the cyclic alkane to the corresponding cyclic ether does not greatly affect the ring strain, the recorded changes being small and negative, except for unsaturated pair cyclopentadiene and furan. Here, the large stabilization gained by replacing the methylene group with oxygen results from furan’s quasiaromatic character, assisted by the lone-pair electrons on the oxygen. Two other compounds are worth mentioning: The ring strain in methyloxirane (the three-membered ring) ΔfH0 = 95 kJ/mol,1
is deduced to be 105 kJ/mol, not greatly different from that of oxirane and cyclopropane. The NIST database also lists a value for the C5H10O four-membered ring, 3,3-dimethyloxetane, ΔfH0= 148 ( 2 kJ/mol,1 for which the ring strain correction term is 100 ( 2 kJ/mol, similar to that for the cyclobutane ring itself. 2f. Effect of Changing a Ring Methylene to a Keto Group. Using the new GAV terms in Tables 1.1, 2.1, and 2.3, the enthalpy change in going from CH2CH2CH2 to CH2(CdO)CH2, in an acyclic species is 114 kJ/mol. For the cyclic compounds listed in Table 2.10, the average change in ΔfH0 when methylene is replaced by a keto function is 116 ( 10 kJ/mol, very close to the acyclic value. This indicates that experimental data are reliably reproduced by additivity. Note that for cyclopropanone, the recently computed ΔfH0 value of 6 kJ/mol20 results in a ring strain of the same magnitude as that in methylenecyclopropane (183 vs 174 kJ/mol, Table 1.6). Provided that the computed value is the best, the increase in ring strain in cyclopropane is functional group independent, when the group is attached via a π-bond (carbonyl, exocyclic methylene, etc.). For the cyclic ketones, these results show that except for cyclopropanone and for cyclohexanone, the ring strain is less than that of the corresponding cycloalkane and is constant for the 8-, 9-, and 10-membered rings. These results closely parallel the effect of insertion of an exocyclic methylene group (see section 1g). 2g. Ring Strain in Epoxy-Cycloalkanes. For the epoxycycloalkanes presented in Table 2.11, except for 1,2-epoxycyclohexane, the ring strain is less than the sum of the ring strains for the individual ring compounds (ethylene oxide and the cycloalkane). In all cases, the ring strain for these molecules are essentially the same and of the same magnitude as that of 10584
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The Journal of Physical Chemistry A ethylene oxide itself (=112 kJ/mol). The datum for 1,2-epoxycycloheptane deserves to be confirmed. 2h. Intramolecular Hydrogen Bonds in Saturated Alcohols. In molecules such as HOCH2CH2OH, CH3OCH2CH2OH, CH3CH(OH)CH2OH, and HOCH2CH2CH2OH, intramolecular H-bonding might contribute to the stabilization of the molecule. Simple GAV summations do not include any term for such interactions, but their magnitude can be estimated by comparing the GAV derived ΔfH0 with the experimental value. For the above four molecules, the averaged NIST (post 1960) ΔfH0 values are 390 ( 4, 387 ( 14, 426 ( 10, and 400 ( 12 kJ/mol, respectively.1 The corresponding GAV derived ΔfH0 values are 384, 366, 419 and 405 kJ/mol. For the 1,4- and 1,5-dihydroxybutane and pentane, respectively, the experimental heats of formation, 426 ( 6 and 445 ( 5 kJ/mol,1 are well reproduced by additivity (426 and 447, respectively) showing no significant hydrogen bonding in the gas phase. Unfortunately, the spread among the experimental data is quite wide and so no definitive conclusion concerning intramolecular H-bond effects can be drawn, except that in general the GAV values tend to be more positive than the experimental data. Until better data are available, no sure correction can be assigned. 2i. Other Intramolecular H-Bonds. As a cistrans model compound we have selected 1-hydroxypropenal, HC(dO)CHd CHOH. Computation at the CBS-QB3 level gives ΔfH0(E/E) = 241 kJ/mol and ΔfH0(Z/Z) = 276 kJ/mol.25 The GAV terms from the above tables yield 246 ( 4 kJ/mol for the trans compound, in satisfactory agreement, and require an H-bond correction for the cis isomer of ca. 30 kJ/mol. In this molecule, the CdO 3 3 3 HO distance is 1.69 Å. A molecule that should be expected to show a similar effect is salicylaldehyde (2-hydroxybenzaldehyde), and experimental values for the 2-, 3-, and 4-hydroxy isomers have been reported recently, namely 246,26 214, and 218 kJ/mol,27 respectively. The GAV terms give 210 ( 4 kJ/mol for no interaction and 240 ( 8 kJ/mol for the 2-isomer, in good agreement. In contrast, recent experimental data for salicylic acid and its meta and para isomers are surprising. The reported ΔfH0 values being 496 (ortho), 469 (meta), and 493 (para) kJ/mol, respectively.1 The GAV result is 474 ( 8 kJ/mol (close to the meta isomer where there are no possible intramolecular interactions), and an ortho H-bond correction of ca. 22 kJ/mol gives agreement. The minimum distance between the ortho-OH and the acid carbonyl and/or carboxyl(OH) oxygen atom is ca. 1.61.7 A. Possibly the para isomer measurement is in error? The ΔfH0 for aspirin, acetylsalicylic acid, has recently been measured as 736 ( 1 kJ/mol.28 GAVs result in 660 ( 8 kJ/mol. A scaled model of the molecule shows that the functional groups have considerable overlap, and so internal H-bonding is the likely cause of the large difference. The GAV result for benzoic acid is 299 kJ/mol; experiment gave 294 kJ/mol.1 However, m- and p-phthalic acids1 are less satisfactory; GAV = 681 kJ/mol and NIST data for the meta acid = 696 kJ/mol and for the para acid, 718 kJ/mol. Later experimental values29 are 652 kJ/mol (ortho), 661 kJ/mol (meta), and 652 kJ/mol (para), indicating no significant evidence for internal H-bonding. In NIST there is only an estimated ΔfH0 for dimethyl-o-phthalate. Although the o-, m-, and p-hydroxybiphenyls are listed in NIST1 with an early (1971) set of ΔfH0 values, 17, 21, and 35.7 kJ/mol, respectively, a much more recent experimental determination (2004)1 for the para derivative, ΔfH0 = 0.0 kJ/mol, is
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well reproduced by the current GAV terms, which give ΔfH0 = 1 ( 10 kJ/mol. Note that the 1971 data give the ortho derivative as the most negative, not in keeping with any steric effect.
’ CONCLUSIONS The Benson group additivity value (GAV) terms for the estimation of ΔfH0 values for a wide variety of C, H, and O containing molecules have been revised and extended. A rationale is provided for the new terms and for all significant changes to older ones. Particular emphasis has been given to the magnitude and sign of the effects of structural change on the enthalpies and the GAV terms. The present results clearly show that additivity is a very effective and simple tool for estimating and assessing experimental and/or computed thermochemical data. As a viable alternative to using the full set of GAV terms, shortcuts can be used where one functional group in a molecule of known ΔfH0, is replaced by different group. ’ ASSOCIATED CONTENT
bS
Supporting Information. Tables that include all of the reevaluated and new group additivity valuers (GAV) and ring strain energies. Note that when the difference between the new value and the earlier GAV is greater than (4 kJ/mol, the latter is also shown for comparison. This material is available free of charge via the Internet at http://pubs.acs.org.
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