Polytrifluoromethylation vs polyfluorination of the isomers of Kekulé benzene and phenol: a theoretical study
Abstract
In the case of valence isomers of benzene and phenol i. e. the structures of prismane, benzvalene, Kekulé and Dewar systems, the study of the polyfluorination and polytrifluoromethylation effects on the electronic structure and intrinsic acidities of the title compounds has been performed using DFTB3LYP and NBO calculations as well as the method of isodesmic analysis.
In aromatic systems (especially if the acidity center is conjugated to the aromatic ring) CF3 groups are significantly more efficient in enhancing acidity than F substituents. At the same time in aliphatic cage-like structures the effects of CF3 and F substituents are comparable. F substituents sometimes outperform CF3 substituents by their acidifying effect. This is similar both for the investigated hydrocarbons and their hydroxy-derivatives.
Some of the compounds investigated in this study perfluorinated and pertrifluoromethylated alcohols, i.e. pertrifluoromethyl 1-prismanol (298 kcalmol-1), pertrifluoromethyl 2- and 5-benzvalenol, etc. are expected to rival the gas-phase acidity of perfluoro-1-adamantanol, the currently most acidic experimentally measured alcohol.
Out of the fluorinated hydroxy derivatives of prismane, Dewar benzene and benzvalene not all are predicted to be stable enough to be able to undergo reversible protonation-deprotonation process. In some derivatives deprotonation is accompanied by rupture of a C-C bond and in some rearrangement to the corresponding phenolate anion.
Isodesmic analysis of substituent effects shows that steric effects in poly-CF3-substitution in the alicyclic cage compounds are small, but saturation of substituent effects is reached faster than in aromatic systems.
Introduction
Polyfluorinated compounds have been attracting the attention of chemists for a long time. Polyfluorination generally increases the thermal stability and often decreases the reactivity of the molecules. In addition they might exhibit unique solubility, surface-wetting, etc properties. These factors put together have led to whole new generations of materials and chemicals, such as fluoropolymers,1 fluorinated catalysts,2 self-assembling structures,3 weakly coordinating anions,4 ionic liquids5 etc. Polyfluorination of a (potentially) acidic molecule can dramatically increases its acidity and thus polyfluorination or introduction of fluorinated substituents is an established approach to design acidic and superacidic molecules12,13. Although, fluorine atom is considered as the most electronegative, the complicated interplay of different properties8 (i.e. resonance/hyperconjugation9-11, electronegativity, polarizability, field(inductive) effect) of fluorine significantly depend on the position of the substitution center relative to the reaction site and the extent of delocalization of the anion lone pair determines the acid-base properties of the molecule. In case of fluorinated organic molecules the acidity increase is mainly due to the stabilization of the carbanion lone pair by C F bond’s ability to delocalize the electron density to its energetically low-lying *-orbital10b. The effect directly countering the acidity increase is p-p lone pair repulsion14 particularly if F is positioned to the atom carrying a nonbonding electron pair.
An interesting group of polyfluorinated molecules are polytrifluoromethylated systems.7 Although CF3 group has field-inductive effect of similar8 strength as F (F of CF3 and F are both 0.44), it is due to the negative hyperconjugation effect9 that lets it act as a -acceptor group10,11, contrary to F, which acts as a resonance donor (R 0.07 and -0.33, respectively). Also, CF3 is more polarizable than F ( -0.25 and 0.13, respectively) and has a tendency to stabilize -bonds8-11. While introduction of multiple F-substituents does not invoke steric strain, multiple CF3-substitution does. In the course of our recent work7 on the synthesis and properties of pentakis-CF3-phenyl derivatives we observed (both experimentally and computationally) that the benzene rings of all C6(CF3)5- compounds were significantly distorted from planarity.7 Nevertheless, all studied compounds were perfectly stable molecules: C6(CF3)5H, C6(CF3)6 and C6(CF3)5OH can be sublimed from sulfuric acid.
If one compares the acidities of e.g. phenols substituted with different patterns by CF3-groups or F atoms then in all cases the respective CF3-substituted phenols are more acidic, both in the gas phase and in solution7,8b,c. Thus, if speaking of acids, polytrifluoromethylation is expected to lead to a more significant stabilization of the conjugate anion and acidity increase than fluorination. When -systems are involved and the resonance effect is of importance, this is really the case. Situation with aliphatic cage-type acids is less straightforward, both with respect to stability and acidity. It is known that polyfluoro- and poly-CF3-substitution can improve the stability of many unstable compounds via steric shielding and withdrawal of electron density7,15. In the cage-type systems with a considerable strain in the ring elements the effect of various orbital overlappings can be very straightforward in terms of releasing the ring strain. Nevertheless, the possibility to prepare stable isomers of C6(CF3)6 has been known for a long time (ref 15, pages 351-353).@Siia veel selle stabiliseerimise kohta. @Sealsamas ütleb Chambers, et perfluorobenseeni isomeerid on madala stabiilsusega.
From the point of view of acidifying effect it is clear that –F and -CF3 behave quite differently in the aliphatic cage structures and it is not easy to predict the effect on acidity nor structural stability.
Polyfluorinated cage compounds and aromatics have been extensively investigated. At the same time the effect of poly-trifluoromethylation on the stability and acidity of aliphatic cage compounds, especially in comparison to polyfluorination has not been so thoroughly investigated. This led us to the idea to investigate the stability and acidity of polyfluorinated and polytrifluoromethylated non-benzenoid isomers of C6H5H and C6H5OH and some related molecules.
Recently a comprehensive study on poly-trifluoromethylated aromatic compounds was published (@viide puudu, lause algsest draftist -> Ivole). Although benzene is by far the most stable and common isomer corresponding to the formula C6H6, it is not the only one. There are three other – non-aromatic – isomers that have been experimentally prepared: benzvalene,18 prismane,19 and Dewar benzene20 (Scheme 1). Choosing the isomers of benzene as the cage compounds is interesting for two reasons: (a) these are quite unstable molecules and it is interesting to see how polysubstitution affects their stability, (b) their identical elemental composition of the respective benzene derivatives allows direct comparisons of stability and acidity.
Scheme 1. Structures of benzene (0), prismane (1), Dewar Benzene (2) and benzvalene (3).
Importantly, due to the geometry of 1-3, the bulky substituents can in these molecules assume orientations significantly less interfering with each other than in the benzene ring.
The goals of this work were the following:
- To investigate the thermodynamic stability and acidity of the polyfluorinated and polytrifluoromethylated isomers of C6H5OH and C6H5H and the dependence of their acidity on the structure.
- To elucidate and compare the acidifying effect and its mode of action in the case of polytrifluoromethylation with that of polyfluorination.
Methods
Computational method
Gacid of an acid HA is the Gibbs’ free energy change on deprotonation of the acid according to the following equilibrium:
HA Gacid A– + H+ (1)
The ΔGacid values were calculated in the usual way21 taking into account zero-point energies, finite temperature (298 K) correction and the pressure-volume work term. Density functional theory (DFT) calculations at B3LYP 6-311+G** level were used. The Gaussian 09 system of programs was used.22 Full geometry optimizations were carried out for all acids and anions. Several different starting geometries were used in doubtful cases. In order to confirm that calculated structures correspond to true minima, frequency calculations were run in all cases and the absence of imaginary frequencies (Nimag = 0) was taken as the criterion of the stability of the species.
Isodesmic reactions analysis method7
The acidity of a pentafluoro (Y = F) or pentakis(trifluoromethyl) (Y = CF3) substituted hydrocarbons 1 to 3 (X = H) or their hydroxy (X = OH) derivatives C6Y5X, denoted as ΔGacid(C6Y5X), can be calculated from the acidity of the unsubstituted compound ΔGacid(C6H5X) and the acidifying/deacidifying effects of different interactions as follows:
ΔGacid(C6Y5X) = ΔGacid(C6H5X) + ΔΔGGAIE + ΔΔGS + ΔΔGRCC + ΔΔGRCX (2)
The ΔΔG values in eq 2 are defined as follows:
ΔΔG = ΔG(anion) – ΔG(neutral)(3)
The ΔG values are defined as follows7:
- ΔGGAIE (defined via eq 5) is the estimate of the gross interaction energy between the reaction center and the substituents Y in the idealized pentasubstituted molecule where there are no steric or other interactions between the substituents themselves and the interactions between the substituents and the reaction center are just as strong as in the respective monosubstituted molecules. Possible steric interactions present in the monosubstituted molecules are also included in ΔGGAIE.
- ΔGS is the energy contribution due to saturation of the substituent effects.
- ΔGRCC is the energy contribution due to steric repulsion between the Y groups.
- ΔGRCX is the energy contribution due to additional steric repulsion between the Y groups adjacent to the group X (or its deprotonated form). This additional contribution has two reasons: (1) in the pentakis-substituted derivative there can be simultaneously several CF3 groups in the vicinity of X and (2) there may be other Y groups that reduce the flexibility of the Y groups in the vicinity of X.
The ΔGGAIE contributions can be estimated from the following series of reactions (in this and the following equations the circle denotes one of the C6H6 isomers, 1 to 3):
(4)
X = OH, HY = F, CF3
In order to obtain the ΔGGAIE the energy effects of this reaction with different substitution pattern are summarized taking into account the symmetry of the molecule. As an example, for 1-OH-Y5-1:
ΔGGAIE = ΔGIE(1,2) + 2 ΔGIE(1,3) + 2 ΔGIE(1,5) (5)
The ΔGRCC is found via the following equation:
(6)
Y = F, CF3
Since the group X is not involved in this reaction this contribution is the same in the respective substituted hydrocarbon and hydroxyl derivative.
No single isodesmic reaction equations can be written for obtaining the remaining two ΔG contributions: the contributions ΔGS + ΔGRCX can in the framework of this isodesmic reaction approach be estimated only jointly. The following series of reactions was used:
(7)
X = OH, HY = F, CF3
denotes for every compound the sum of the five possible isomers of C6H4XY. The negative free energy change of these reactions can be expressed as follows:
ΔGSRR = ΔGS + ΔGRCC + ΔGRCX(8)
From eqs 6 and 8 follows that:
ΔGS + ΔGRCX = ΔGSRR – ΔGRCC(9)
Results
The proton affinities, inherent gas-phase acidities together with calculated acidifying and substituent additivity effects as well as relative stabilities of the systems that remained intact are presented in Table 1 and 2. The corresponding energies (HF, H, G) of all systems calculated are presented in Supporting Information.
All neutrals calculated were stable with respect to the geometry optimization of the used computational method, except pentakistrifluoromethylated 1-hydroxyderivative of 2, whose frequency calculations resulted in one imaginary frequency. However, in some of the anions significant bond elongations or bond ruptures took place. The monosubstituted 5-F- and 5-CF3- and also perfluorosubstituted hydroxy anions of 1 went through a C(5) – C(6) bond cleavage. Perfluorinated and pertrifluoromethylated 1-hydroxy anions of 2 underwent rearrangement (rupture of the 1-4 bond) to give the corresponding phenolate derivatives. Several anions of the monosubstituted 1-OH- and 5-OH-derivatives as well as 1-OH persubstituted and 5-OH perfluoro derivatives of 3 also rearranged into monosubstituted phenolate ions.
Table 1. The results of calculations of the unsubstituted and pentakis-substituted systems and their respective acidifying effects and relative stabilities towards the corresponding parent isomers of benzenes (all values in kcalmol-1).
Acid / PA(A-) / GA / Acidifying effect / Rel stab Neutral / Rel stab Neutral / Rel stab Anion / Rel stab AnionGA / H / G / H / G / Comments
H-0 / 400.7 / 393.3
H-F5-0 / 354.4 / 346.5 / -46.8
H-(CF3)5-0 / 341.9 / 332.4 / -60.9
H-1 / 402.6 / 394.7 / 122.1 / 122.6 / 124.0 / 124.1
H-F5-1 / 343.1 / 335.6 / -59.2 / 131.2 / 131.0 / 119.9 / 120.1
H-(CF3)5-1 / 341.8 / 334.9 / -59.8 / 82.2 / 77.9 / 82.1 / 80.4
1-H-2 / 390.0 / 382.2 / 82.9 / 82.8 / 72.3 / 71.7
1-H-F5-2 / 358.1 / 350.0 / -32.2 / 70.7 / 70.3 / 74.4 / 73.8
1-H-(CF3)5-2 / 327.8 / 319.9 / -62.3 / 46.4 / 41.1 / 32.3 / 28.6
2-H-2 / 397.7 / 389.9 / 82.9 / 82.8 / 80.0 / 79.4
2-H-F5-2 / 362.4 / 354.5 / -35.4 / 70.1 / 69.8 / 78.1 / 77.8
2-H-(CF3)5-2 / 344.1 / 336.3 / -53.6 / 47.4 / 43.3 / 49.6 / 47.2
1-H-3 / 401.5 / 393.6 / 80.1 / 80.4 / 80.9 / 80.7
1-H-F5-3 / 355.1 / 347.2 / -46.4 / 93.1 / 92.8 / 93.7 / 93.6
1-H-(CF3)5-3 / 346.2 / 338.3 / -55.4 / 51.2 / 47.7 / 55.5 / 53.6
2-H-3 / 401.0 / 393.2 / 80.1 / 80.4 / 80.4 / 80.2
2-H-F5-3 / 362.2 / 354.3 / -38.9 / 92.8 / 92.6 / 100.5 / 100.3
2-H-(CF3)5-3 / 346.9 / 339.8 / -53.4 / 51.7 / 48.3 / 56.7 / 55.7
5-H-3 / 391.4 / 383.5 / 80.1 / 80.4 / 70.8 / 70.6
5-H-F5-3 / 345.4 / 337.6 / -45.9 / 91.5 / 91.4 / 82.5 / 82.5
5-H-(CF3)5-3 / 333.6 / 326.1 / -57.4 / 54.8 / 52.0 / 46.5 / 45.7
OH-0 / 346.8 / 339.2
OH-F5-0 / 323.5 / 315.7 / -23.5
OH-(CF3)5-0 / 297.8 / 290.2 / -49.0
OH-1 / 357.6 / 350.0 / 124.1 / 124.6 / 135.0 / 135.4
OH-F5-1 / 133.3 / 132.9 / Anion: C1-C5 broken
OH-(CF3)5-1 / 305.4 / 298.0 / -52.0 / 79.8 / 75.3 / 87.3 / 83.0
1-OH-2 / 361.3 / 353.5 / 85.5 / 85.4 / 100.1 / 99.7
1-OH-F5-2 / 68.7 / 68.4 / Anion: C1-C4 broken
1-OH-(CF3)5-2 / Anion: broken, Neutral: Nimag=1
2-OH-2 / 343.6 / 336.2 / 80.4 / 80.1 / 77.2 / 77.1
2-OH-F5-2 / 314.7 / 307.2 / -29.1 / 64.9 / 64.5 / 56.1 / 55.9
2-OH-(CF3)5-2 / 296.5 / 288.5 / -47.8 / 34.0 / 30.0 / 32.8 / 28.2
1-OH-3 / 358.5 / 350.7 / 83.9 / 84.1 / 95.6 / 95.5
1-OH-F5-3 / 94.9 / 94.5 / Anion: changed the order of C's
1-OH-(CF3)5-3 / 49.5 / 46.0 / Anion: bond broken
2-OH-3 / 350.9 / 342.9 / 81.3 / 81.6 / 85.4 / 85.3
2-OH-F5-3 / 324.4 / 317.0 / -26.0 / 93.5 / 92.9 / 94.4 / 94.2
2-OH-(CF3)5-3 / 303.9 / 296.2 / -46.7 / 42.7 / 39.4 / 48.8 / 45.3
5-OH-3 / 351.0 / 343.1 / 82.6 / 82.8 / 86.8 / 86.7
5-OH-F5-3 / 93.0 / 92.2 / Anion: Bond C5-C6 broken
5-OH-(CF3)5-3 / 298.7 / 290.5 / -52.6 / 52.1 / 49.2 / 52.9 / 49.5 / Anion: single subst. broken
Table 2. Results of computations of the mono-substituted systems that remained intact and their stabilities towards the corresponding parent isomers of benzene (all values in kcalmol-1).
Acid / PA(A-) / GA / Acidifying effect / Rel stab Neutral / Rel stab NeutralGA / H / G
1-H-2-F-1 / 390.7 / 382.9 / -11.8 / 124.9 / 125.3
1-H-3-F-1 / 394.3 / 386.4 / -8.3 / 124.9 / 125.3
1-H-5-F-1 / 389.5 / 381.6 / -13.1 / 124.9 / 125.3
1-H-2-F-2 / 381.2 / 373.4 / -8.9 / 81.5 / 81.3
1-H-3-F-2 / 385.5 / 377.6 / -4.6 / 81.5 / 81.3
1-H-4-F-2 / 381.8 / 373.9 / -8.3 / 83.7 / 83.6
2-H-1-F-2 / 389.3 / 381.4 / -8.5 / 83.7 / 83.6
2-H-3-F-2 / 388.5 / 380.6 / -9.3 / 81.5 / 81.3
2-H-4-F-2 / 389.0 / 381.1 / -8.8 / 83.7 / 83.6
2-H-5-F-2 / 392.6 / 384.7 / -5.2 / 81.5 / 81.3
2-H-6-F-2 / 391.8 / 383.9 / -6.0 / 81.5 / 81.3
1-H-2-F-3 / 391.2 / 383.3 / -10.3 / 83.6 / 83.8
1-H-3-F-3 / 396.3 / 388.3 / -5.3 / 83.6 / 83.8
1-H-4-F-3 / 392.9 / 384.9 / -8.7 / 82.8 / 83.1
1-H-5-F-3 / 391.1 / 383.2 / -10.5 / 85.3 / 85.4
2-H-1-F-3 / 390.9 / 383.1 / -10.1 / 82.8 / 83.1
2-H-3-F-3 / 391.3 / 383.5 / -9.7 / 83.6 / 83.8
2-H-4-F-3 / 393.9 / 386.1 / -7.1 / 82.8 / 83.1
2-H-5-F-3 / 395.1 / 387.2 / -6.0 / 85.3 / 85.4
5-H-1-F-3 / 382.2 / 374.3 / -9.2 / 82.8 / 83.1
5-H-2-F-3 / 385.9 / 378.0 / -5.6 / 83.6 / 83.8
5-H-6-F-3 / 373.0 / 364.9 / -18.7 / 85.3 / 85.4
1-H-2-CF3-1 / 387.6 / 379.8 / -14.9 / 118.0 / 119.2
1-H-3-CF3-1 / 389.1 / 381.2 / -13.5 / 118.0 / 119.2
1-H-5-CF3-1 / 385.8 / 377.9 / -16.9 / 118.0 / 119.2
1-H-2-CF3-2 / 376.3 / 368.4 / -13.8 / 80.6 / 80.8
1-H-3-CF3-2 / 372.6 / 364.7 / -17.6 / 80.6 / 80.8
1-H-4-CF3-2 / 375.6 / 367.9 / -14.3 / 80.5 / 81.1
2-H-1-CF3-2 / 384.5 / 376.7 / -13.2 / 80.5 / 81.1
2-H-3-CF3-2 / 379.9 / 372.0 / -17.8 / 80.6 / 80.8
2-H-4-CF3-2 / 385.3 / 377.5 / -12.4 / 80.5 / 81.1
2-H-5-CF3-2 / 386.5 / 378.5 / -11.3 / 80.6 / 80.8
2-H-6-CF3-2 / 387.5 / 379.5 / -10.4 / 80.6 / 80.8
1-H-2-CF3-3 / 388.1 / 380.3 / -13.3 / 77.6 / 78.3
1-H-3-CF3-3 / 388.0 / 380.1 / -13.5 / 77.6 / 78.3
1-H-4-CF3-3 / 388.8 / 380.9 / -12.8 / 78.8 / 79.7
1-H-5-CF3-3 / 386.6 / 378.7 / -15.0 / 78.1 / 78.8
2-H-1-CF3-3 / 388.0 / 380.3 / -12.9 / 78.8 / 79.7
2-H-3-CF3-3 / 382.1 / 374.3 / -18.9 / 77.6 / 78.3
2-H-4-CF3-3 / 389.2 / 381.4 / -11.8 / 78.8 / 79.7
2-H-5-CF3-3 / 389.2 / 381.4 / -11.8 / 78.1 / 78.8
5-H-1-CF3-3 / 376.3 / 368.4 / -15.1 / 78.8 / 79.7
5-H-2-CF3-3 / 378.7 / 370.8 / -12.8 / 77.6 / 78.3
5-H-6-CF3-3 / 374.9 / 367.0 / -16.5 / 78.1 / 78.8
1-OH-2-F-1 / 351.0 / 343.1 / -6.8
1-OH-3-F-1 / 350.2 / 342.4 / -7.6
2-OH-1-F-2 / 338.5 / 331.0 / -5.2
2-OH-3-F-2 / 340.4 / 332.8 / -3.4
2-OH-4-F-2 / 333.4 / 326.0 / -10.3
2-OH-5-F-2 / 338.7 / 331.3 / -5.0
2-OH-6-F-2 / 338.6 / 331.2 / -5.1
2-OH-1-F-3 / 345.1 / 337.3 / -5.6
2-OH-3-F-3 / 345.2 / 338.1 / -4.9
2-OH-4-F-3 / 341.9 / 334.0 / -9.0
2-OH-5-F-3 / 343.7 / 336.2 / -6.8
1-OH-2-CF3-1 / 345.1 / 337.3 / -12.7
1-OH-3-CF3-1 / 346.2 / 338.5 / -11.5
2-OH-1-CF3-2 / 333.1 / 325.5 / -10.7
2-OH-3-CF3-2 / 327.9 / 319.8 / -16.4
2-OH-4-CF3-2 / 332.7 / 325.2 / -11.0
2-OH-5-CF3-2 / 334.5 / 326.9 / -9.3
2-OH-6-CF3-2 / 335.6 / 328.0 / -8.2
1-OH-2-CF3-3 / 349.0 / 341.1 / -2.0
1-OH-3-CF3-3 / 345.2 / 337.7 / -5.5
2-OH-1-CF3-3 / 339.9 / 332.1 / -11.1
2-OH-3-CF3-3 / 333.7 / 325.3 / -17.8
2-OH-4-CF3-3 / 340.2 / 332.1 / -11.0
2-OH-5-CF3-3 / 339.6 / 331.7 / -11.5
5-OH-6-CF3-3 / 335.5 / 328.5 / -14.6
As seen from the results in SI (@arvutusandmete terviktabel SI’sse), the rupture of the bonds in the anions is accompanied by a significant release of steric strain and additional stabilization of the system. Because the process is not a reversible Brønsted acid-base equilibrium such acidities can only be called apparent. In order to carry out the analysis of substituent effects according to the above presented scheme it is necessary that all the neutral and anionic species involved in equations 4, 6 and 8 can be computed and that no bond ruptures take place. If a species with disrupted bond (and thus with several tens of kcalmol-1 extra stabilization) were included in the analysis, the whole analysis would immediately be meaningless. Given the bond rupture problems outlined above it was possible to carry out the analysis of isodesmic reactions only for H-(CF3)5-1, H-F5-1, 1-H-(CF3)5-2, 1-H-F5-2, 2-H-(CF3)5-2, 2-H-F5-2, 2-OH-(CF3)5-2, 2-OH-F5-2, 1-H-(CF3)5-3, 1-H-F5-3, 2-H-(CF3)5-3, 2-H-F5-3, 5-H-(CF3)5-3, 5-H-F5-3, 2-OH-(CF3)5-3 and 2-OH-F5-3 species. OH-(CF3)5-1was also included with certain reservations. The single-substituted anion with substituent in position 5 had a bond rupture, so the energies of the corresponding derivative with substituent in position 2 were used instead. Results of the analysis of substituent effects using the isodesmic reactions approach described above are presented in Table 3.
Table 3. Results of the analysis of substituent effects according to eq 5-9 (all G values in kcalmol-1).
Neutral / AnionGGAIE / GSRR / GRCC / GS+GRCX / GGAIE / GSRR / GRCC / GS+GRCX / Additivity (%)
H-F5-0 / 0 / 24.0 / 24.0 / 0 / -48.2 / 25.4 / 24.0 / 1.4 / 97.1
H-F5-1 / 0 / 19.0 / 19.0 / 0 / -54.7 / 14.6 / 19.0 / -4.4 / 108.1
1-H-F5-2 / 0 / 16.9 / 16.9 / 0 / -35.3 / 19.9 / 16.9 / 3.0 / 91.4
2-H-F5-2 / 0 / 14.0 / 14.0 / 0 / -47.8 / 26.5 / 14.0 / 12.4 / 74.0
1-H-F5-3 / 0 / 16.7 / 16.7 / 0 / -45.2 / 15.5 / 16.7 / -1.2 / 102.6
2-H-F5-3 / 0 / 17.1 / 17.1 / 0 / -43.1 / 21.3 / 17.1 / 4.2 / 90.2
5-H-F5-3 / 0 / 17.6 / 17.6 / 0 / -48.3 / 19.9 / 17.6 / 2.3 / 95.1
H-(CF3)5-0 / 0 / 56.0 / 56.0 / 0 / -68.5 / 63.6 / 56.0 / 7.6 / 88.9
H-(CF3)5-1 / 0 / 28.9 / 28.9 / 0 / -75.6 / 44.6 / 28.9 / 15.8 / 79.1
1-H-(CF3)5-2 / 0 / 24.2 / 24.2 / 0 / -77.1 / 39.0 / 24.2 / 14.8 / 80.8
2-H-(CF3)5-2 / 0 / 26.1 / 26.1 / 0 / -65.1 / 37.6 / 26.1 / 11.5 / 82.3
1-H-(CF3)5-3 / 0 / 31.4 / 31.4 / 0 / -69.5 / 45.6 / 31.4 / 14.2 / 79.6
2-H-(CF3)5-3 / 0 / 30.5 / 30.5 / 0 / -67.1 / 44.2 / 30.5 / 13.7 / 79.6
5-H-(CF3)5-3 / 0 / 34.7 / 34.7 / 0 / -72.3 / 49.6 / 34.7 / 14.8 / 79.5
OH-F5-0 / 3.7 / 26.0 / 24.0 / 2.1 / -19.2 / 25.4 / 24.0 / 1.5 / 102.6
2-OH-F5-2 / 2.3 / 14.8 / 14.0 / 0.7 / -26.6 / 14.6 / 14.0 / 0.6 / 100.6
2-OH-F5-3 / 10.1 / 11.9 / 17.1 / -5.2 / -22.9 / 18.9 / 17.1 / 1.8 / 78.7
OH-(CF3)5-0 / 3.8 / 61.1 / 56.0 / 5.0 / -56.5 / 72.4 / 56.0 / 16.3 / 81.3
OH-(CF3)5-1a / 3.2 / 29.9 / 28.9 / 1.0 / -57.8 / 38.9 / 28.9 / 10.1 / 85.2
2-OH-(CF3)5-2 / -4.3 / 28.6 / 26.1 / 2.5 / -60.0 / 36.6 / 26.1 / 10.4 / 85.7
2-OH-(CF3)5-3 / -1.2 / 30.4 / 30.5 / -0.1 / -63.0 / 45.5 / 30.5 / 15.0 / 75.6
aFor the position 5 the energies of the corresponding anion with substituent in position 2 were used
The gross acidifying effect among the fluorinated valence isomers of Kekulé benzene is higher than in the case of0only in case of the prismane (by 6.5 kcalmol-1) and significantly lower (12.9 kcalmol-1) in case of the 1-H-F5-2. The respective trifluoromethyl derivatives had up to 7.1 kcalmol-1higher GGAIE when the reaction center was not on the bridgehead of the double bond. If the gross acidifying effects among hydrocarbons and hydroxyl derivatives are comparedthen the effect among the latter ones was markedly lower. Nevertheless, in the cases where the HO-systems remained intact, the isomerisation improved the gross acidifying effect.GRCC parameters followed the pattern where the strongest repulsion was detected between the substituents of 0 and other isomers were ordered as follows: prismane > benzvalene > dewar benzene. The GS + GRCX values were noted generally larger in case of the CF3-substituted systems. For all but 2-H-F5-2 (12.4 kcalmol-1) of the fluorine derivatives they remained below 5 kcalmol-1. Two of them were even negative. In several cases very low values of substituent effect saturation were accompanied by additivity rates above 100%.
Discussion
Since acidities refer first of all to Gibbs' free energies, the discussion below uses Gibbs' free energies (unless specifically stated otherwise). Using enthalpies leads in all cases to the same conclusions.
The stability of the calculated derivatives is evaluated with respect to the corresponding isomeric benzene derivatives.
Stability of the derivatives of 1-3
The unsubstituted hydrocarbons 1-3 are significantly less stable than benzene (0).Prismane is the most strained of them and is by even by 123 kcalmol-1 less stable than benzene. Relative stabilities of the pentakis-fluorinated 1 and 3 as well as the corresponding alcohols with respect to pentafluorobenzene and pentafluorophenol, respectively, are even less stable with strain energies: 131.0 kcalmol-1 in the case of 1-H-F5-1 and 132.9 for 1-OH-F5-1.The latter is predicted as the least stable compound investigated in this work. For Dewar benzene and benzvalene the relative (in)stabilities are 82.8 and 80.4 kcalmol-1, respectively.
In most cases the relative stabilities of the parent (fluoro)hydrocarbons and the corresponding alcohols are fairly similar. This cannot be generalized to their anions. As opposed to the deprotonated hydrocarbons, deprotonated OH derivatives of1-3can be significantly destabilized if the OH group is not attached to a double bond, e. g. 11 to 28 kcalmol-1 lower relative stabilities towards the respective derivatives of0 than the respective deprotonated hydrocarbons and were accompanied by significant (>0.1 Å) C(α) – C(β) bond lengthening.This was also observed with C10H15O– derivatives in ref 6. In two cases (OH-(CF3)5-1and 5-OH-(CF3)5-3) when CF3-substituents were used the X = H/OH difference of relative stabilities remained under 4 kcalmol-1. The rest of the deprotonated fluoro- and CF3-derivatives of 1-OH- and 5-OH- alcohols were unstable as indicated by the cleavage of a C(α) – C(β) bond from the X.
The situation is quite different if OH-group is in position 2. Especially in the fluorinated and trifluoromethylated structure of Dewar benzene this positionseems to favor the stabilizing orbital interactions between the–O–fragment and substituents. The anion of 2-OH-F5-2 had 21.2 kcalmol-1 better relative stabilization against –O-F5-0 than the unsubstituted2 against phenolate anion–O-0. One reason is obviously the resonance between the –O–center and the unsaturated part that is visible from the stability of the anions of both 2 and 3 and which is also confirmed by the results of NBO calculations where the E(2) energy of the oxygen lone pair interaction with C(2) – C(3) antibonding orbital is in the range of 100 kcalmol-1 (@Ivo: kas tõesti nii palju?Lauri: Jah.). This resonance interaction is visible also with the neutrals of the respective hydroxyl compound resulting in E(2) energies around 30 kcalmol-1.
It has been demonstrated that polyfluorination stabilizes the anions of strained alcohols.6 First of all this occurs via the partial transfer of electron density from the nonbonding orbitals of the –O– center to the * orbitals of the adjacent C-C bonds of the hydrocarbon skeleton. This effect causes elongation of the C-C bonds and partial release of the steric strain. In derivatives of 1-3the exact stabilizing effect significantly depends on the orientation and distance between interacting substituent, functional group and C-C bond acceptor orbitals as well as the position of the double bond relative to the reaction centre. Certain substitution patterns can easily result in decomposition of the molecular structure.
The substituted hydrocarbons are generally slightly less stable then the corresponding hydroxyl derivatives. Nevertheless, the former are less prone to decomposition on protonation. In many cases the relative stabilities even improve considerably with deprotonation: H-F5-1 (-10.9 kcalmol-1), 1-H-2 (-11.1 kcalmol-1), 1-H-(CF3)5-2 (-12.5 kcalmol-1) and the derivatives of 5-3 (-6.3…-9.8 kcalmol-1). Notable exceptions are the pentakis-substituted derivatives with the -H at the bridgehead of a double bond, where deprotonation results in ca8 kcalmol-1 worse relative stability.