Structure of Methyl 2-Methylbenzoate: Steric Effect of a Smallortho-Substituent

Hnyk, Drahomı́r; Borisenko, Konstantin B.; Samdal, Svein; Exner, Otto

doi:10.1002/1099-0690(200006)2000:11<2063::aid-ejoc2063>3.0.co;2-0

Cited by 8 publications

(9 citation statements)

References 23 publications

(39 reference statements)

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“…The possibility of significant ring distortions and widening of the adjoining angles has been generally underestimated or omitted in previous discussions of SIR. However, a joint GED/ab initio investigation of 1-CH 3 COO-2-CH 3 -C 6 H 4 did pay attention to the possibility of these geometrical features and found a lengthening of the C(1)−C(2) distance in the ring and a widening of the adjacent C(2)−C(1)−C(7) and C(1)−C(2)−C(10) angles, as well as other deformations of the ring . In 1 , the same geometrical features are observed (see Table ).…”

Section: Resultsmentioning

confidence: 72%

“…However, the change in R factor on varying p 28 about its calculated value was negligible (note that the free energy difference gives p 28 as 93%), highlighting the limited information in the data about the presence of the second form. The parameter p 28 was therefore fixed at its MP2(full)/6-311++G** value, as was the case in the study of methyl 2-methylbenzoate . Many refinements were also tried with different starting values (e.g., those provided by various functionals) for p 22 .…”

Section: Resultsmentioning

confidence: 99%

“…According to the classic theory, it was assumed ,− that ϕ varies continuously between 0 and 90° according to the strength of the steric hindrance. However, this view was recently challenged by us ,, on the basis that certain less-hindered compounds evidently exist in planar conformations and as an equilibrium of two conformers, as shown in 1B ⇋ 1C . Besides the X-ray analysis of 2-methyl-5-nitrobenzoic acid, the most important direct experimental proof was the gas electron diffraction (GED) structure of methyl 2-methylbenzoate .…”

Section: Introductionmentioning

confidence: 99%

“…However, this view was recently challenged by us ,, on the basis that certain less-hindered compounds evidently exist in planar conformations and as an equilibrium of two conformers, as shown in 1B ⇋ 1C . Besides the X-ray analysis of 2-methyl-5-nitrobenzoic acid, the most important direct experimental proof was the gas electron diffraction (GED) structure of methyl 2-methylbenzoate . However, this study of a compound with 21 atoms, two axes of rotation, and two rotamers in equilibrium was already at the limits of possibility of the GED method.…”

Section: Introductionmentioning

confidence: 99%

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Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

et al. 2010

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The structure and conformations of 2-methylacetophenone (1) have been investigated by ab initio calculations carried out at the MP2(full)/6-311++G** level and by gas electron diffraction (GED). According to both methods, 1 exists predominantly as a form with the C=O bond synclinal with respect to the C(ar)-C(O) bond (1B), with a torsional angle [C(6)-C(1)-C=O] of 32.7(24) degrees as determined by GED and 26.0 degrees from MP2(full)/6-311++G**. Calculations also predict the presence of a second conformer, the anticlinal structure (1C), with phi = 140.0 degrees, with an abundance of less than 6%, an amount hardly detectable by GED. Different DFT computational protocols both support a nonplanar form of the predominant conformer (B2PLYP) and are in contradiction (B3LYP, M052x, B98, B97-D) with this experimental finding. The GED results, supported by the calculations that involve long-range correlation, are in a good agreement with (13)C NMR spectroscopic investigations, UV spectra, and dipole moment studies. However, previous claims that assumed steric inhibition of resonance caused by a significantly nonplanar conformation with phi close to 90 degrees have been disproved. Steric crowding is evident from the geometrical parameters, particularly from the C(1)-C(2) bond length and from the C(1)-C(2)-C(H(3)) and C(2)-C(1)-C(O) bond angles. It is concluded that any explanation of reactivity by steric inhibition of resonance and by other steric factors must be supported by experimental and/or theoretical investigation of the actual molecular shape.

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Section: Resultsmentioning

confidence: 72%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

et al. 2010

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“…Planar conformation of 2 was discussed together with further methyl derivatives 6 and supported by electron diffraction of methyl 2methylbenzoate. 28 Most remarkably, the conformation of the anions is not always the same as that of the acids. In the anions of 2 and 10 the carboxylate group is twisted (by 31 and 59 , respectively) while in the anion of 4 it is coplanar.…”

Section: Conformations and Their Populationmentioning

confidence: 99%

Analysis of the ortho effect: acidity of 2-substituted benzoic acids

2004

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The classical term ortho effect was quantitatively analyzed and decomposed into its real or supposed components, using the acidity of eleven 2-substituted benzoic acids as a sample. The substituent effects were evaluated by calculations at the B3LYP/6-311þG(d,p) level by means of isodesmic reactions, separately for the acid molecules and for their anions. An intramolecular hydrogen bond, affecting the acidity moderately, was found only in the case of 2-methoxy-and 2-dimethylaminobenzoic acids. For the other acids, the oftenemployed interpretation invoking a hydrogen bond is wrong: the pertinent form was not revealed by calculations or by infrared spectroscopy. Another widespread explanation relying on the steric inhibition of resonance of the COOH group is also not correct in most cases: only some acid molecules are nonplanar (with substituents tert-Bu, CH 2 Cl, CF 3 , NO 2 , Cl) and the steric inhibition exists also in the anions, thus partly cancelling this effect.The main part of the ortho effect consists of polar effects (inductive and resonance) transmitted through the ring and of purely steric effects. Their tentative separation was based on comparison with the 4-substituted isomers. Polar effects are most important for the acidity and are not very different in the ortho and para positions (rough estimate 0.8:1). Steric effects are strong but mostly of similar magnitude in the acid as in the anion; hence they are of restricted importance for the acidity. They may be better described as van der Waals interactions rather than as short-range field effects. The whole analysis leads to the conclusion that the acidity of ortho-substituted benzoic acids cannot be interpreted by a simple universal theory.Benzene ortho derivatives have received less attention compared to their meta and para isomers. While in the latter the dependence of the reactivity and properties on structure is simple and can be approximated by the Hammett equation, effects of ortho substituents are much more complex. 1,2 Mostly they have been assumed to arise from two contributions: one similar as in the meta and para isomers, called inductive effect and resonance, 1,3 the other commonly called steric. The latter was further divided into several tentative components, 2,4,5 which were not always exactly identified: primary steric effect, steric inhibition to resonance (SIR), short-range field effect, hydrogen bonds (HB), steric inhibition to solvation and others. In numerous treatments of this subject, certain of these effects were more or less stressed and sometimes they were expressed quantitatively by an empirical equation with limited success. 4 Note that some of these effects are only simplified models, sometimes poorly defined: for instance, one can hardly distinguish a short-range field effect from a primary steric effect. As an extreme opinion, reactivities of ortho derivatives were explained without referring to any steric effects at all, 3 either by a blend of inductive and resonance effects 3a,b or most simply, by correlation with the...

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¹³C and ¹H nuclear magnetic resonance of methyl‐substituted acetophenones and methyl benzoates: steric hindrance and inhibited conjugation

Budêšínský

Kulhánek

Böhm

et al. 2004

Magnetic Reson in Chemistry

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The 1H and 13C NMR spectra of 14 methyl-substituted acetophenones and 14 methyl-substituted methyl benzoates were assigned and interpreted with respect to the conformation of the C(ar)-C(O) bond. The substituent effects are proportional in the two series and can be divided into polar and steric: each has different effects on the 13C SCS of the individual atoms. In the case of C atoms C(O), C(1) and CH3(CO), the steric effects were quantitatively separated by comparing SCS in the ortho and para positions. The steric effects are proportional for the individual C atoms and also to steric effects estimated from other physical quantities. However, they do not depend simply on the angle of torsion phi of the functional group as anticipated hitherto. A better description distinguishes two classes of compounds: sterically not hindered or slightly hindered planar molecules and strongly sterically hindered, markedly non-planar. In order to confirm this reasoning without empirical correlations, the J(C,C) coupling constants were measured for three acetophenone derivatives labeled with 13C in the acetyl methyl group. The constants confirm unambiguously the conformation of 2-methylacetophenone; their zero values are in accord with the conformation of 2,6-dimethylacetophenone. The zero values in the unsubstituted acetophenone are at variance with previous erroneous report but all J(C,C) values are in accord with calculations at the B3LYP/6-311++G(2d,2p)//B3LYP/6-311+G(d,p) level.

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Structure of Methyl 2-Methylbenzoate: Steric Effect of a Smallortho-Substituent

Cited by 8 publications

References 23 publications

Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

Analysis of the ortho effect: acidity of 2-substituted benzoic acids

¹³C and ¹H nuclear magnetic resonance of methyl‐substituted acetophenones and methyl benzoates: steric hindrance and inhibited conjugation

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Structure of Methyl 2-Methylbenzoate: Steric Effect of a Smallortho-Substituent

Cited by 8 publications

References 23 publications

Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

Analysis of the ortho effect: acidity of 2-substituted benzoic acids

13C and 1H nuclear magnetic resonance of methyl‐substituted acetophenones and methyl benzoates: steric hindrance and inhibited conjugation

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¹³C and ¹H nuclear magnetic resonance of methyl‐substituted acetophenones and methyl benzoates: steric hindrance and inhibited conjugation