Determination of a new scale of ortho-steric parameters S 0 from N-methylation of pyridines

Berg, Ulf; Gallo, R.; Klatte, Gerd; Metzger, Jacques

doi:10.1039/p29800001350

Cited by 37 publications

(11 citation statements)

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“…Quantitative studies of steric effects of substituents must separate these effects from electronic effects . Note that the ratios of the two fragment ions of Fe + -bound dimers containing unhindered pyridines are very similar to those in proton-bound dimers.…”

Section: Resultsmentioning

confidence: 98%

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe⁺ Affinities of Pyridines

Wong

Yang

et al. 1996

J. Am. Chem. Soc.

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The relative molecular, molecular pair, and molecular triplet Fe + affinities of pyridines are determined by the kinetic method through examination of the competitive dissociation of Fe + -bound dimer, trimer, and tetramer cluster ions. The molecular Fe + affinities vary strongly with the electron donating power of the substituent on the pyridine. Similar behavior is seen in the molecular pair and molecular triplet affinities, the first triplet affinity values determined by the kinetic method. The absolute Fe + affinity of pyridine itself is estimated to be 49 ( 3 kcal/mol. A linear correlation (slope ) 0.38) is demonstrated between the relative Fe + affinities and the corresponding proton affinities of the meta-and para-substituted pyridines. The slope together with data from previous studies falls in the order of OCNCO + ≈ SiCl 3 + > Cl + > CN + > SiCl + > Fe + , which indicates that the strength of binding to pyridines falls in the same order. The relative molecular pair and molecular triplet Fe + affinities also correlate linearly with the number averaged proton affinities of the pyridines. Lower than expected affinities are observed for systems containing ortho-substituted pyridines due to intramolecular steric effects between the ortho-substituted alkyl group(s) and the central Fe + cation. A set of gas-phase steric parameters (S k ), measured by the deviation of the experimental data from the regression line, shows much larger steric effects in the crowded trimers and tetramers.

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Section: Resultsmentioning

confidence: 98%

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe⁺ Affinities of Pyridines

Wong

Yang

et al. 1996

J. Am. Chem. Soc.

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“…Berg et al (23) discussed the empirical and theoretical parameters that describe steric and solvent effects on aromatic systems. In one of his examples, a methyl derivative was 2.5 times faster than an ethyl compound.…”

Section: Structure1mentioning

confidence: 99%

Effect of chain length on the rate of quaternization of alkyl amines

Friedli¹

1990

J Americ Oil Chem Soc

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Trialkyl amines from triethyl to tristearylamine were quaternized with benzyl chloride and compared for the effect of chain length on the rate of quaternization. Triethylamine reacted about twice as fast as tripropylamine with virtually no chain-length effect from propyl to stearyl. Results are compared to previous rate studies on steric hindrance and chain-length dependence.In the course of studying process improvements on alkylation of tertiary amines to quaternary ammonium salts, the reaction properties of trialkyl amines (Structure 1) were examined. R3N STRUCTURE1Industrially, amines from trimethylamine [R = Me) to tristearylamine (R --ClsH37) are readily available. Molecular models of the long-chain analogs (Structure 2) give the distinct impression of an aquatic organism. STRUCTURE 2 a 3-legged octopus.The flexibility and possible conformations are almost limitless. Depending on the conformations most favorable under a particular condition or solvent system, it is possible to theorize either equal quaternization rates for the short-and long-chain homologs of the tertiary amine series, or vastly differing rates. Three basic types of conformations seem possible in solution: (i) parallel (Structure 3): where the hydrophobic tails cling together with no solvent molecules separating them: N STRUCTURE3 (ii) separated (Structure 4): where the hydrocarbon chains are widely separated by solvent molecules: .j STRUCTURE4 (iii) ball (Structure 5): where the chains are coiled and twisted around the nitrogen: STRUCTURE 5Structures 3 and 4 should offer no particular steric hindrance to quaternization, while Structure 5 should be very slow to quaternize due to steric problems. It is known that quaternizations occur best in solvents with high dipole moments and dielectric constants {1). Acetonitrile, ethanol, or tetrahydroforan are much better reaction media than hexane or toluene. The coiled conformation should create a non-polar hydrocarbon environment around the reactive nitrogen, disfavoring formation of polar transition states typical in SN2 displacements. The effect could further slow the rate of reaction of Structure 5.While long-chain trialkyl amines do have appreciable solubility (at temperatures above their melting points) in polar organic solvents, it seems likely that they are not totally compatible on a molecular level. This conjecture is the basis for the three suggested conformation types. Rapid conversion between the types likely occurs with the important feature being the equilibrium ratio of the conformers. NMR evidence on the conformations of the amines in solution cannot be determined with certainty. The techniques are very difficult and not fully developed at this time (2).The literature shows that the effect of steric hindrance on rates of various quaternizations has been studied. Pyridine derivatives (3-11), aromatic systems (12-14), thiazoles (15,16), polymers (17), and miscellaneous nitrogen and phosphorous systems (18-22) all exhibit a rate change with general steric crowding.Berg et al. (23) discu...

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“…The troublesome points are Et for RC0,Me and Pr' for MeC0,R. Effects of size and shape are stressed by comparing MeC0,R [aqueous 40% dioxane; equation (lo)] with 2 5 PhC0,R [aqueous 60% dioxane; equation (12)]. In the latter case, V" and G have less effect than in the former, presumably because Ph in the substrate poses more severe steric requirements than does Me.…”

Section: This Contrasts With Values Formentioning

confidence: 99%

Molecular mechanics and molecular shape. Part 4. Size, shape, and steric parameters

Meyer¹

1986

J. Chem. Soc., Perkin Trans. 2

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To deal with steric effects of substituents on reaction rates, t w o types of descriptor are proposed. Shape is characterised by the ratio (G) of the substituent's surface area to its volume. Bulk is characterised by the volume ( V a ) of the portion of the substituent that is within 0.3 nm of the reaction centre. For alkyl substituents, rate constants for several series of reactions (including the classical series, acid hydrolysis of the esters RC0,Et) correlate fairly well with Va and G. Taft's steric parameters (E,) correlate less well.Values of €s for hetero-substituents are found to be contaminated by non-steric factors. It is possible to construe whether the contaminant acts to hinder or t o enhance reaction.

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Determination of a new scale of ortho-steric parameters S 0 from N-methylation of pyridines

Cited by 37 publications

References 1 publication

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe⁺ Affinities of Pyridines

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe⁺ Affinities of Pyridines

Effect of chain length on the rate of quaternization of alkyl amines

Molecular mechanics and molecular shape. Part 4. Size, shape, and steric parameters

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Determination of a new scale of ortho-steric parameters S 0 from N-methylation of pyridines

Cited by 37 publications

References 1 publication

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe+ Affinities of Pyridines

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe+ Affinities of Pyridines

Effect of chain length on the rate of quaternization of alkyl amines

Molecular mechanics and molecular shape. Part 4. Size, shape, and steric parameters

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Product

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Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe⁺ Affinities of Pyridines

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe⁺ Affinities of Pyridines