Benzene Protons Research Articles

Protonated benzofuran, anthracene, naphthalene, benzene, ethene, and ethyne: measurements and estimates of pK(a) and pK(R).

Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants k(p) for the deprotonation and k(H2O) for the nucleophilic reaction based on the "azide clock". Combining these with rate constants for (a) the H(+)-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pK(R) = -6.0 and -9.4 and pK(a) = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK(R) = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK(a) = -20.4 for the 2-protonated naphthalene from combining k(p) with an exchange rate constant. The differences between pK(R) and pK(a) correspond to pK(H2O), the equilibrium constant for hydration of the aromatic molecule (pK(H2O) = pK(R) - pK(a)). For naphthalene and anthracene values of pK(H2O) = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK(a) = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK(a) with calculated values of pK(H2O) gives pK(R) = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK(a) for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK(H2O) then yields pK(R) = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.

Arene-mercury complexes stabilized by aluminum and gallium chloride: catalysts for H/D exchange of aromatic compounds.

Dissolution of Hg(arene)(2)(MCl(4))(2) [arene = C(6)H(5)Me, C(6)H(5)Et, o-C(6)H(4)Me(2), C(6)H(3)-1,2,3-Me(3); M = Al, Ga] in C(6)D(6) results in a rapid H/D exchange and the formation of the appropriate d(n)-arene and C(6)D(5)H. H/D exchange is also observed between C(6)D(6) and the liquid clathrate ionic complexes, [Hg(arene)(2)(MCl(4))][MCl(4)], formed by dissolution of HgCl(2) and MCl(3) in C(6)H(6), m-C(6)H(4)Me(2), or p-C(6)H(4)Me(2). The H/D exchange reaction is found to be catalytic with respect to Hg(arene)(2)(MCl(4))(2) and independent of the initial arene ligand. Reaction of a 1:1 ratio of C(6)H(5)Me and C(6)D(6) with <0.1 mol % Hg(C(6)H(5)Me)(2)(MCl(4))(2) results in an equilibrium mixture of all isotopic isomers: C(6)H(5-x)D(x)Me and C(6)D(6-x)H(x) (x = 0-5). DFT calculations on the model system, Hg(C(6)H(6))(2)(AlCl(4))(2) and [Hg(C(6)H(6))(2)(AlCl(4))](+), show that the charge on the carbon and proton associated with the shortest Hg...C interactions is significantly higher than that on uncomplexed benzene or HgCl(2)(C(6)H(6))(2). The protonation of benzene by either Hg(C(6)H(6))(2)(AlCl(4))(2) or [Hg(C(6)H(6))(2)(AlCl(4))](+) was calculated to be thermodynamically favored in comparison to protonation of benzene by HO(2)CCF(3), a known catalyst for arene H/D exchange. Arene exchange and intramolecular hydrogen transfer reactions are also investigated by DFT calculations.

On the applicability of the Redfield high-temperature theory for proton dipolar order relaxation in liquid crystals

Nuclear magnetic resonance experiments on the Larmor frequency dependence of the intrapair dipolar order relaxation time T1D showed the existence of discrepancies between the usual two-spin theory and the experimental data. Field-cycling experiments of T1D in methyl deuterated para azoxyanisole (PAA-d6) in the nematic phase [J. Chem. Phys. 110, 8155 (1999)] showed that the difference cannot be assigned to the common assumption of isolated spin pairs or to the neglecting of the alkyl chain protons in the theoretical interpretation. Though the applicability of the spin temperature and short correlation time assumptions have not been justified for nematic liquid crystals, they have been used in all the available theoretical approaches for dipolar order relaxation. In this work, we calculate T1D of PAA-d6 within the Redfield high-temperature theory of spin–lattice relaxation but avoid the usual assumptions of short correlation times and random phase. First, we find that this fact does not alter the expression for T1D in the simple case of an ensemble of isolated spin pairs. Then we take the four benzene protons as the spin system and find that this refinement gives no relevant contribution able to explain the discrepancies between theory and experiments. Also, we find that the random phase assumption becomes redundant in the calculation of T1D, whenever the relevant spin Hamiltonian is nondegenerate. Thus, in these cases the spin temperature hypothesis is justified in liquid crystals, where the spin system contains few spins. We conclude that the Redfield high-temperature relaxation theory fails to describe T1D in thermotropic liquid crystals, since it underestimates the prominent role of the slow cooperative motions.

Nuclear spin-lattice relaxation in the triply rotating frame and ultraslow molecular motions in solids

A method for measuring nuclear magnetic spin-lattice relaxation in solids in the effective field He3 acting in the triply rotating frame (TRF) is described. The method advances the previously described techniques whereby nuclear magnetic resonance and relaxation in the rotating (RF) and doubly rotating frames (DRF) are measured directly. In the present work, the RF and DRF are employed for suppressing the secular part of nuclear dipole-dipole (DD) interactions in the first two orders. As a result, the higher-order DD interactions (four- and five-particle ones) were separated, and their contribution to the nuclear spin-lattice relaxation in the TRF was studied experimentally. The experiments were carried out on protons in polycrystalline benzene. With the introduced technique, an overall spin-lattice relaxation decay in the TRF was recorded continuously during a single radio-frequency pulse with a length not exceeding 1 s. The contribution of multiproton nonsecular DD interactions to the proton spin-lattice relaxation in the TRF was observed selectively as a pronounced local minimum in the temperature dependence of the relaxation timeT 1ϱϱϱ. This contribution corresponds to ultraslow motion of benzene molecules with a rate about γHe3 ≈ 2π · (101-103) s-1 and is determined quantitatively by specific correlation functions corresponding to the multiparticle nonsecular DD interactions of protons. The prospects of using this method for studying ultraslow atomic and molecular dynamics in solids are discussed.

Theoretical prediction of proton chemical shift in supercritical water using gas-phase approximation

Chemical shifts of the OH proton in supercritical water referenced to the benzene proton have been estimated theoretically using the ab initio molecular orbital (MO) method. The degree of dissociation from hydrogen-bonded water clusters to monomers calculated using the CCSD (T)/6-31+G(d)//MP2 (frozen core)/6-31+G(d) level of theory indicates that supercritical water is comprised of 80% monomer and 20% dimer at the critical point ( T c=647.1 K, P c=22.06 MPa). On the basis of this supercritical water composition, the chemical shift of the OH proton is determined to be −6.19 ppm at the MP2 (frozen core)/6-31+G(d)// MP2 (frozen core)/6-31+G(d) level of theory, which reproduces the recent NMR experimental results well.

Protonation of archetypal aromatic and antiaromatic systems – G2 studies of benzene and cyclobutadiene

The problem of protonation of benzene ( B) and cyclobutadiene ( CB) is addressed by the ab initio G2 benchmark computations. It appears that the σ-complexes are the most favourable protonated forms in both cases. The proton affinity (PA) of ( CB) is higher by 45 kcal/mol than that of B, which is rationalized by a decrease in the antiaromatic destabilization of the four-membered ring and stabilization by the allylic cation formation upon protonation accompanied by the ring puckering, which enables an effective additional homoconjugative interaction. The barrier for the hydrogen ring walk in B is estimated to be 8.6 kcal/mol leading to seven equivalent hydrogens on the NMR time scale. In contrast, this barrier in CB is 43.6 kcal/mol implying that the ring walk is precluded in this cationic system. The ring flipping inversion barrier of the σ-protonated CB is estimated to be 7 kcal/mol thus being in good accordance with experiment. The striking structural features of σ and π-protonated configurations of B and CB involve degrees of pyramidalization and tetrahedral distortions of carbon atoms, which are briefly discussed. Some practical models, alternative to the G2 scheme, designed for studying the PA of aromatic and antiaromatic compounds are examined and recommended.

A comprehensive investigation of variations in melting ranges and NMR data of 2,4-dinitrophenylhydrazine derivatives

The well-known characterization method of aldehydes and ketones utilizing their 2,4-dinitrophenylhydrazones (DNPs) often gives variable melting ranges and conflicting results for a single derivative. We have found that the DNPs melting point discrepancies and the irreproducibility of the method are mainly due to syn–anti (E–Z) isomerization caused by traces of acids which remain in the products when prepared by the conventional methods. A bicarbonate washing of the DNPs removes the acid, stabilizes the melting range and reproducibly gives higher-melting derivatives. In the DNP preparations of aldehydes and some unsymmetrical ketones only the kinetically favored (and also thermodynamically more stable) syn (E) isomer is formed. The six different melting ranges of acetaldehyde DNP (AA-DNP) previously reported by other investigators are correlated with the amounts of acid present in their DNP crystals. Derivatives with higher concentrations of acid(s) give lower melting ranges. In the presence of small amounts of acid, E–Z isomerization occurs in most solvents but not in methanol or dimethyl sulfoxide (DMSO). Larger amounts of acid cause the isomerization in all solvents, but the process is much slower in the aforementioned solvents. Alcohols, but not chlorinated solvents, are suitable for the DNPs recrystallization. E–Z isomerization of AA-DNP also occurs thermally by first order kinetics when its pure E isomer is heated at its melted state. An equilibrium ratio of 2∶1 (E∶Z) is reached after 10 minutes. A substantial decrease (up to 2.05 ppm) is observed for the NMR chemical shifts of the AA-DNPs protons in benzene and electron rich aromatic solvents compared to those obtained in chloroform, acetonitrile, acetone, methanol, DMSO or even cyclohexane and electron deficient nitrobenzene. An explanation is given for the drastic chemical shift changes.

Deuteration of closo-1,2- and 1,7-C2B10H12 using C6D6/AlCl3: Mechanistic considerations

Regioselective deuteration of 1-X-C2B10H12 (X = 2, 7) cage systems with C6D6/AlCl3 is correlated to ab initio calculational results on a [C2B10H13]+ intermediate. Full geometry optimizations of pertinent [C2B10H13]+ isomers, derived from each of the two 1-X-C2B10H12 carborane isomers, result in cage geometries not unlike the (nearly) icosahedral starting carborane. Each isomer contains a BH2 group having an acute H-B-H angle, long B–H bonds, and a very short H · · · H distance, hinting that the pertinent boron shares the electrons of a hydrogen molecule σ pair. It is suggested that the structural differences between the BH2 group of [C2B10H13]+ and the CH2 group of the benzenium ion, [C6H7]+ (the intermediate strongly intimated upon protonation of benzene), can be explained, in part, by (a) the availability of the π-ring electrons for bonding to the (extra) proton in the latter and (b) the unavailability of π electrons from the carborane. Thus, the C2B10H12 cage is most probably very reluctant to give up a cage electron pair in order to assist in bonding to an (externally bound) proton. Instead, it is more probable that “hydridic” B–H sigma electrons could very well play the important role in providing bonding to the attacking proton. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:95–102, 1998

Solid-State NMR Studies of Fullerene C60/Benzene Solvates

Stable van der Waals-bonded C60/benzene solvates were prepared using a novel crystallization technique. Solid-state NMR measurements on highly crystalline solvate under liquid benzene show that there is very little interaction between benzene and the C60 molecules and that the rotation of C60 at room temperature is not hindered. The magnetization transfer between benzene protons and local C60 molecules is highly inefficient. Three benzene species have been found in the solvate: two, nearly freely mobile, in wide and narrow porous structures, and the third constrained by C60 molecules in lattice defects. Premelting of deuterated benzene within the C60 solvate has been observed at Tm − 10.5 K.

Longitudinal nuclear spin relaxation in solids in a triply rotating coordinate system: Selective observation of the multispin dipole contribution

The longitudinal nuclear spin relaxation in an effective magnetic field H e3 acting in a triply rotating coordinate system is recorded. Rotating and doubly rotating coordinate systems are employed for strong suppression of the secular nuclear dipole interactions in the first two orders and for separation of higher-order interactions (four-and five-spin). Experiments on protons in polycrystalline benzene showed that the contribution of such multispin dipole interactions to this relaxation can be observed selectively as a pronounced local minimum in the temperature dependence of the relaxation time. This contribution correponds to ultraslow molecular motions with rates ≃ γH e3≃2π(101−103) s−1 and can be employed to study such motions in detail, including for purposes of identification of the form of the motion.

Wettability and T1 proton relaxation times of sandstone rocks

The spin-lattice relaxation times of water and hydrocarbon (hexane, benzene) protons in saturated model and natural core plug samples have been measured before and after their hydrophobization. The results do not show large differences between water-wet and oil-wet samples.

Kinetics and mechanism of proton transfer reactions from NH acid to 1,2-bis(dialkylaminomethyl)benzene in acetonitrile

Deprotonation of the Nb̀H acid 1,8-bis(4-toluenesulphonamido)-2,4,5,7-tetranitronaphthalene by proton sponges 1,2-bis(di-R-aminomethyl)benzene (R  Me, Et, Pr, Bu) has been studied by kinetic and spectroscopic methods. Mixtures of the proton sponges with the NH acid in acetonitrile solution give products which are cations and anions with [NH ⋯ N] + and [NH ⋯ N] − homoconjugated intramolecular hydrogen bonds. The kinetic parameters in acetonitrile were determined by the bulk of the alkyl groups in the 1,2-bis(dialkylaminomethyl)benzene proton sponges.

The relationship between partial bond fixation induced by through-bond and/or through-space perturbations in nonplanar benzene derivatives and proton spin-spin coupling constants

4 J Me-C-C-H = 4 J OB , the spin-spin coupling constant over four formal bonds between methyl protons and a proton bonded to an sp carbon, previously established 9-17 was a probe for bond order, was determined for 6-methyl-1,4-dihydro-1,4-methanonaphthalene (1) and for a number of reference compounds 2-5. Self-consistent field (SCF) calculations at the 3-21G level for 1-4 predict partial bond fixation in the benzene ring for all molecules in the sense predicted by interpretation of previously-reported photoelectron spectra of closely-related molecules

Chemical shift changes on conversion of benzofurans to dihydrobenzofurans; the large effect on the proton ortho to the furan oxygen

AbstractA comparison of the 1H NMR spectra of benzofuran and its dihydrofuran shows that the benzene proton ortho to the furan oxygen undergoes a larger change in chemical shift than those for the other protons. This observation is useful for locating the fusion sites of the furan ring in naturally occurring benzofuran derivatives.

Lack of protonation of benzene and toluene by trifluoromethanesulfonic acid and its significance for evaluating superacid strengths

AbstractDistribution between pentane and trifluoromethanesulfonic acid (TFMSA) and carbon‐13 NMR measurements showed that benzene and toluene are not protonated to any significant extent in TFMSA. This finding contradicts previous reports, and validates the ranking of superacids based on the extent of benzene protonation.

Determination of the stabilities and structures of X −(C 6H 6) clusters (X = Cl, Br and I)

The temperature dependence of the equilibria X −+C 6H 6⇄X −(C 6H 6) (X = Cl, Br and I) was measured using a pulsed electron beam mass spectrometer. The observed -Δ H 0 values agree with theoretical binding energies from second-order Møller-Plesset perturbation calculations. Cl − and B − are found to bond to the benzene proton linearly, while I − is bridge bonded to two protons.

A NMR method to probe the nature of liquid clathrates

The liquid clathrate formed from [N(CH3)4][Al2(CH3)6I] and benzene has been studied using 200 MHz1H FT-NMR. Two resonances, corresponding to the free and guest benzene are observed. The1H spin lattice relaxation times,T 1, for these molecules were measured and found to be distinctly different. The guest benzene protons relax nearly four times faster than the free molecules which is consistent with a more ordered structure of the benzene molecules within the clathrate.

ChemInform Abstract: EVALUATION OF SUPER ACID STRENGTH FROM THE PROTONATION OF BENZENE. COMPARISON OF HF‐SBF5, HF‐TAF5, AND HBR‐ALBR3 SYSTEMS

Chemischer InformationsdienstVolume 13, Issue 31 Article ChemInform Abstract: EVALUATION OF SUPER ACID STRENGTH FROM THE PROTONATION OF BENZENE. COMPARISON OF HF-SBF5, HF-TAF5, AND HBR-ALBR3 SYSTEMS D. FARCASIU, D. FARCASIUSearch for more papers by this authorS. L. FISK, S. L. FISKSearch for more papers by this authorM. T. MELCHIOR, M. T. MELCHIORSearch for more papers by this authorK. D. ROSE, K. D. ROSESearch for more papers by this author D. FARCASIU, D. FARCASIUSearch for more papers by this authorS. L. FISK, S. L. FISKSearch for more papers by this authorM. T. MELCHIOR, M. T. MELCHIORSearch for more papers by this authorK. D. ROSE, K. D. ROSESearch for more papers by this author First published: August 3, 1982 https://doi.org/10.1002/chin.198231082AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume13, Issue31August 3, 1982 RelatedInformation

Evaluation of super acid strength from the protonation of benzene. Comparison of HF-SbF5, HF-TaF5, and HBr-AlBr3 systems

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEvaluation of super acid strength from the protonation of benzene. Comparison of HF-SbF5, HF-TaF5, and HBr-AlBr3 systemsD. Farcasiu, Susan L. Fisk, M. T. Melchior, and K. D. RoseCite this: J. Org. Chem. 1982, 47, 3, 453–457Publication Date (Print):January 1, 1982Publication History Published online1 May 2002Published inissue 1 January 1982https://doi.org/10.1021/jo00342a015RIGHTS & PERMISSIONSArticle Views322Altmetric-Citations34LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (742 KB) Get e-Alerts Get e-Alerts

The role of solvent in the mechanism of protonation of benzene and ethylene

The hypersurfaces for protonation of benzene and ethylene by H+ and H3O+ species have been studied by means of the MINDO/3 method. Proton solvation reduces the heat of the exothermic reactions, and changes their mechanism. Electrophilic attack is accompanied by desolvation of the proton with this process beginning late in the reaction, the transition state having been passed long before.