Kinetics and mechanism of the bromination of aromatic compounds by N-bromosuccinimide in solution

The kinetics of bromination of several aromatic compounds (anilides, anisoles, and phenols) was investigated in 80% aqueous acetic acid (v/v) in the temperature range 20–50°C using N-bromosuccinimide (NBS) as the reagent. The reaction was found to be first order in the aromatic substrate (ArH), and zero order in NBS, the overall order being 1. Stoichiometry of the reaction was 1:1. An increase in solvent polarity increased the reaction rate, and chloride ions were found to be specific catalysts for the reaction. Arrhenius activation energy remained almost constant for all the substrates. A probable mechanism explaining all these observed facts was proposed. The mechanism involved an attack by Br⁺ or more probably by a solvated Br⁺ ion on the aromatic substrate.     

Regioselective bromination of aromatic compounds with Br2/SO2Cl2 over microporous catalysts

A new selective brominating system Br₂/SO₂Cl₂/zeolite, has been discovered. Partially cation-exchanged Ca²⁺-Y zeolite efficiently catalyzes the selective para-bromination of neat chlorobenzene (CB) by Br₂/SO₂Cl₂ affording a CB conversion of ∼89% and a para-selectivity of ∼97%. During the bromination reaction, SO₂Cl₂ oxidizes HBr, prevents its accumulation within the zeolite pores and yields a more active brominating species. The Ca²⁺-Y catalyst was found to be stable under the bromination conditions, and can easily be regenerated by calcination. The Br₂/SO₂Cl₂/Ca²⁺-Y brominating system could be applicable to other activated aromatic compounds such as o-xylene, toluene and fluorobenzene.     

Substitution électrophile aromatique dans l'anhydride sulfureux liquide. Influence de la concentration en bromure et rôle du solvant sur la réactivité des anisoles au cours de la réaction de bromation

Electrophilic Aromatic Substitution in Liquid Sulfur Dioxide. Kinetic Dependance of Rate on the Bromide Concentration and Influence of the Solvent during the Course of the Reaction On the reported data for bromination of anisole and eleven of its derivatives in liquid SO₂, it was shown that, with a large excess of bromide, the rate of reaction, obeys a first-order law. Rate constants thus obtained do not discriminate between the two different forms of bromide, e.g. Br₂ and Br^?₃ present as the A⁺Br^?₃ form, and corrections were made by use of the apparent equilibrium constant K′ for tribromide formation. The variations of rate constants with initial concentration of bromide has been studied and the effect results in a retardation of the bromination rate. Moreover, the ratio [Br₂] [A⁺Br^?]_T, which is constant during an experiment, varies with initial bromide concentrations, this variation affecting the total rate. To account for the bromide effect on the reactivity, variations of k^o,p_g {1 + K′[A⁺Br^?]_T}VS[A⁺Br^?]_T were studied over a 0.01 to 1M range of bromide concentration. The mechanism proposed shows that liquid SO₂ helps the reactive intermediate to be deprotonated and because of solvation of reactive species this step would probably be rate determining. Bromination by molecular bromine is more sensitive to substituent effects in liquid SO₂ than in water. This result is ascribed to the +M effect of the methoxy group which increase the conjugation of ortho-substituted derivatives (p⁺_p = ?7.83; p⁺_o= ?10.47).     

The solubility of C<Subscript>60</Subscript>Br<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 6, 8, 24) in organic solvents

The polythermal (over the temperature range 20–80°C) solubility of fullerene bromo derivatives C₆₀Brn (n = 6, 8, 24) in several solvents (o-xylene, o-dichlorobenzene, n-decanol-1, and enanthic acid) was studied. The corresponding solubility polytherms are given.     

Synthesis and Application of Poly(diallyldimethylammonium tribromide)as a Novel Polymeric Brominating Agent

In this study, the synthesis and applications of a new supported tribromide reagent based on poly(diallyldimethylammonium chloride) is reported. This supported tribromide is used in α‐bromoacetalization of ketones, bromination of alkenes and regioselective bromination of activated aromatic compounds. This method is mild and no Br₂ and HBr were used. Other advantages of this reagent are stability, high efficiency, simple filtera‐ bility and reusability.     

Synthesis and identification of bromofullerenes C70Br8 and C70Br10 and their solubility in some aromatic solvents

Bromofullerenes C₇₀Br₈ and C₇₀Br₁₀ were synthesized and identified by electronic absorption and infrared spectroscopy. Their solubility in aromatic solvents (benzene, toluene, o-xylene, and o-dichlorobenzene) was studied in the temperature range from 20 to 60°C.     

HBr or not HBr? That is the question: crystal structure of 6‐hydroxy‐1,4‐diazepane‐1,4‐diium dibromide redetermined

Liu et al. [Chin. J. Struct. Chem. (1996). 15 , 371–373] reported the structure of 6‐hydroxy‐1,4‐diazepane di(hydrogen bromide), C₅H₁₂N₂O·2HBr, which was interpreted in terms of neutral diazepane and HBr molecules. We found, however, ample evidence that the formation of an organic salt, consisting of a diammonium cation and two bromide anions, is more plausible. This interpretation is also in agreement with thermogravimetric analysis and with the observed solution behaviour. The crystal structure of 6‐hydroxy‐1,4‐diazepane‐1,4‐diium dibromide, C₅H₁₄N₂O²⁺·2Br^?, measured at 142 K, crystallized in the orthorhombic space group P2₁2₁2₁. The structure displays O—H…Br and N—H…Br hydrogen bonding. Contact distances are given. A search in the Cambridge Structural Database for the singly‐bonded H—Br moiety revealed a total of 69 structures. The question, whether these structures really include HBr as neutral molecules or rather Br^? anions and a protonated substrate such as an amine, is addressed.     

Synthesis and thermal stability of xylene-formaldehyde resins

o-,p- andm-xylene have been condensed with formaldehyde using paratoluene sulphonic acid as a catalyst and acetic acid/acetic anhydride as a solvent. The xylene-formaldehyde resins thus obtained have been characterised by IR and NMR spectra. The average molecular weight of the resin samples was determined by vapour pressure osmometry. Kinetic parameters were determined by differential thermal analysis, thermogravimetry and differential scanning calorimetry.     

Mild,Efficient, and Regioselective Monobromination of Arylamines and Phenols Using [BBIm]Br3 as a New Reagent

We report here an efficient method for the synthesis and characterization of the room-temperature ionic liquid 1,3-di-n-butylimidazolium tribromide ([BBIm]Br₃) (2) and its application as an efficient reagent and solvent for regioselective bromination of arylamines and phenols under mild conditions. The bromination was carried out in the absence of organic solvents, and in most cases, the only extraction solvent needed was water. The spent 1,3-di-n-butylimidazolium bromide (1) was easily recycled.     

On the substituent effects of halogens in aromatic substitutions

The general kinetic features of the bromination of 4-bromophenol by molecular bromine in acetic acid medium has been studied andArrhenius activation energies for the bromination of fourpara substituted phenols of the general formulaRC₆H₄OH (R=CH₃, F, Cl, and Br) have been accurately determined. These values, which indicate an anomalous order in substituent effect of halogens, are discussed.

---

Der Substituenten-Effekt von Halogenen bei der aromatischen Substitution

Zusammenfassung Die Bromierung von 4-Bromphenol mit Br₂ in Essigsäure wurde untersucht. Außerdem wurden dieArrhenius-Aktivierungs-Energien für die Bromierung von vierp-substituierten PhenolenRC₆H₄OH (R=CH₃, F, Cl und Br) genau bestimmt. Die beobachteten Substituenten-Effekte werden diskutiert und interpretiert.

     

First steps of the reactions of alkanes and arenes with adamantyl cations in sulfuric acid in comparison with other reagents

Solutions of 1-adamantanol in sulfuric acid at T < 100 °C interact with alkanes (RH, [H₂SO₄] > 85%) and arenes (ArH, [H₂SO₄] > 59%). The data on the kinetics, kinetic isotope effect (KIE), effects of the structure of RH and ArH and acidity of the medium, and the observation of 1,4-cis-dimethylcyclohexane isomerization indicate that adamanyl cations (Ad⁺) serve as reactive species. In the reactions with alkanes, the Ad⁺ cation abstracts the hydride ion from RH in the rate-determining step. Compensation dependences appear between the activaion parameters for the KIE and “effect 5/6” (ratio of the rate constants for the C–H bond cleavage in cyclopentane and cyclohexane) in the reactions of cycloalkanes with Ad⁺ and other electrophilic reagents, such as “anthracene” (An₂)H⁺ and hydroxymethyl (CH₂OH)⁺ cations and Hg^II ions, including the points of the lower selectivity limit (k _H/k _D) = 1.4, (“5/6”) = 1. In the reactions with the Ad⁺ cation, the bond selectivity 3⁰: 2⁰ of alkanes is higher, while 2⁰: 2⁰ is lower compared to other reagents. In the first case, the selectivity is probably determined predominantly by the energies of the cleaved C–H bonds, whereas in the second case it is determined by steric hindrances. Judging by the kinetic and selectivity data in the series benzene—toluene—o-xylene—m-xylene and the absence of the reaction with p-xylene, mesitylene, and pseudocumene, it can be concluded that the main contribution to the Ad⁺ + ArH interaction is made by adamantylation to the para- and meta-positions of the benzene ring, whereas the ortho-positions are inaccessible to the attack because of steric hindrances. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1581–1596, August, 2008.     

The quantitative electrochemical generation of bromine in acetic acid

The quantitative electrochemical generation of bromine at a platinum electrode in acetic acid is described. Coulometric methods for the determination of hydroquinone and 2-methylhydroquinone are reported. The best results are obtained with 0.7–1.1 M potassium acetate solutions as supporting electrolyte, and biamperometric end-point detection. The effects of water and acetic anhydride on the accuracy of titrations are discussed. Determination of the formal redox potential of the Br₂/Br^- system in a 0.9 M potassium acetate solution in acetic acid showed that bromide is oxidized directly to bromine at a platinum electrode with 100% current efficiency.     

An efficient vanadium-catalyzed bromination reaction

An efficient catalytic oxidative bromination of arenes, alkenes, and alkynes in aqueous media was achieved under relatively mild conditions by using NH₄VO₃ catalyst combined with H₂O₂, HBr, and KBr. Dodecyltrimethylammonium bromide was found to serve as an efficient surfactant to facilitate the NH₄VO₃-catalyzed bromination in aqueous media.     

Synthetic Attempts towards Polymers with Pentafulvene Structural Units

Synthetic attempts towards fully conjugated polymers 9 with pentafulvene-diyl structural units are described. Cationic polymerization of pentafulvenes 1a (R = X = Me) and 1b (R = X = MeS) nearly quantitatively gives polymers 8a and 8b with typical M_n and M_w values of 38800 and 53750, respectively, for 8a , and 12000 and 35900, respectively, for 8b . Key step of the conversion 8a → 9a (Scheme 6) is a quantitative bromination 8a → 32a , the structure of 32a being confirmed by analytical data as well as by spectroscopic comparison with model compound 23 . Best results in view of two-fold the HBr elimination 32a → 9a are obtained with Et₃N, but so far elimination has not been complete. Synthetic sequences are optimized with model compound 21 (Scheme 4). Here again, bromination 21 → 23 is quantitative, while two-fold HBr elimination 23 → 22 with Et₃N proceeds in 51% yield. Dibromide 23 easily undergoes HBr elimination followed by a Br shift to give bromide 29 . Contrary to cationic polymerization, anionic polymerization of simple pentafulvenes 1 to 2 (which would be attractive in view of the conjugated polymers 3 ) is not successful: For pentafulvene 1b (R = X = MeS), the main reaction is Diels-Alder-type dimerization 1b → 15b (Scheme 2), even under anionic conditions.     

New insights into bromination process: effective preparation of Ambroxol

Ambroxol used as an expectorant in treating respiratory diseases was effectively prepared with a total yield of 62%, with o-toluidine as the feedstock via successive procedures of electrophilic bromination, acetylation, radical benzylic bromination, N-alkylation and hydrolysis processes. The addition of aqueous hydrogen peroxide could enhance the utilisation of liquid bromine in the electrophilic bromination of o-toluidine, avoiding the hazardous HBr generated as a by-product. In addition, liquid bromine promoted by MnO₂ was used efficiently for the radical benzylic bromination of N-acetyl-N-(2,4-dibromo-6-methylphenyl)acetamide under mild conditions.     

Synthesis and Structure of 2,3-Bis(5-tert-butyl-2-methoxyphenyl)buta-1,3-diene by Bromine Elimination of (Z)-1,4-Dibromo-2,3-bis(5-tert-butyl-2-methoxyphenyl)-2-butene

2,3-Bis(5-tert-butyl-2-methoxyphenyl)buta-1,3-diene was prepared by bromination of (Z)- and (E)-2,3-bis(5-tert-butyl-2-methoxyphenyl)-2-butene followed by treatment with zinc powder in a mixture of CH₂Cl₂ and acetic acid, which was converted to the corresponding o-terphenyl skeleton by the condensation with dimethyl acetylenedicarboxylate followed by oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone.     

Oxidative Bromination of Aromatic Compounds Using O-Iodoxybenzoic Acid with Tetraethylammonium Bromide

A mild and selective procedure for the bromination of activated arenes using o-iodoxybenzoic acid and tetraethylammonium bromide is presented. The reactions were carried out at room temperature and gave moderate to excellent yields.     

An efficient, rapid, and regioselective bromination of anilines and phenols with 1-butyl-3-methylpyridinium tribromide as a new reagent/solvent under mild conditions

1-Butyl-3-methylpyridinium tribromide, [BMPy]Br₃ proves to be a highly efficient, regioselective reagent/solvent for nuclear bromination of various anilines and phenols. The synthesis and characterization of the room temperature ionic liquid [BMPy]Br₃ (2) is described. The bromination was carried out in the absence of organic solvents and in most cases the only extraction solvent needed was water. The spent 1-butyl-3-methylpyridinium bromide (1) was easily recycled.     

Anodic bromination of aromatic compounds in acetonitrile

The electrochemical bromination of benzene, toluene and p-xylene has been investigated in acetonitrile; even if the mechanism was found to be somewhat similar to that postulated in a previous paper for acetic acid as the solvent, the yields of the brominated products in this case are near to 100% because the solvent is not brominated in these conditions.