Mechanisms of bromination of uracil derivatives. 5. Reaction of uracil and 5-bromouracil via their anions in weakly acidic aqueous solution

Tee, Oswald S.; Berks, Charles G.

doi:10.1021/jo01293a014

Cited by 22 publications

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“…Normally, bromination reactions are monitored at the tribromide ion maximum at 265 nm (13). In the present case, we used 275 nm to minimize any interference from the enolization of bromocyclohexadienones, which is apparent at shorter wavelengths (5-7).…”

Section: Methodsmentioning

confidence: 99%

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Kinetics and mechanism of the bromination of phenols in aqueous solution. Evidence of general base catalysis of bromine attack

Tee¹,

Iyengar²

1990

Can. J. Chem.

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This paper is dedicated to Professor Ross Stewart ot1 tlze occasion of his 65th birthda!.OSWALD S. TEE and N. RANI IYENGAR. Can. J. Chem. 68, 1769 (1990).The reactions of bromine with phenol, 4-bromophenol, and 4-methylphenol (p-cresol) in aqueous solution are catalyzed by carboxylate anions, confirming the suggestions of earlier work. The results are consistent with deprotonation of the phenol hydroxyl group by a general base occurring at more or less the same time as electrophilic attack by molecular bromine. 1769 (1990) Les rCactions du brome avec le phenol, le 4-bromophCno1 et le 4-mCthylphCno1 (p-cresol), en solution aqueuse, sont catalyskes par les anions carboxylates; cette observation confirme les suggestions faites anter~eurement. Les resultats sont en accord avec un mCcanisme impliquant une dCprotonation du groupement hydroxyle du phCnol par une base gCnCrale qui se produirait pratiquement en m&me temps que I'attaque Clectrophile par le brome moleculaire. On d~scute des origines possible de la catalyse basique gCnCrale. Si I'on combine les rCsultats actuels avec ceux obtenus antkrieurement, on peut suggerer qu'une I cyclohexadi&none protone'e n'est pas un intermediaire nCcessaire dans la bromation du phCnol; cet intermidiaire peut &tre evite par la formation et par l'enolisation de l'intermediaire cyclohexadi6none par une catalyse gCnCrale.Mots cle's : bromation, phenol, mkcanisme, catalyse, cinktique.[Traduit par la revue]The importance of acid-base catalysis for a large number of reactions occurring in aqueous solution is firmly established (1). Many reactions which are sluggish or non-existent in other media are facilitated by the ease of proton transfers in water, 1 thereby allowing highly unfavorable intermediates to be stabil-I ized or avoided (1,2). As reported herein, such may be the case I with phenol bromination in aqueous solution. ISeveral recent studies in this laboratory have been concerned with the mechanism of bromination of phenol derivatives in I aqueous solution (3-9). Some of these studies dealt mainly with electrophilic bromine attack (3, 4), while others were devoted to the enolization or debromination of transient bromocyclohexadienones (5-9). Three studles, in particular, suggested that bromine attack may be catalyzed by general bases.Firstly, we found that salicylate (o-hydroxybenzoate) anions (1) have an enhanced reactivity towards bromine and a reduced selectivity (less negative p+), when compared to analogous, monosubstituted phenols (36). As a result, we argued that the attack of bromine on 'salicylate anions is intramolecularly catalyzed by their ortho carboxylate groups (reaction [l]). Complementary to these conclusions, we also found that debromination of the ipso-dienone 5 by bromide ion is strongly catalyzed by the internal carboxyl group (reaction [3]) (36, 6,9).'Since three, quite distinct studies strongly suggested that the attack of bromine on phenols is catalyzed by general bases, we have looked for buffer catalysis in the aqueous bromination of phenol and tw...

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Section: Methodsmentioning

confidence: 99%

“…Reported values were the averages of 4-5 determinations. In some cases values of kIob" was converted to second-order constants (k20b") by correction for the substrate concentration and the fraction of free bromine in equilibrium with tribromide ion (13). The dissociation constant of tribromide ion was taken as 0.0625 M (I = 1.0 M) (23).…”

Section: Methodsmentioning

confidence: 99%

Kinetics and mechanism of the bromination of phenols in aqueous solution. Evidence of general base catalysis of bromine attack

Tee¹,

Iyengar²

1990

Can. J. Chem.

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“…Above pH 5 the rate of brornination decreases due to the formation of HOBr (15 For personal use only.…”

Section: Product Studiesmentioning

confidence: 99%

“…The ratc of reaction of furan with brominc was measured by monitoring the disappearance of bromine (A = 267 nm, tribromide ion band) under pseudo first-order conditions (furan = lo-'' M, bromine = lo-' M) in acetatc buffer (pH = 3.98) and in 0.1 M KBr, at 25°C (15). The second-order ratc constant givcn in the text was obtained from the observed first-order rate constant (45.2 s-') by correcting for the substrate conccntration and for thc actual concentration of free bromine in the normal way (15).…”

Section: Kirzeticsmentioning

confidence: 99%

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On the reaction of furan with bromine in aqueous solution. Observation of the slow hydration of malealdehyde

Tee¹,

Swedlund²

1983

Can. J. Chem.

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Thc rate of rcaction of furan with brominc in aqueous solution has bcen measurcd and thc ratc constant obtaincd by Williamson and Collcr is confirmed. Howcvcr. it is now found that thc rcaction of brominc with furan in watcr has two distinct stages: attack of brominc leading to the formation of malealdehyde 3 and hydration of 3 to a 2: 1 mixture of thc geometrical isomers of its cyclic hydrate 4. This second, slower process is observable by stopped-flow uv spectrophoton~etry occurring aftcr the consumption of bromine. The observed kinetics are vcry similar to thosc rcccntly found for thc analogous hydration of phthalaldchydc by McDonald and Martin.OSWALD S. TEE et B. E. SWEDLUND. Can. J. Chem. 61, 2171Chem. 61, (1983.On a mesuri la vitesse de la rCaction du furanne avec Ic brome en solution aqueusc et on a confirm6 la constantc de vitesse obtenue par Williamson et Coller. Cependant on trouve maintenant que la rCaction du brome avec le furanne dans I'cau a deux Ctapes distinctes: I'attaque du brome qui conduit B la formation du malCaldChyde 3 et I'hydratation du composC 3 qui conduit ? I un mClange 2: 1 des isomkres gComCtriques de son hydrate cyclique 4. Cc sccond processus. le plus lent, peut Ctre obscrvd par la spectrophotomdtrie uv a flux stoppd que I'on utilise apres la disparition du brome. Les cindtiqucs observees sont scmblables h cclles trouvCes rCcemment pour I'hydratation analogue du phthalaldehyde par McDonald ct Martin.[Traduit par le journal]The halogenation of furans is complex with the nature of products varying widely with the substrate, reagent, solvent, and reaction conditions (1 -4). However, two broad classes of reaction may be recognized, viz. addition and substitution, with the former often leading to ring-opening (1 -4).Bromination of furan in CCI, gives a mixture of 2-bromoand 2,5-dibromofuran (5). Using ' Hmr spectroscopy, Baciocchi et nl. (6) observed the formation of 1,4-and 1,2-dibromo adducts from furan and bromine in the nonnucleophilic solvent carbon disulphide, at low temperature. With time, and on warming, the adducts underwent elimination to 2-bromofuran (6). In hydroxylic solvents the reaction of furan (1) follows a different course. With bromine (or chlorine) in an alcohol solvent at low temperatures, reaction leads to the formation of a that bromination of methyl furoate in 50% aqueous acetic acid gives the substitution product, eq. [3], the same product as can be obtained under other reaction conditions (12).We have begun to study the bromination of furans systematically to try to ascertain why some derivatives undergo addition, whereas others show substitution. Our initial approach to this problem was to study the effect of substituents and solvents upon the rate of br~mination.',~ To anchor a correlation of results for aqueous solution a reliable value for furan was required.' During measurement of this value we found that another, slower process is observable, which is the subject of the present paper. Results Kinetic studies 1 2We first measured the second-order rate c...

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1985

Chemistry of Heterocyclic Compounds: A Series of Monographs

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Mechanisms of bromination of uracil derivatives. 5. Reaction of uracil and 5-bromouracil via their anions in weakly acidic aqueous solution

Cited by 22 publications

References 3 publications

Kinetics and mechanism of the bromination of phenols in aqueous solution. Evidence of general base catalysis of bromine attack

Kinetics and mechanism of the bromination of phenols in aqueous solution. Evidence of general base catalysis of bromine attack

On the reaction of furan with bromine in aqueous solution. Observation of the slow hydration of malealdehyde

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