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Article: Change of rate limiting step in general acid-catalyzed benzo[a]pyrene diol epoxide hydrolysis

TitleChange of rate limiting step in general acid-catalyzed benzo[a]pyrene diol epoxide hydrolysis
Authors
KeywordsAqueous Solution
Article
Catalysis
Chemical Reaction Kinetics
Hydrolysis
Ph
Pka
Issue Date1998
PublisherAmerican Chemical Society. The Journal's web site is located at http://pubs.acs.org/journals/jacsat/index.html
Citation
Journal of the American Chemical Society, 1998, v. 120 n. 18, p. 4327-4333 How to Cite?
AbstractThe rates of reaction of (±)-7β,8α-Dihydroxy-9α,10α-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (1) in 1:9 dioxane-water buffer solutions containing primary amines whose pK(a) values span the range of 5.410.7 have been determined. For those amines with pK(a) values < ca. 8, only the acid form (RNH3/+) gives rise to a kinetic term for reaction with 1. The rate- limiting step of this reaction is general acid-catalyzed epoxide ring opening to yield a discrete α-hydroxycarbocation, followed by a fast reaction of this intermediate with solvent. The intermediate α-hydroxycarbocation is sufficiently stable so that its reactions with external nucleophiles and bases compete with its reaction with solvent. For those amines with pK(a) > ca. 8, both acid and base forms of the buffer react with 1. The magnitude of the kinetic term in the base form of the amine (RNH2) increases with amine pK(a) and is attributed to nucleophilic addition of the amine to the epoxide group. Curvatures in plots of the kinetic term due to buffer (k(buff)) as a function of the mole fraction of buffer acid for substituted ammonium ions with pK(a) > 8 are interpreted in terms of a change in rate-limiting step of the general acid-catalyzed pathway from epoxide ring opening at low amine base concentrations to reaction of amine base acting as either a general base or nucleophile with an α-hydroxycarbocation at higher amine base concentrations. Thus, epoxide ring opening of I in buffer solutions of the more basic amines is a reversible reaction. Rate and product studies of the reaction of 1 in acid solutions (pH 5.5) and in Tris buffer solutions containing sodium azide show that azide ion is effective in trapping the α- hydroxycarbocation intermediate, subsequent to its rate-limiting formation by reaction of 1 with either H+ or Tris-H+. These results demonstrate that the intermediate formed from epoxide ring opening of 1 with acids has a sufficient lifetime so that its reaction with azide ion competes with its reaction with solvent.
Persistent Identifierhttp://hdl.handle.net/10722/91125
ISSN
2015 Impact Factor: 13.038
2015 SCImago Journal Rankings: 7.123
ISI Accession Number ID
References

 

DC FieldValueLanguage
dc.contributor.authorLin, Ben_HK
dc.contributor.authorIslam, Nen_HK
dc.contributor.authorFriedman, Sen_HK
dc.contributor.authorYagi, Hen_HK
dc.contributor.authorJerina, DMen_HK
dc.contributor.authorWhalen, DLen_HK
dc.date.accessioned2010-09-17T10:13:24Z-
dc.date.available2010-09-17T10:13:24Z-
dc.date.issued1998en_HK
dc.identifier.citationJournal of the American Chemical Society, 1998, v. 120 n. 18, p. 4327-4333en_HK
dc.identifier.issn0002-7863en_HK
dc.identifier.urihttp://hdl.handle.net/10722/91125-
dc.description.abstractThe rates of reaction of (±)-7β,8α-Dihydroxy-9α,10α-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (1) in 1:9 dioxane-water buffer solutions containing primary amines whose pK(a) values span the range of 5.410.7 have been determined. For those amines with pK(a) values < ca. 8, only the acid form (RNH3/+) gives rise to a kinetic term for reaction with 1. The rate- limiting step of this reaction is general acid-catalyzed epoxide ring opening to yield a discrete α-hydroxycarbocation, followed by a fast reaction of this intermediate with solvent. The intermediate α-hydroxycarbocation is sufficiently stable so that its reactions with external nucleophiles and bases compete with its reaction with solvent. For those amines with pK(a) > ca. 8, both acid and base forms of the buffer react with 1. The magnitude of the kinetic term in the base form of the amine (RNH2) increases with amine pK(a) and is attributed to nucleophilic addition of the amine to the epoxide group. Curvatures in plots of the kinetic term due to buffer (k(buff)) as a function of the mole fraction of buffer acid for substituted ammonium ions with pK(a) > 8 are interpreted in terms of a change in rate-limiting step of the general acid-catalyzed pathway from epoxide ring opening at low amine base concentrations to reaction of amine base acting as either a general base or nucleophile with an α-hydroxycarbocation at higher amine base concentrations. Thus, epoxide ring opening of I in buffer solutions of the more basic amines is a reversible reaction. Rate and product studies of the reaction of 1 in acid solutions (pH 5.5) and in Tris buffer solutions containing sodium azide show that azide ion is effective in trapping the α- hydroxycarbocation intermediate, subsequent to its rate-limiting formation by reaction of 1 with either H+ or Tris-H+. These results demonstrate that the intermediate formed from epoxide ring opening of 1 with acids has a sufficient lifetime so that its reaction with azide ion competes with its reaction with solvent.en_HK
dc.languageengen_HK
dc.publisherAmerican Chemical Society. The Journal's web site is located at http://pubs.acs.org/journals/jacsat/index.htmlen_HK
dc.relation.ispartofJournal of the American Chemical Societyen_HK
dc.subjectAqueous Solutionen_HK
dc.subjectArticleen_HK
dc.subjectCatalysisen_HK
dc.subjectChemical Reaction Kineticsen_HK
dc.subjectHydrolysisen_HK
dc.subjectPhen_HK
dc.subjectPkaen_HK
dc.titleChange of rate limiting step in general acid-catalyzed benzo[a]pyrene diol epoxide hydrolysisen_HK
dc.typeArticleen_HK
dc.identifier.emailLin, B:blin@hku.hken_HK
dc.description.naturelink_to_subscribed_fulltext-
dc.identifier.doi10.1021/ja972982men_HK
dc.identifier.scopuseid_2-s2.0-0032513721en_HK
dc.relation.referenceshttp://www.scopus.com/mlt/select.url?eid=2-s2.0-0032513721&selection=ref&src=s&origin=recordpageen_HK
dc.identifier.volume120en_HK
dc.identifier.issue18en_HK
dc.identifier.spage4327en_HK
dc.identifier.epage4333en_HK
dc.identifier.isiWOS:000073645600010-

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