title: Toward rational design of amine solutions for PCC applications: the kinetics of the reaction of CO2(aq) with cyclic and secondary amines in aqueous solution.
abstract: The kinetics of the fast reversible carbamate formation reaction of CO(2)(aq) with a series of substituted cyclic secondary amines as well as the noncyclic secondary amine diethanolamine (DEA) has been investigated using the stopped-flow spectrophotometric technique at 25.0 °C. The kinetics of the slow parallel reversible reaction between HCO(3)(-) and amine has also been determined for a number of the amines by (1)H NMR spectroscopy at 25.0 °C. The rate of the reversible reactions and the equilibrium constants for the formation of carbamic acid/carbamate from the reactions of CO(2) and HCO(3)(-) with the amines are reported. In terms of the forward reaction of CO(2)(aq) with amine, the order with increasing rate constants is as follows: diethanolamine (DEA) < morpholine (MORP) ~ thiomorpholine (TMORP) < N-methylpiperazine (N-MPIPZ) < 4-piperidinemethanol (4-PIPDM) ~ piperidine (PIPD) < pyrrolidine (PYR). Both 2-piperidinemethanol (2-PIPDM) and 2-piperidineethanol (2-PIPDE) do not form carbamates. For the carbamate forming amines a Brønsted correlation relating the protonation constant of the amine to the carbamic acid formation rate and equilibrium constants at 25.0 °C has been established. The overall suitability of an amine for PCC in terms of kinetics and energy is discussed.
authors.affiliation: Department of Chemistry, The University of Newcastle, Callaghan, NSW 2308, Australia. william.conway@uon.edu.au
authors.forename: William
authors.initials: W
authors.lastname: Conway
authors.forename: Xiaoguang
authors.initials: X
authors.lastname: Wang
authors.forename: Debra
authors.initials: D
authors.lastname: Fernandes
authors.forename: Robert
authors.initials: R
authors.lastname: Burns
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Lawrance
authors.forename: Graeme
authors.initials: G
authors.lastname: Puxty
authors.forename: Marcel
authors.initials: M
authors.lastname: Maeder
mesh_terms: D000588:Amines / Q000737:chemistry*; D002219:Carbamates / Q000737:chemistry*; D002245:Carbon Dioxide / Q000737:chemistry*; D004983:Ethanolamines / Q000737:chemistry; D007700:Kinetics; D012996:Solutions; D013053:Spectrophotometry; D014867:Water / Q000737:chemistry*
chemical_list: D000588:Amines; D002219:Carbamates; D004983:Ethanolamines; D012996:Solutions; D014867:Water; D002245:Carbon Dioxide; C020283:diethanolamine