Limits...
The Gibbs free energy of formation of halogenated benzenes, benzoates and phenols and their potential role as electron acceptors in anaerobic environments.

Dolfing J, Novak I - Biodegradation (2014)

Bottom Line: The sequence of redox reactions in the natural environment generally follows the electron affinity of the electron acceptors present and can be rationalized by the redox potentials of the appropriate half-reactions.Answering the question how halogenated aromatics fit into this sequence requires information on their Gibbs free energy of formation values.The results underline the need to take speciation into consideration when evaluating redox potentials at pH 7 and highlight the fact that halogenated aromatics are excellent electron acceptors in aqueous environments.

View Article: PubMed Central - PubMed

Affiliation: School of Civil Engineering and Geosciences, Newcastle University, Newcastle, NE1 7RU, England, UK, jan.dolfing@ncl.ac.uk.

ABSTRACT
The sequence of redox reactions in the natural environment generally follows the electron affinity of the electron acceptors present and can be rationalized by the redox potentials of the appropriate half-reactions. Answering the question how halogenated aromatics fit into this sequence requires information on their Gibbs free energy of formation values. In 1992 Gibbs free energy data for various classes of halogenated aromatic compounds were systematically explored for the first time based on Benson's group contribution method. Since then more accurate quantum chemical calculation methods have become available. Here we use these methods to estimate enthalpy and Gibbs free energy of formation values of all chlorinated and brominated phenols. These data and similar state-of-the-art datasets for halogenated benzenes and benzoates were then used to calculate two-electron redox potentials of halogenated aromatics for standard conditions and for pH 7. The results underline the need to take speciation into consideration when evaluating redox potentials at pH 7 and highlight the fact that halogenated aromatics are excellent electron acceptors in aqueous environments.

Show MeSH

Related in: MedlinePlus

Relationships between thermodynamic parameters of chlorophenols. a Relationship between ΔGfogas and ΔHfogas, b relationship between ΔGfoaq and ΔHfoaq, and c relationship between ΔGfoaq and ΔGfogas
© Copyright Policy - OpenAccess
Related In: Results  -  Collection


getmorefigures.php?uid=PMC4305373&req=5

Fig2: Relationships between thermodynamic parameters of chlorophenols. a Relationship between ΔGfogas and ΔHfogas, b relationship between ΔGfoaq and ΔHfoaq, and c relationship between ΔGfoaq and ΔGfogas

Mentions: Table 1 shows the ΔHfo and ΔGfo values for all 19 environmentally relevant chlorophenols for both the gaseous and the aqueous phase. The ΔGfo values range between −75.7 and −137.9 kJ/mol for the gas phase and between −95.2 and −144.3 kJ/mol for the aqueous phase. These values are lower than those previously reported (Dolfing and Harrison 1992). There is considerable scatter in plots of the new versus these “old” data (Fig. 1). This is not surprising since quantum mechanical methods incorporate interactions that are not taken into account by group contribution methods which rely on transferability and averaging of properties of a particular functional group. The correlations between ΔHfogas and ΔGfogas, and between ΔHfoaq and ΔGfoaq (Fig. 2) are excellent, while the correlation between ΔGfogas and ΔGfoaq (Fig. 2) is less perfect, which reflects inter alia the influence of molecular structure on solvent solute interactions.Fig. 1


The Gibbs free energy of formation of halogenated benzenes, benzoates and phenols and their potential role as electron acceptors in anaerobic environments.

Dolfing J, Novak I - Biodegradation (2014)

Relationships between thermodynamic parameters of chlorophenols. a Relationship between ΔGfogas and ΔHfogas, b relationship between ΔGfoaq and ΔHfoaq, and c relationship between ΔGfoaq and ΔGfogas
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4305373&req=5

Fig2: Relationships between thermodynamic parameters of chlorophenols. a Relationship between ΔGfogas and ΔHfogas, b relationship between ΔGfoaq and ΔHfoaq, and c relationship between ΔGfoaq and ΔGfogas
Mentions: Table 1 shows the ΔHfo and ΔGfo values for all 19 environmentally relevant chlorophenols for both the gaseous and the aqueous phase. The ΔGfo values range between −75.7 and −137.9 kJ/mol for the gas phase and between −95.2 and −144.3 kJ/mol for the aqueous phase. These values are lower than those previously reported (Dolfing and Harrison 1992). There is considerable scatter in plots of the new versus these “old” data (Fig. 1). This is not surprising since quantum mechanical methods incorporate interactions that are not taken into account by group contribution methods which rely on transferability and averaging of properties of a particular functional group. The correlations between ΔHfogas and ΔGfogas, and between ΔHfoaq and ΔGfoaq (Fig. 2) are excellent, while the correlation between ΔGfogas and ΔGfoaq (Fig. 2) is less perfect, which reflects inter alia the influence of molecular structure on solvent solute interactions.Fig. 1

Bottom Line: The sequence of redox reactions in the natural environment generally follows the electron affinity of the electron acceptors present and can be rationalized by the redox potentials of the appropriate half-reactions.Answering the question how halogenated aromatics fit into this sequence requires information on their Gibbs free energy of formation values.The results underline the need to take speciation into consideration when evaluating redox potentials at pH 7 and highlight the fact that halogenated aromatics are excellent electron acceptors in aqueous environments.

View Article: PubMed Central - PubMed

Affiliation: School of Civil Engineering and Geosciences, Newcastle University, Newcastle, NE1 7RU, England, UK, jan.dolfing@ncl.ac.uk.

ABSTRACT
The sequence of redox reactions in the natural environment generally follows the electron affinity of the electron acceptors present and can be rationalized by the redox potentials of the appropriate half-reactions. Answering the question how halogenated aromatics fit into this sequence requires information on their Gibbs free energy of formation values. In 1992 Gibbs free energy data for various classes of halogenated aromatic compounds were systematically explored for the first time based on Benson's group contribution method. Since then more accurate quantum chemical calculation methods have become available. Here we use these methods to estimate enthalpy and Gibbs free energy of formation values of all chlorinated and brominated phenols. These data and similar state-of-the-art datasets for halogenated benzenes and benzoates were then used to calculate two-electron redox potentials of halogenated aromatics for standard conditions and for pH 7. The results underline the need to take speciation into consideration when evaluating redox potentials at pH 7 and highlight the fact that halogenated aromatics are excellent electron acceptors in aqueous environments.

Show MeSH
Related in: MedlinePlus