Brian G. Cox
- Published in print:
- 2013
- Published Online:
- May 2013
- ISBN:
- 9780199670512
- eISBN:
- 9780199670512
- Item type:
- book
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199670512.001.0001
- Subject:
- Physics, Condensed Matter Physics / Materials
Acids and bases are ubiquitous in chemistry. Our understanding of them, however, is dominated by their behaviour in water. Transfer to non-aqueous solvents leads to profound changes in acid-base ...
More
Acids and bases are ubiquitous in chemistry. Our understanding of them, however, is dominated by their behaviour in water. Transfer to non-aqueous solvents leads to profound changes in acid-base strengths and to the rates and equilibria of many processes: for example, synthetic reactions involving acids, bases, and nucleophiles; isolation of pharmaceutical actives through salt formation; formation of zwitter-ions in amino acids; and chromatographic separation of substrates. This book seeks to enhance our understanding of acids and bases by reviewing and analysing their behaviour in non-aqueous solvents. The behaviour is related where possible to that in water, but correlations and contrasts between solvents are also presented. Fundamental background material is provided in the initial chapters: quantitative aspects of acid–base equilibria, including definitions and relationships between solution pH and species distribution; the influence of molecular structure on acid strengths; and acidity in aqueous solution. Solvent properties are reviewed, along with the magnitude of the interaction energies of solvent molecules with (especially) ions; the ability of solvents to participate in hydrogen bonding and to accept or donate electron pairs is seen to be crucial. Experimental methods for determining dissociation constants are described in detail. In the remaining chapters, dissociation constants of a wide range of acids in three distinct classes of solvent are discussed: protic solvents, such as alcohols, which are strong hydrogen-bond donors; basic, polar aprotic solvents, such as dimethylformamide; and low-basicity and low-polarity solvents, such as acetonitrile and tetrahydrofuran. Dissociation constants of individual acids vary over more than twenty orders of magnitude among the solvents, and there is a strong differentiation between the response of neutral and charged acids to solvent change. Ion-pairing and hydrogen-bonding equilibria, such as between phenol and phenoxide ions, play an increasingly important role as the solvent polarity decreases, and their influence on acid–base equilibria and salt formation is described.Less
Acids and bases are ubiquitous in chemistry. Our understanding of them, however, is dominated by their behaviour in water. Transfer to non-aqueous solvents leads to profound changes in acid-base strengths and to the rates and equilibria of many processes: for example, synthetic reactions involving acids, bases, and nucleophiles; isolation of pharmaceutical actives through salt formation; formation of zwitter-ions in amino acids; and chromatographic separation of substrates. This book seeks to enhance our understanding of acids and bases by reviewing and analysing their behaviour in non-aqueous solvents. The behaviour is related where possible to that in water, but correlations and contrasts between solvents are also presented. Fundamental background material is provided in the initial chapters: quantitative aspects of acid–base equilibria, including definitions and relationships between solution pH and species distribution; the influence of molecular structure on acid strengths; and acidity in aqueous solution. Solvent properties are reviewed, along with the magnitude of the interaction energies of solvent molecules with (especially) ions; the ability of solvents to participate in hydrogen bonding and to accept or donate electron pairs is seen to be crucial. Experimental methods for determining dissociation constants are described in detail. In the remaining chapters, dissociation constants of a wide range of acids in three distinct classes of solvent are discussed: protic solvents, such as alcohols, which are strong hydrogen-bond donors; basic, polar aprotic solvents, such as dimethylformamide; and low-basicity and low-polarity solvents, such as acetonitrile and tetrahydrofuran. Dissociation constants of individual acids vary over more than twenty orders of magnitude among the solvents, and there is a strong differentiation between the response of neutral and charged acids to solvent change. Ion-pairing and hydrogen-bonding equilibria, such as between phenol and phenoxide ions, play an increasingly important role as the solvent polarity decreases, and their influence on acid–base equilibria and salt formation is described.
Brian G. Cox
- Published in print:
- 2013
- Published Online:
- May 2013
- ISBN:
- 9780199670512
- eISBN:
- 9780199670512
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199670512.003.0008
- Subject:
- Physics, Condensed Matter Physics / Materials
Acid–base equilibria involving salt formation and the generation of reactive intermediates, such as enolates, are discussed. The absence of the solvated proton in the equilibria means that, in ...
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Acid–base equilibria involving salt formation and the generation of reactive intermediates, such as enolates, are discussed. The absence of the solvated proton in the equilibria means that, in contrast to individual ionization constants, they are relatively insensitive to solvent basicity. The intrinsic tendency to salt formation between carboxylic acids and amines is drastically reduced in most solvents compared with water. At higher concentrations in alcohols and alcohol–water mixtures, however, this is compensated for by high levels of ion-pair formation. In polar aprotic solvents, charge-forming equilibria are strongly inhibited by a combination of low equilibrium constants and a weaker tendency for ion-association. The ionization of carbon acids, such as ketones, by neutral bases, even the strongest known neutral bases, phosphazines, is severely limited. In non-polar solvents, such as chlorobenzene, chloroform, and carbon tetrachloride, there is no observable formation of free ions, but a high degree of proton transfer between common acids and bases to form ion-pairs frequently occurs.Less
Acid–base equilibria involving salt formation and the generation of reactive intermediates, such as enolates, are discussed. The absence of the solvated proton in the equilibria means that, in contrast to individual ionization constants, they are relatively insensitive to solvent basicity. The intrinsic tendency to salt formation between carboxylic acids and amines is drastically reduced in most solvents compared with water. At higher concentrations in alcohols and alcohol–water mixtures, however, this is compensated for by high levels of ion-pair formation. In polar aprotic solvents, charge-forming equilibria are strongly inhibited by a combination of low equilibrium constants and a weaker tendency for ion-association. The ionization of carbon acids, such as ketones, by neutral bases, even the strongest known neutral bases, phosphazines, is severely limited. In non-polar solvents, such as chlorobenzene, chloroform, and carbon tetrachloride, there is no observable formation of free ions, but a high degree of proton transfer between common acids and bases to form ion-pairs frequently occurs.
Brian G. Cox
- Published in print:
- 2013
- Published Online:
- May 2013
- ISBN:
- 9780199670512
- eISBN:
- 9780199670512
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199670512.003.0005
- Subject:
- Physics, Condensed Matter Physics / Materials
Protic solvents have hydrogen bound directly to electronegative atoms, such as oxygen or nitrogen. They are characterized by their ability to form strong hydrogen bonds with suitable acceptors, ...
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Protic solvents have hydrogen bound directly to electronegative atoms, such as oxygen or nitrogen. They are characterized by their ability to form strong hydrogen bonds with suitable acceptors, particularly simple anions. They include alcohols, formamide and other primary and secondary amides, and formic acid. In methanol, dissociation constants of carboxylic acids, phenols, and protonated nitrogen bases show excellent correlations with corresponding values in water. The largest differences occur for carboxylic acids, which are typically 5 pK-units weaker than in water. Acids become increasingly weak in the higher alcohols, especially t-butanol, because of poorer ion solvation. Homohydrogen-bond formation is generally weak, but ion-pair formation becomes progressively stronger as the solvent polarity decreases. Formamide contains a polar carbonyl group in addition to the ability to hydrogen-bond to anions, and displays pKa-values close to those in water. Formic acid hydrogen-bonds strongly with anions, but poor solvation of the proton, which inhibits the dissociation of acids, normally prevails.Less
Protic solvents have hydrogen bound directly to electronegative atoms, such as oxygen or nitrogen. They are characterized by their ability to form strong hydrogen bonds with suitable acceptors, particularly simple anions. They include alcohols, formamide and other primary and secondary amides, and formic acid. In methanol, dissociation constants of carboxylic acids, phenols, and protonated nitrogen bases show excellent correlations with corresponding values in water. The largest differences occur for carboxylic acids, which are typically 5 pK-units weaker than in water. Acids become increasingly weak in the higher alcohols, especially t-butanol, because of poorer ion solvation. Homohydrogen-bond formation is generally weak, but ion-pair formation becomes progressively stronger as the solvent polarity decreases. Formamide contains a polar carbonyl group in addition to the ability to hydrogen-bond to anions, and displays pKa-values close to those in water. Formic acid hydrogen-bonds strongly with anions, but poor solvation of the proton, which inhibits the dissociation of acids, normally prevails.
