Homogeneous Acid Catalysis in Nonasymmetric Synthesis

Department of Chemistry, Durham University, South Road, Durham DHl 3QX, UK 

The definition of pH (eqn (3.1)) was coined as early as 1909 by Sorensen of the Carlsberg breweries in Copenhagen. Sometime later in the 1920s, Bronsted and Lowry established the fundamental definitions of acidity and 

Sustainable Catalysis Without Metals or Other Endangered Elements, Part 1 

For a large number of organic functionalities, significant protonation is only achieved in more concentrated acid solutions e.g. alcohols, ethers, ketones, esters, sulfides, sulfoxides). More concentrated acid solutions cannot be treated as ideal, and Ka values cannot be measured in terms of concentrations as in eqn (3.4). In strong acid media, the significantly decreased water concentration results in additional solvent effects on pA a that are not accounted for by the pH scale. To account for acid-base behaviour in strong acid media, a number of acidity functions have been established. One of the earliest examples was the Hammett Ho acidity function based on a pairwise comparison of spectrophotometric changes in a series of aniline bases in concentrated acid solution. However, this scale could only be applied for structurally similar bases with similar protonation behaviour. Several other acidity functions have been proposed for other classes of bases such as the Hr acidity function for the ionisation of alcohols. As recently reviewed by Scorrano and More O Ferrall, later treatments by Bunnett and 

Olsen/ Marziano, and Cox and Yates attempted to link and extend previous acidity functions to a more broadly applicable description of the behaviour of strong acids in aqueous solution. 

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