H , acidity function

Rochester, C.H. Acidity Functions , Academic Press London, 1970. 

Several cyditol derivatives of varying ring size, for example, (69)/(70), have been prepared based on an enzymatic aldolization as the initial step. Substrates carrying suitably installed C,H-acidic functional groups such as nitro, ester, phosphonate (or halogen) functionalities made use of facile intramolecular nucleophilic (or radical) cyclization reactions ensuing, or subsequent to, the enzyme-catalyzed aldol addition (Figure 10.27) [134—137]. 

Rochester, C.H. Acidity Functions Academic Press NY, 1970. For discussion of the basicity of such compounds, see Liler, M. Reaction Mechanisms in Sulfuric Acid Academic Press NY, 1971, p. 118. 

Values up to pKa 33 (diphenylmethane) are those reported for equilibrium methods, and were measured either directly using the H acidity function or by comparing acidity with 9-phenyl-fluorene. Above pA 33, we assume a = 0.3 for toluene, cumene, and tripticene, and base other values on pKa = 43 for benzene and an assumed a of 0.9. The scale is based on the Langford and Burwell value of 18.5 for pKa of 9-phenylfluorene. 

Many other scales of electron-pair donation abilities have been proposed over the years, which are in general in good correlation with DN e g., the heat of complexation of the solvent molecules with boron trifluoride in dichloromethane (Maria and Gal 1985), and P e.g., SB, the solvatochromism of 5-nitroindoline compared with l-methyl-5-nitroindoline in neat solvents (Catalan el al. 1996) scales. The latter, the SB scale, has the advantage that the N-H acid function of the 5-nitroindoline probe has only a single hydrogen atom, contrary to the nitroanilines used for the P scale, that have two. It was devised quite recently for... 

In contrast, the acidity function approach, which involves a study of the dependence of the ionisation ratio ([S ]/[HS]) on an appropriate acidity function, was recently used for substituted acetophenones and some aliphatic ketones (Cockerill et al., 1974 Earls et al., 1975 Kankaanpera et al., 1978). All these studies used the property that the addition of DMSO increases the basicity of aqueous hydroxide ion solutions p/ s values were determined from (46) by plotting the ionisation ratio versus the H acidity function for... 

An H acidity function scale has been constructed for methoxide ion in methanol and its mixtures with DMSO (10-80%, v/v) using the dissociation of 11 amides (114) as the anchors for the scale.The degradation pathways of the anti-flammatory and analgesic lomoxicam (115), which contains an amide bond, have been examined recently. In acid, cleavage of the amide bond was the main reaction path and in alkaline and neutral solution the proton shift of the enolic hydroxyl initiated the major degradation pathway. The mechanism of hydrolysis of some A-nitrobenzamides (116) in strong acid follow an -1 mechanism with O-protonation but, in more moderate acid, they exhibit a neutral water-catalysed mechanism. /V-Methyl-/V-nitroacetamide (117) shows only the neutral water-catalysed process. Nitrourea follows an, 4-1 acid-catalysed mechanism. ... 

Vicinal C-H acidic functionality can promote the rearrangement of halocyclopropanes by either of two different mechanisms. 

Non-aqueous solvents are more of a problem than aqueous acids. We expect their medium effects to be less uniform and more unpredictable, especially in non-hydrogen-bonding solvents. It may be recalled that one-colour indicators are just those substances whose medium effects may include a sizeable contribution from London dispersion forces. For acidic solvents such as formic acid H functions have been investigated and reference will be made to these later. Current interest " centres more on H acidity functions which are useful for strongly basic media, such as ethylenediamine or mixtures of methanol and sodium methoxide. More details on indicator acidity functions will be given when we consider specific solvents later in this chapter. 

Ethanolamine in aqueous solution is approximately as basic as ammonia, pATb = 4.6 vs. 4.76 for NH3. The value of the H acidity function in... 

In view of the diversity of medium effects between water and dimethylsulphoxide, we must expect any acidity function that is anchored to the pH scale in water to apply only to acids with very similar structures. Dolman and Stewart have reported values of the H acidity function in dimethylsulphoxide-water mixtures based on 24 substituted aniline and diphenylamine indicators. Dimethylsulphoxide containing 0.4 mol per cent water has a H value of 26.2. 

Spectrophotometry has been used to establish Ho and H acidity function scales in pure sulpholane and sulpholane-water mixtures. Very strongly basic solutions (H = 20) have been prepared by dissolving phenyltrimethylammonium hydroxide in sulpholane containing 5 mol per cent water. Solutions of acids in sulpholane have Hq... 

NHg-R-COOH NHa-R-COO + H (acid function) NHa-R-COOH + H2O NHa-R-COOH + OH (basic function)... 

The H Acidity Function. The H. acidity function is defined as a measure of the ability of the basic solution to abstract a proton from an acidic neutral solute (1). 

Rochester, C. H., Acidity Functions in Organic Chemistry, a series of monographs, Vol. 17, Academic Press, New York, 1970. 

DMSO-H O mixtures also impart a strong basicity to the hydroxide ion. Quantitatively, this is expressed by means of the H acidity function, that is. 

H+ acidity function 30-55% and 75-95% H2SO4 in water Bonner and Lockhart, 1957... 

---