Alcohol, acidity constants

By slow distillation of the alcohol with constant boiling point (48 per cent.) hydrobromic acid, for example ... 

Table 17.1 Acidity Constants of Some Alcohols and Phenols...

Fatty alcohols were sulfated with concentrated sulfuric acid before World War II, most frequently involving oleyl alcohols or alcohols derived from sperm and whale oil. Low temperatures were often suggested. Oleyl alcohol was sulfated by adding the alcohol to concentrated sulfuric acid, precooled to a temperature only slightly above the melting point of the alcohol [19], or by addition of cooled sulfuric acid (<95.12%) mixed with diethyl ether and phosphorus pentoxide to the alcohol, under constant stirring, for 3 h at 6-7°C, giving a yield of 98% [20]. Oleyl alcohol was also sulfated either by sulfuric acid or by oleum at 15°C [21]. 

Values of Kadd for the addition of water (hydration) of alkenes to give the corresponding alcohols. These equilibrium constants were obtained directly by determining the relative concentrations of the alcohol and alkene at chemical equilibrium. The acidity constants pATaik for deprotonation of the carbocations by solvent are not reported in Table 1. However, these may be calculated from data in Table 1 using the relationship pA ik = pATR + logA dd (Scheme 7). 

In the multimedia models used in this series of volumes, an air-water partition coefficient KAW or Henry s law constant (H) is required and is calculated from the ratio of the pure substance vapor pressure and aqueous solubility. This method is widely used for hydrophobic chemicals but is inappropriate for water-miscible chemicals for which no solubility can be measured. Examples are the lower alcohols, acids, amines and ketones. There are reported calculated or pseudo-solubilities that have been derived from QSPR correlations with molecular descriptors for alcohols, aldehydes and amines (by Leahy 1986 Kamlet et al. 1987, 1988 and Nirmalakhandan and Speece 1988a,b). The obvious option is to input the H or KAW directly. If the chemical s activity coefficient y in water is known, then H can be estimated as vwyP[>where vw is the molar volume of water and Pf is the liquid vapor pressure. Since H can be regarded as P[IC[, where Cjs is the solubility, it is apparent that (l/vwy) is a pseudo-solubility. Correlations and measurements of y are available in the physical-chemical literature. For example, if y is 5.0, the pseudo-solubility is 11100 mol/m3 since the molar volume of water vw is 18 x 10-6 m3/mol or 18 cm3/mol. Chemicals with y less than about 20 are usually miscible in water. If the liquid vapor pressure in this case is 1000 Pa, H will be 1000/11100 or 0.090 Pa m3/mol and KAW will be H/RT or 3.6 x 10 5 at 25°C. Alternatively, if H or KAW is known, C[ can be calculated. It is possible to apply existing models to hydrophilic chemicals if this pseudo-solubility is calculated from the activity coefficient or from a known H (i.e., Cjs, P[/H or P[ or KAW RT). This approach is used here. In the fugacity model illustrations all pseudo-solubilities are so designated and should not be regarded as real, experimentally accessible quantities. 

Compounds with an acidity constant, pK, in the range of 4 to 10, i.e. weak organic acids or bases, are present in two species forms at ambient pH. This pA a.i. range includes aromatic alcohols and thiols, carboxylic acids, aromatic amines and heterocyclic amines [15]. Conversely, alkyl-H and saturated alcohols do not undergo protonation/deprotonation in water (pA iw 14). 

The acid constant of phenol, 1.7 X 10 10, is much larger than that of the aliphatic alcohols. This we attribute to resonance with the structures Fj G, and H,... 

Alkyl iodides are the most easily formed of the alkyl halides and the slow distillation of the alcohol with constant boiling hydriodic acid is a general method of preparation (e.g. Expt 5.57). As with the corresponding chlorides and bromides (q.v.), the yields of the required alkyl iodides in this reaction may be diminished in the case of certain (tertiary and secondary) alcohols as a result of skeletal rearrangement. 

Table 5 shows the rate constants for hydrogen exchange in the CH3 group of quinaldine with alcohols at 120° (fc120). In addition, values of the relative acidity constants (Kf) are also given. The latter were determined by the indicator method (Hine and Hine, 1952). The... 

The review starts with a discussion of the mechanism of keto-enol tautomerisation and with kinetic data. Included in this section are results on stereochemical aspects of enolisation (or enolate formation) and on regioselec-tivity when two enolisation sites are in competition. The next section is devoted to thermodynamic data (keto-enol equilibrium constants and acidity constants of the two tautomeric forms) which have greatly improved in quality over the last decade. The last two sections concern two processes closely related to enolisation, namely the formation of enol ethers in alcohols and that of enamines in the presence of primary and secondary amines. Indeed, over the last fifteen years, data have shown that enol-ether formation and enamine formation are two competitive and often more favourable routes for reactions which usually occur via enol or enolate. 

Amines are considerably more basic than alcohols, ethers, or water. When an amine is dissolved in water, an equilibrium is established in which water acts as an acid and transfers a proton to the amine. Just as the acid strength of a carboxylic acid can be measured by defining an acidity constant Ka (Section 2.8), the base strength of an amine can be measured by defining an analogous basicity constant K. The larger the value of ifb (and the smaller the value of piTj,), the more favorable the proton-transfer equilibrium and the stronger the base. 

Cobalt(III) complexes are reduced readily to Co(II) by radiation-generated reducing radicals" including aliphatic radicals derived from alcohols, acids, amines, and aldehydes". However, the rate constant for the reaction of a-hydroxyalkyl radicals with Co(NH3)jP is pH-dependent (high in neutral solution, low in acidic solution) indicating an inner-sphere mechanism" the yield of Co follows the same pH dependence. ... 

This substance is made in the same way as butyl nitrite, with a few variations. The nitrite-water solution in the flask has 76 grams sodium nitrite in 240 ml water. The alcohol-sulfuric acid solution is made by diluting 60 ml of absolute alcohol (65 ml of 190 proof vodka) with an equal volume of water. Then the chemist carefully adds 28 ml of concentrated sulfuric acid to it. He swirls while adding. Then he dilutes this solution to 240 ml total volume by adding water. He cools both solutions to about IO0C, and adds the alcohol-acid solution to the nitrite solution slowly with constant stirring over a period of about half an hour. 

The acidity constants of electrically neutral acids (HA) and of acid anions (HA, HA , etc.) diminish in going from water to alcohol, whereas cation acids (ammonium ion, quinine ion, aluminum ion) show an increased acidity constant. These qualitative conclusions have been substantiated by experiment. 

L. Michaelis and M. Mizutani, and later Mizutani alone, have measured with the hydrogen electrode the pH of solutions of weak acids with their salts in the presence of varying amounts of alcohol. They assumed in their calculations that the constant of the hydrogen electrode remained unchanged by the addition of alcohol. Probably, only a small error enters when solutions with higher alcohol concentrations are involved. Michaelis and Mizutani were unable to calculate the true dissociation constants of the particular acids from their measurements because the activity of the anions was not known with sufficient accuracy. The quantity which they determined was the acidity constant (cf. page 90). 

---