Alcohols hydroxyl value determination

Reaction with acetic anhydride is used in pharmacopoeial assays of benzyl alcohol and dienestrol and determination of hydroxyl values of castor oil, cetosteryl alcohol and cetomacrogol. 

R. 0. Crisler and A. M. Burrill, Determination of Hydroxyl Value of Alcohols by Near-Infrared Spectroscopy, Anal, Chem, 31, 2055,1959. 

The pH dependence of the reaction of m-tolyl acetate with cyclohexa-amylose implies a pAa of 12.1 for the catalytically active secondary hydroxyl group (Van Etten et al., 1967b). Although this pK at first appears low for the ionization of an aliphatic alcohol, it is consistent with the value of 12.35 determined thermodynamically for the ionization of the secondary hydroxyl groups of the ribose moiety of adenosine (Izatt et al., 1966 Christensen et al., 1966), and with the value of 12.2 reported by Lach for the... 

A glance at the pKa values in Table 3.1 reveals that many classes of compounds can act either as acids or as bases, depending on the reaction environment. Such materials are termed amphoteric. They must have an acidic proton (i.e., a proton attached to an electronegative element or group) and unshared pairs of electrons that can be donated to a proton. For example, water, alcohols, and other hydroxylic compounds as well as amines and amides are all amphoteric materials. Comparing the p.Ka s of these materials permits an assessment of the predominant behavior in a given environment. For example, if an amine is dissolved in water, it could function as an acid or a base. To determine which behavior will predominate, the position of the equilibrium can be determined for each process. Comparison of these values will indicate which will be the principal behavior. Thus, as an acid, the amine would donate a proton to water to give an amide anion and the hydronium ion. 

Absolute rate constants have been determined for the reaction of the hydroxyl radical with a variety of aromatic compounds in aqueous solution. The rate constants obtained are significantly higher than values previously reported. Rate constants for the reaction of the hydroxyl radical with methyl alcohol and ethyl alcohol have also been determined by competition kinetics using three of these absolute rate constants as reference values. Comparison of our results with the published values from competition kinetics suggests that the rate constants for the reaction of hydroxyl radicals with iodide ion and thiocyanate ion are significantly higher than reported in earlier work. The ultraviolet absorption bands of the various substituted hydroxycyclohexadienyl radicals formed have been observed. 

Rate constants for methanol and ethyl alcohol relative to those for benzoate ion, phenylacetate ion and p-nitrobenzoate ion are shown in Table III. Each value in the table consists of experiments at five separate concentration ratios. The random uncertainty in each value is less than 10%. In determining these rate constants from optical density ratios it was necessary to make a small correction for the contribution to the optical density by the H-adduct free radical. The molar extinction coefficients at 340-350 m/x for the H-adduct and OH-adduct are similar for benzoic acid (22) and were assumed to be comparable for the other two aromatic ions in the table. The correction is necessary since the rate constants for the reaction of hydrogen atoms with the alcohols used are two orders of magnitude lower than the rate constants for hydrogen atom addition to the aromatic ring, while the analogous hydroxyl rate constants are roughly comparable. 

As a result of the reaction the hydrogen in the hydroxyl group in alcohol is replaced by the acetyl group. If a diatomic alcohol is used two acetyl groups are introduced. The reaction is of great analytical value as by means of its use the number of hydroxyl groups present in a substance of unknown structure can be determined. 

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