Acidity and Basicity of Alcohols

In this section, we study the acidity and basicity of alcohols, their dehydration to alkenes, their conversion to haloalkanes, and their oxidation to aldehydes, ketones, or carboxylic acids. 

The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

West, R., and R. H. Baney Hydrogen Bonding Studies. II. The Acidity and Basicity of Silanols Compared to Alcohols. J. Amer. chem. Soc. 81, 6145-6148 (1959). 

In view of the means available to vary acidity and basicity of zeolites, studies of mechanistic detail using model alcohols might be very rewarding. 

Osmont and Schreiber introduced interaction parameter as a. measure of acidity and basicity of glass fibers [56]. This parameter was calculated by using specific retention volumes of the injected probes, i.e. n-butanol and butylamine. For acidic surfaces, where the specific retention volume for the base exceeds that for the acidic alcohol ... 

N. C. J. Stutchbury and D. L. Cooper, J. Chem. Phys., 79, 4967 (1983). Charge Partitioning by Zero-Flux Surfaces Tbe Acidities and Basicities of Simple Aliphatic Alcohols and Amines. 

The measurement of acidity and basicity of silicon compounds follows quite well-defined phases. The first studies were conventional acid-base titrations in which pKa values were determined. The solvents varied, but were usually aqueous or water/alcohol mixtures. These studies were limited by solubility and resistance to attack by protic solvents. 

Here also the equilibrium is far to the left. Although alcohols are very weak acids and bases, their acidity and basicity like that of water far exceeds the acidity and basicity of hydrocarbons. 

On the other hand, the activity for dehydrogenation of isopropyl alcohol to acetone (ra) is assumed to be proportional to the acidity and basicity of a catalyst, since the dehydrogenation is considered to proceed by a concerted mechanism, for example "- >... 

Production of ally alcohol by the isomerization of propylene oxide is an industrialized process. Lithium phosphate is specifically selective for this reaction. It is believed that the appropriate bdance of the acidity and basicity of catalyst is essential for high selectivity. If the acidity is dominant, isomerization to aldehyde becomes the main reaction (eq. 9a), and a path to acetone is favored on basic catalysts (eq. 9c), as shown below. 

Hydrolysis of vinyl acetate is catalyzed by acidic and basic catalysts to form acetic acid and vinyl alcohol which rapidly tautomerizes to acetaldehyde. This rate of hydrolysis of vinyl acetate is 1000 times that of its saturated analogue, ethyl acetate, ia alkaline media (15). The rate of hydrolysis is minimal at pH 4.44 (16). Other chemical reactions which vinyl acetate may undergo are addition across the double bond, transesterification to other vinyl esters, and oxidation (15—21). 

These reactions ate carried out in the presence of acidic and basic catalysts. The acid-cataly2ed addition of ethyl alcohol to acetylene or to a vinyl ether produces acetals (diethers of 1,1-dihydroxyethane). The acid-cataly2ed reaction of ethyl alcohol with an aldehyde or ketone also gives acetals. 

Hydroxy-l-alkenyl diisopropylcarbamates 2 (X = OCb), in this respect, occupy a medium position since they are stable in strongly acidic and basic protic solvents. For deblocking vinyl carbamates, the presence of catalytic amounts of mercuric or palladium(II) salts is required. Due to this stability, several reactions of homoallylic alcohols, proceeding with high diastereo-selectivity, e g., epoxidation, are applicable in order to introduce further hetero-substituents. 

Chloramine-B (CAB, PhS02NClNa) and chloramine-T (CAT, p-Me-C6H4S02NClNa) have also been used for the oxidation of sulphoxides107-115. The required sulphone is produced after initial attack by the sulphoxide sulphur atom on the electrophilic chlorine-containing species, forming a chlorosulphonium intermediate as shown in equation (34). These reactions take place at room temperature, in water and aqueous polar solvents such as alcohols and dioxane, in both acidic and basic media. In alkaline solution the reaction is slow and the rate is considerably enhanced by the use of osmium tetroxide as a catalyst115. 

Oxime carbamates have high polarity and solubility in water and are relatively chemically and thermally unstable. They are relatively stable in weakly acidic to neutral media (pH 4-6) but unstable in strongly acidic and basic media. Rapid hydrolysis occurs in strongly basic aqueous solutions (pH > 9) to form the parent oxime/alcohol and methylamine, which is enhanced at elevated temperature. Additionally, oxime carbamates are, generally, stable in most organic solvents and readily soluble in acetone, methanol, acetonitrile, and ethyl acetate, with the exception of aliphatic hydrocarbons. Furthermore, most oxime carbamates contain an active -alkyl (methyl) moiety that can be easily oxidized to form the corresponding sulfoxide or sulfone metabolites. 

Photolabile linkers play an important role in solid-phase organic synthesis (SPOS) due to their stability under both acidic and basic conditions. The ONb photolabile linker was modified to improve cleavage rates and yields Fmoc-Tos-OFI was released in 87% yield after 23 h (Scheme 4) [24]. Specifically, the primary alcohol was changed to a secondary benzylic alcohol and the attachment to the resin was through an alkyl chain as opposed to an amide function. Linker 20 was used for the production of carboxylic acids or carbohydrates. A second example... 

Amphoteric molecules of this type, where the acidic and basic sites are relatively close to each other but cannot interact directly, can heterolytically cleave H-X and C-X bonds where X is a halide, alkoxide, amide, alcohol, thiol, trimethylsilyl, or alkyl group.18,18a The ability to effect changes in the reactivity of borollide complexes by adjusting metal oxidation states and ligands allows fine-tuning of catalytic and other properties, which in turn advances the application of these compounds in synthesis. 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

---