Alcohols relative acidity

Another example of the effect of resonance is in the relative acidity of carboxylic acids as compared to alcohols. Carboxylic acids derived from saturated hydrocarbons have ipK values near 5, whereas saturated alcohols have pA values in the range 16-18. This implies that the carboxylate anion can accept negative charge more readily than an oxygen on a saturated carbon chain. This can be explained in terms of stabilization of the negative charge by resonance, ... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

If this intermediate, in turn, is more n idly attacked by water or hydroxide ion than the original ester, the overall reaction will be faster in the presence of the nucleophile than in its absence. These are the requisite conditions for nucleophilic catalysis. Esters of relatively acidic alcohols (in particular, phenols) are hydrolyzed by the nucleophilic catalysis mechanism in the presence of imidazole ... 

Nucleophilic participation is important only for esters of alcohols that have pK <13. Specifically, phenyl and trifluoroethyl esters show nucleophilic catalysis, but methyl and 2-chloroethyl esters do not. This result reflects the fete of the tetrahedral intermediate that results fi om nucleophilic participation. For relatively acidic alcohols, the alkoxide group can be eliminated, leading to hydrolysis via nucleophilic catalysis ... 

The hydrates of aldehydes and ketones are considerably more acidic than normal alcohols (pAl Fs 16-19). How would you account for this fact Some reported values are shown below. Explain the order of relative acidity. 

Reactant and product structures. Because the transition state stmcture is normally different from but intermediate to those of the initial and final states, it is evident that the stmctures of the reactants and products should be known. One should, however, be aware of a possible source of misinterpretation. Suppose the products generated in the reaction of kinetic interest undergo conversion, on a time scale fast relative to the experimental manipulations, to thermodynamically more stable substances then the observed products will not be the actual products of the reaction. In this case the products are said to be under thermodynamic control rather than kinetic control. A possible example has been given in the earlier description of the reaction of hydroxide ion with ester, when it seems likely that the products are the carboxylic acid and the alkoxide ion, which, however, are transformed in accordance with the relative acidities of carboxylic acids and alcohols into the isolated products of carboxylate salt and alcohol. 

Benzyl esters are readily prepared by many of the classical methods (see the introduction to this chapter), as well as by many newer methods, since benzyl alcohol is unhindered and relatively acid stable. 

Reaction orders in alcohol and add were obtained from a study of the first of these reactions under non-stoichiometric conditions. This is not possible for a polyesterification, carried out in an excess of one of the reactants since in this case stoichiometry is required. The orders in acid and alcohol relative to the reaction of 1-dodecanol with dodecanoic add in dodecyl dodecanoate are 2 and 1 respectively which, according to Solomon, corresponds to an Aac2 mechanism. Since the dielectric constant is low, the ions are assumed to be associated as ion pairs ... 

Various quantitative methods have been developed to express the relative stabilities of carbocations. One of the most common of these, though useful only for relatively stable cations that are formed by ionization of alcohols in acidic solutions, is based on the equation ... 

If only the O-H, as opposed to the carbonyl, of a carboxyl group has to be masked, it can be readily accomplished by esterification. Alkaline hydrolysis is the usual way for regenerating the acid. t-Butyl esters, which are readily cleaved by acid, can be used if alkaline conditions must be avoided. 2,2,2-Trichloroethyl esters, which can be reductively cleaved with zinc, are another possibility.267 Some esters can be cleaved by treatment with anhydrous TBAF. These reactions proceed best for esters of relatively acidic alcohols, such as 4-nitrobenzyl, 2,2,2-trichloroethyl, and cyanoethyl.268... 

Shortly after, the same authors reported the results on the relative acidities of 144 as compared with 1,3-dithiacycloheptane (150) and the open-chain model bis-(vinylthio)methane (151)m. In refluxing /erf-butyl alcohol-d in the presence of... 

A potentially useful variant of the synthesis of some Mo(CHR)(NAr)(OR )2 complexes (R=CMe3 or CMe2Ph) consists of addition of two equivalents of a relatively acidic alcohol (R OH=a fluorinated alcohol or phenol) to Mo(NAr)(N-t-Bu)(CH2R)2 [78,79], a variation of the reaction of that type that was first reported in 1989 [80]. The more basic t-butylimido ligand is protonated selectively. This synthesis avoids the addition of triflic acid to Mo(NAr)2(CH2R)2 to give Mo(CHR)(N-t-Bu)(triflate)2(dimethoxyethane), the universal precursor to any Mo(CHR)(NAr)(OR )2 complex. Unfortunately, the method does not appear to succeed when R OH does not have a relatively high pK,. 

Protection of alcohols. /-Butyldiphenylsilyl ethers (6, 51) are useful for pro-tecton of alcohols, but are more resistant to acid hydrolysis and fluorolysis than /-butoxydiphenylsilyl ethers. In contrast, /-butoxydiphenylsilyl ethers are relatively acid-stable but are readily cleaved by fluoride ion in CFLCU. The ethers are stable to most alkyllithiums and to Swern or PCC oxidation. 

Wood contains a small proportion (usually less than 5%) of components which are extractable by organic solvents such as ethanol or dichloromethane. The proportion of these extractives varies in hardwoods and softwoods and also between species. Although many of these substances are removed during the chemical pulping process, some may still be retained in the final sheet of paper. Their chemical composition is very varied, and they include alkanes, fatty alcohols and acids (both saturated and unsaturated), glycerol esters, waxes, resin acids, terpene and phenolic components. The proportion which remains in pulp and paper depends upon the pulping process used. In general, acidic components such as the resin and fatty acids are relatively easily removed by alkali by conversion to their soluble... 

The nature of the solvent also determines the chemoselective outcome in the reaction products. Products arising from the incorporation of one solvent molecule are formed (besides dibromides) in alcohols, acetic acid and acetonitrile (Id-e), whereas dibromo derivatives are formed exclusively in chlorinated solvents, nitromethane and in ionic liquids. (9) Chemoselectivity depends on the relative nucleophilicity of the solvent and the counterion, although it is affected also by other phenomena (ion pairing, and ion dissociation) in methanol the addition process gives quasi-exclusively bromo-methoxy adducts, whereas in acetic acid dibromides are the main products, formed in addition to smaller amounts of the bromo-acetoxy derivatives. (70)... 

Pure alumina catalyst prepared either by hydrolysis of aluminum isopropoxide or by precipitation of aluminum nitrate with ammonia, and calcined at 600-800°, contains intrinsic acidic and basic sites, which participate in the dehydration of alcohols. The acidic sites are not of equal strength and the relatively strong sites can be neutralized by incorporating as little as 0.1 % by weight of sodium or potassium ions or by passing ammonia or organic bases, such as pyridine or piperidine, over the alumina. 

Some thought about relative acidity and basicity is also sensible ionization of alcohols does not occur without a strong base, as suggested in the example. 

Alkylation of enolate is an important synthetic method.27 The alkylation of relatively acidic compounds such as /i-dikctoncs, /i-ketoesters, and esters of malonic acid can be carried out in alcohols as solvents using metal alkoxides as bases. The presence of two electron-withdrawing substituents facilitates formation of the enolate resulting from removal of a proton from the carbon situated between them. Alkylation then occurs by an Sn2 process. Some examples of alkylation reactions involving relatively acidic carbon acids are shown in Scheme 1.5. These reactions are all mechanistically similar in that a... 

The effects of exchange depends on the relative acidities of the alcohol (or other protonic substance) and the polymeric alcohol. The exchange reaction occurs throughout the course of the polymerization if the acidities of the two alcohols are approximately the same. The polymerization rate is unaffected while the molecular weight decreases (Eq. 7-15), but the molecular weight distribution (MWD) is Poisson. 

Reinitiation by RO Na+ would usually be slower, since ROH is relatively acidic. This results in a decreased polymerization rate and a broadening of the polymer molecular weight. The rate of polymerization will be relatively unaffected during most of the polymerization for the case in which ROH is less acidic than the polymeric alcohol. Exchange will occur in the later stages of reaction with a broadening of the MWD. 

Figure 6-13 6-311+G vs. Experimental Aqueous-Phase Relative Acidities of Alcohols, Phenols and Related Compounds...

It was shown that the patterns of the relative rates of reaction of DPC and DPC with alcohols are essentially identical and followed the relative acidity of the alcohols (MeOH > /-PrOH > f-BuOH) and showed a kinetic deuterium isotope effect on reaction with the OH bond (Table 9.13). These results indicate that DPC attacks the O—H bond rather than the C—H bond of the alcohol. If the C—H bonds of the alcohol were attacked by DPC, as in the ground triplet state reaction, then one would expect 2-propanol would react faster than methanol. Lack of any dis-cernable quenching of DPC by diethyl ether and THF indicates that DPC ... 

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