Acidities of alcohols

Alcohols are weakly acidic. In fact, they are weaker in acidity than water. Alcohols have K values around 10 

Alcohols give off the acidic hydrogen to form alkoxide ions. Alkoxide ion is the conjugate base of alcohols and they are strong bases. 

Among alcohols, primary alcohols are the most acidic, and tertiary alcohols are the least acidic. 

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Additional gas-phase reactivity data, such as gas-phase acidities of alcohols [41], proton affinities of alcohols and ethers [41], and proton affinities of carbonyl compounds [42] could equally well be described by similar equations. 

The strong electronegativity of the fluorinated substituents is reflected in the effect that this group has upon the acidity of alcohols and carboxylic acids, as well as the effect it has on the basicity of amines (Tables 1.3-1.5). 

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

The order of decreasing acidity of alcohols, CH30H> 1°>2°>3°, is attributed to electron-releasing R s. These intensify the charge on the conjugate base, RO, and destabilize this ion, making the acid weaker. 

A good illustration of the relationship between the acidity of alcohols and their reactivity towards sulfur tetrafluoride can be found in the reaction of nitro alcohols. The conversion of 2,2,2-trinitroethanol into 2-fluoro-l,l,l-trinitroethane (la) proceeds at ambient temperature, while 2,2-dinitropropanol and 2,2-dinitropropane-l,3-diol are less reactive and require elevated temperatures for their conversion into l-fluoro-2,2-dinitropropane (lb) and 1.3-difluoro-2,2-dinitropropane(2), respectively. 2-Methyl-2-nitropropanol, however, does not react with sulfur tetrafluoride.43... 

IEs on acidity of alcohols have been little studied, primarily because they are not sufficiently acidic to be titratable in water, and no studies have been reported in DMSO. One exception is aqueous trifluoroethanol-cA, which is less acidic than trifluoroethanol, according to NMR pH titration, with a ApAa of 0.056.45... 

The acidity of alcohols decreases while going from primary to secondary to tertiary. This decrease in acidity is due to two factors an increase of electron density on the oxygen atom of the more highly-substituted alcohol, and steric hindrance (because of the alkyl groups, which inhibit solvation of the resulting alkoxide ion). Both of these situations increase the activation energy for proton removal. 

Contrast the acidity of alcohols and thiols. Also contrast their reactivity toward oxidizing agents. 

The most complete measurements of the acidity of alcohols in water were made some time ago by Long and Ballinger (1,2 3) using conductivity methods. The pKa values for substituted methanols (RCH OH) (2 ) are a linear function of the Taft a constants (4, 5 for the R substituents, allowing the prediction of the actual pKa by using the formula pKa = 15.9 - 1.42 a. ... 

Structure and Classification of Alcohols 425 10-3 Nomenclature of Alcohols and Phenols 427 10-4 Physical Properties of Alcohols 430 10-5 Commercially Important Alcohols 433 10-6 Acidity of Alcohols and Phenols 435 10-7 Synthesis of Alcohols Introduction and Review 438 Summary Previous Alcohol Syntheses 438 10-8 Organometallic Reagents for Alcohol Synthesis 440 10-9 Addition of Organometallic Reagents to Carbonyl Compounds 443... 

The acidities of alcohols vary widely, from alcohols that are about as acidic as water to some that are much less acidic. The acid-dissociation constant, Ka, of an alcohol is defined by the equilibrium... 

Table 10-4 shows that substitution by electron-withdrawing halogen atoms enhances the acidity of alcohols. For example, 2-chloroethanol is more acidic than ethanol because the electron-withdrawing chlorine atom helps to stabilize the 2-chloroethoxide ion. 

Acidities of / -Dicarbonyl Compounds Table 22-1 compares the acidities of some carbonyl compounds with the acidities of alcohols and water. Notice the large increase in acidity for compounds with two carbonyl groups beta to each other. The a protons of the jS-dicarbonyl compounds are more acidic than the hydroxyl protons of water and alcohols. This enhanced acidity results from increased stability of the enolate ion. The negative charge is delocalized over two carbonyl groups rather than just one, as shown by the resonance forms for the enolate ion of diethyl malonate (also called malonic ester). 

The energy of stabilization of the anionic complex increases when acidity of alcohols increases (Figure 21-17). For anionic complexes for which we identified two minima corresponding to U HOR and UH OR structures, the structure with protonated uracil is more stable. The vertical detachment energy of anionic complex systematically increases when deprotonation energy of alcohol decreases. There is a discontinuity in VDE of ca. 0.5 eV, which is a manifestation of intermolecular proton transfer (Figure 21-18). 

The discovery of Brauman and Blair in 1968 [34] that the acidities of aliphatic alcohols are completely reversed on going from bulk solution to the gas phase was a landmark in the interpretation of solvent and substituent effects on acid/base equihbria. The gas-phase acidity of alcohols increases in the following order [34, 125, 126] ... 

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