Acidity, of hydrocarbons

In the discussion of the relative acidity of carboxylic acids in Chapter 1 (p. 53-54), the thermodynamic acidity, expressed as the acid dissociation constant in aqueous solution, was taken as the measure of acidity. Determining the dissociation constants of carboxylic acids in aqueous solution by measuring the titration curve with a pH-sensitive electrode is straightforward, but determination of the acidity of hydrocarbons is more difficult. As most are quite weak acids, very strong bases are required 

The acidity of a hydrocarbon can be determined in an analogous way. If the electronic spectra of the neutral and anionic forms are sufficiently different, the concentration of each can be determined directly in a solution of known H the equilibrium constant for 

01 M NaOMe in 10 1 DMSO-MeOH 18.0 0.01 M NaOEt in 20 1 DMSO-EtOH 21.0 

It has been found that there is often a correlation between the rate of proton abstraction (kinetic acidity) and the thermodynamic stability of the carbanion (thermodynamic acidity). Owing to this relationship, kinetic measurements can be used to extend scales of hydrocarbon acidities. These kinetic measurements have the advantage of not requiring the presence of a measurable concentration of the carbanion instead, the relative ease of carbanion formation is judged by the rate at which exchange occurs. This method is applicable to weakly acidic hydrocarbons for which no suitable base will generate a measurable carbanion concentration. 

The kinetic method of determining relative acidity suffers from one serious complication, however, which has to do with the fate of the ion pair that is formed immediately on abstraction of the proton. If the ion pair separates and diffuses rapidly into the solution, so that each deprotonation results in exchange, the exchange rate is an accurate measure of the rate of deprotonation. Under many conditions of solvent and base, however, an ion pair may return to reactants at a rate exceeding protonation of the carbanion by the solvent, a phenomenon known as internal return. 

as is frequently the case, the electronic spectrum is not informative with respect to the hydrocarbon and its anion, one of the indicators used to determine the basicity of the medium is used and its spectrum monitored. The equilibrium established between the indicator and hydrocarbon in the basic medium 

When the acidities of hydrocarbons are discussed in terms of the relative stabilities of neutral and anionic forms, particularly with respect to the extent of electron delocalization in the anion, it follows that equilibrium measures of acidity are required. We have just seen how such data may be obtained. In many instances, it is not possible to obtain equilibrium data. In such cases, it may be possible to determine which proton in a molecule is abstracted most easily, or to determine 

CHAPTER 7 CARBANIONS AND OTHER NUCLEOPHILIC CARBON SPECIES 

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 

When the acidities of hydrocarbons are discussed in terms of the relative stabilities of neutral and anionic forms, particularly with respect to the extent of electron delocalization in the anion, the appropriate data are equilibrium acidity measurements. We have just seen 

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Murphy GL, JJ Perry (1983) Incorporation of chlorinated alkanes into fatty acids of hydrocarbon-utilizing mycobacteria. / BactcnoZ 156 1158-1164. 

Polar head of Non-polar tail sulphonic acid of hydrocarbon chain... 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

A carbanion can be regarded as the conjugate base of the corresponding carbon acid and thus the thermodynamic acidities of hydrocarbons and substituted hydrocarbons provide a very convenient quantitative measure of the extent of carbanion formation in the generalized reaction... 

The order of sequence (3) is that of thermodynamic acidity, and not one of kinetic acidity. However, it appears5 that the order of kinetic acidities of hydrocarbons rank as follows... 

It will be shown below that with the aid of liquid ammonia solutions of potassium amide, the acidity of hydrocarbons (including even saturated ones) cannot only be detected but also determined quantitatively from measurements of hydrogen isotope exchange rates. 

The rate of hydrogen exchange depends on the protolytic properties of both the solvent and the substrate. In fact there is a correspondence between the magnitude of the rate constants for deuterium exchange with ND3 and the conventional ionization constants of hydrocarbons which were used by Conant and Wheland (1932) and by McEwen (1936) to obtain the first quantitative estimates of the acidity of hydrocarbons. To do this, they determined the equilibrium of metallation of hydrocarbons by organo-alkali metal compounds. This reaction was described by Shorygin (1910) and is represented by the equation... 

We have described the use of liquid ammonia and liquid ammonia solutions of potassium amide in studies of hydrogen isotope exchange reactions in organic compounds. This method has been used to obtain new data on the reactivity of organic compounds, the effect of the reagents upon it, and the acidity of hydrocarbons. 

Streitwieser, A. Mares, F. Acidity of hydrocarbons. XXIX. Kinetic acidities of benzal fluoride and 9-fluorofluorene. A pyramidal benzyl anion./. Am. Chem. Soc. 1968, 90, 2444—2445. 

We apply the principle The more 4 character in the orbital used by the C of the C—H bond, the more acidic is the H. Therefore the order of acidity of hydrocarbons is... 

A rough correlation between the relative rates of deprotonation and the Jq h for the bridgehead protons in the series 7, 8 and 9, was observed by Gloss and Larrabee. Taking the C-H coupling constant as a measure of the acidity of hydrocarbons, the acidity of the proton in bicyclobutane is expected to be between that of acetylene and ethylene (Jq-h values for acetylene, ethylene and bicyclobutane are 248,156 and 205 Hz respectively). 

Thus we have the following sequences of acidity of hydrocarbons and basicity of their anions ... 

One test method (ASTM D-1093) is used solely for the qualitative determination of the acidity of hydrocarbon liquids and their distillation residues. The results are qualitative. Basicity determination can also be performed with a small change in the procedure (see below). In the test method, a sample is shaken with water and the aqueous layer is tested for acidity with methyl orange indicator (red color). Basicity can be determined with phe-nolphthalein indicator (pink color) instead of the methyl orange indicator. 

Figure 3. Plot of acidities of hydrocarbons in the gas phase vs. (CH3)2SO solution phase. (Reproduced with permission from reference 21. Copyright 1986 Weizmann Science Press.)...

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