Acidities of some hydrocarbons

A similar prediction can be made on the basis of qualitative considerations of the orbital occupied by the unshared electron pair. In a planar carbanion, the lone pair would occupy a p-orbital. In a pyramidal geometry, the orbital would have substantial 5-character. Since the electron pair would be of lower energy in an orbital with some 5-character, it would be predicted that a pyramidal geometry would be favored. 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2 phenylbutane has also been studied. When potassium r-butoxide is used, the exchange occurs with retention of configuration in r-butanol, but with racemization in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule attached to the metal ion acts as the proton donor  

In DMSO, symmetrical solvation is achieved prior to protonation, and complete racemization is observed. 

Ion-pair effects have also been implicated in hydrogen-deuterium exchange reactions proceeding through highly delocalized fluorenyl anions. The stereochemical course of exchange of deuterium for protium in the chiral fluorene derivative 2 

Several of the simple organolithium compounds, such as methyl, n-butyl, /-butyl, and phenyl derivatives, are commercially available as hydrocarbon or ether solutions. There are other preparative methods, but we will defer consideration of these until Part B, Chapter 5. 

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Approximate Acidities of Some Hydrocarbons and Reference Materials... 

Fig. 2.8 Acidities of some hydrocarbons, calculated by the CBS-4 method. For the methyl-substituted molecules the proton being removed is one on the methyl group. Methane is extremely weak as an acid and methylpyramidane is relatively strong...

An extensive series of hydrocarbons has been studied in cyclohexylamine, with the use of cesium cyclohexylamide as base. For many of the compounds studied, spectroscopic measurements were used to determine the relative extent of deprotonation of two hydrocarbons and thus establish relative acidity. For other hydrocarbons, the acidity was derived by kinetic measurements. It was shown that the rate of tritium exchange for a series of related hydrocarbons is linearly related to the equilibrium acidities of these hydrocarbons in the solvent system. This method was used to extend the scale to hydrocarbons such as toluene for which the exchange rate, but not equilibrium data, can be obtained. Representative values of some hydrocarbons withpAT values ranging from 16 to above 40 are given in Table 7.2. 

The following table gives exchange rates in methanolic sodium methoxide for a number of hydrocarbons and equilibrium acidities for some. Determine whether there is a correlation between kinetic and thermodynamic acidity in this series of compounds. If so, predict the thermodynamic acidity of the hydrocarbons for which no values are listed. 

TABLE 5. Gas-phase acidities of some cyclic and bicyclic unsaturated hydrocarbons"... 

Although ketene is a likely intermediate in the combustion of some hydrocarbon derivatives, and is perhaps a precursor of the acids sometimes detected in combustion products, its combustion has only recently been invest ted [5 —59]. 

An interesting feature of Scheme 24, devised for palladium, is that it also provides a straightforward explanation for the bond shift isomerization on platinum. An adsorbed metallocyclobutane complex is similar to the trimethylene di-tr-complex of platinum, which is readily formed from hexa-chloroplatinic acid and cyclopropane (75). Also, platinum is known to promote easily a-y exchange of some hydrocarbons with deuterium (57, 34, 76). Therefore, in the case of platinum, the direct formation of metal-locyclobutanes, without intermediacy of 7t-allylic species, would explain the remarkable ability of this metal to promote the isomerization of neopentane to isopentane (Scheme 25). 

The reactions occurring at the sea surface and in the euphotic zone, photo-oxidation, oxidation processes and association of some hydrocarbons with orgemic complexes such as humic or fulvic acids (Khsm and Schnitzer, 1972 Gagosian and Stuermer, 1977), tend to reduce the concentration of more labile compounds, especially the unsaturated hydrocarbons. Oxidation processes lead to the formation of alcohols, acids, alkyl and arylethers, carbonyl compounds and sulfoxides (Kawahara, 1969 Hansen, 1977). 

The tenn terpenes originates from turpentine lat. balsamum terebinthinae). Turpentine, the so-called "resin of pine trees", is the viseous pleasantly smelling balsam which flows upon cutting or carving the bark and the new wood of several pine tree species (Pinaceae). Turpentine contains the "resin acids" and some hydrocarbons, which were originally referred to as terpenes. Traditionally, all natural compounds built up from isoprene subunits and for the most part originating from plants are denoted as terpenes (section 1.2). 

The reaction goes to the right if the hydrocarbon R H is more acidic than RH (or RjNH). In Table 3.3 the pK values of some hydrocarbons, a measure of their acidities, are listed. Phenylethyne, for example, reacts with either butyl- or phenyllithium... 

The term terpene was given to the compounds isolated from turpentine (Latin balsamum terebinthinae), a volatile liquid isolated from pine trees. Turpentine contains the resin acids and some hydrocarbons, which were referred to as terpenes. The term terpene was originally coined to describe a mixture of isomeric plant hydrocarbons of the molecular formula CioHig occurring in the essential oils. 

The alicyclic secondary alcohol, cycZohexanol, may be dehydrated by concentrated sulphuric acid or by 85 per cent, phosphoric acid to cyciohexene. It has a higher boiling point (82-83°) than amylene and therefore possesses some advantage over the latter in.the study of the reactions of unsaturated hydrocarbons. 

Table 14 2 repeats some approximate data presented earlier m Table 1 7 for the acid strengths of representative hydrocarbons and reference compounds... 

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