Acidity of alkanes

Carbon Acidity of Strained Hydrocarbons The difference in the formal hybridization at carbon accounts for the relative acidities of alkanes versus alkenes versus akynes. The greater the s-character of the carbon of Ihe C-H bond of interest, the more acidic is that proton. Of interest here is extension of this idea to strained compounds, which may employ greater or lesser s-character to accommodate the unusual geometries. Will these compounds have acidities that reflect their hybridization Are strained hydrocarbons more acidic than their unstrained analogs ... 

Hydrogens on carbons adjacent to carbonyl or related functional groups are, as a class, the most acidic of alkane-like hydrogens. A short list of representative pK ... 

Metal salts of alkanes, e.g. RLi, have long been used as sources of carbanions R in synthetic organic chemistry and metalation of a carbon acid R H by an organo-lithium compound RLi gives a measure of the relative acidities of alkanes RH and RH (equation 21). Unfortunately this metalation reaction is not quite so simple because both RLi and R Li usually exist as aggregates, even in coordinating solvents and in the gas phase... 

The relative acidity of alkanes, amines, alcohols, and hydrogen halides. 

As might be expected, the hybridization at nitrogen itself also drastically affects basicity, in the order jNHs > R2C=NR > RC=N , a phenomenon that we already enconntered in the discussion of the relative acidity of alkanes, alkenes, and alkynes (Section 13-2). Thus, iminium ions (Section 17-9) have valnes estimated to be of the order of 7 to 9 A-protonated nitriles (Section 20-8) are even more acidic (pa s < -5). Table 21-2 summarizes the pAa values of the conjugate acids of some representative amines. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Acid-catalyzed isomerization reactions of alkanes as well as alkylation and condensation reactions are initiated by protolytic ionization. Available evidence indicates nonlinear but not necessarily triangular... 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

The perfluoroalkane sulfonic acids were fkst reported ki 1954. Trifluoromethanesulfonic acid was obtained by the oxidation of bis(ttifluoromethyl thio) mercury with aqueous hydrogen peroxide (1). The preparation of a series of perfluoroalkanesulfonic acids derived from electrochemical fluotination (ECF) of alkane sulfonyl haUdes was also disclosed ki the same year (2). The synthetic operations employed when the perfluoroalkanesulfonic acid is derived from electrochemical fluotination, which is the best method of preparation, are shown ki equations 1—3. 

Reaction conditions depend on the reactants and usually involve acid or base catalysis. Examples of X include sulfate, acid sulfate, alkane- or arenesulfonate, chloride, bromide, hydroxyl, alkoxide, perchlorate, etc. RX can also be an alkyl orthoformate or alkyl carboxylate. The reaction of cycHc alkylating agents, eg, epoxides and a2iridines, with sodium or potassium salts of alkyl hydroperoxides also promotes formation of dialkyl peroxides (44,66). Olefinic alkylating agents include acycHc and cycHc olefinic hydrocarbons, vinyl and isopropenyl ethers, enamines, A[-vinylamides, vinyl sulfonates, divinyl sulfone, and a, P-unsaturated compounds, eg, methyl acrylate, mesityl oxide, acrylamide, and acrylonitrile (44,66). 

The major use of alkan olamines in agricultural products is as a neutralizer for acidic herbicides. They also contribute increased water solubiUty, reduced volatility, and more uniform solutions. Various ethan olamines are reported in formulations to improve potato tuber size (219) and enhance the resistance to salt of some crops (220). 

Deep fluorinalion of alkanes, ethers, acid fmlides, esters, alkyl chlorides, most ketones, ketals, orthoesters, and combinations of these functional groups produces principally the perfluonnated analogues (Table 2) Chlorine substituents (or chloro groups) usually survive fluorination... 

An illustration of the tendency of alkane 1,2- and alkane-1,3-dicarboxylie acids to a ring closure during the reaction with sulfur tetrafluoride is the reaction of propane-1,2,3-tricarboxylie acid The corresponding six- and five-membered cyclic ethers are formed in a 1 4 ratio [211] (equation 106)... 

Oxidative reactions frequently represent a convenient preparative route to synthetic intermediates and end products This chapter includes oxidations of alkanes and cycloalkanes, alkenes and cycloalkenes, dienes, aromatic fluorocarbons, alcohols, phenols, ethers, aldehydes and ketones, carboxylic acids, nitrogen compounds, and organophosphorus, -sulfur, -selenium, -iodine, and -boron compounds... 

---