Alkanes relative acidity

In addition to stable cations, triarylmethyl compounds form stable carbanions. Because of this the corresponding hydrocarbons are relatively acidic compared to simple alkanes. They react readily with strong bases such as sodium amide, and the resulting carbanions, like the cations, are intensely colored ... 

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 ... 

These are equilibria whose positions depend on the relative acidities of the proton sources. The more acidic compound provides the greater amount of organolithium and so the greater the difference in the acidity, the larger the equilibrium constant. However, kinetic acidities must be considered as well as thermodynamic (pK values) e.g., PhH (pKj 37) is not metallated by alkyllithiums [pK of alkanes > 40 in tetrahydrofuran (THF)], despite large difference in pK values. The more reactive RLi-TMED (tetra-methylethylenediamine) complexes, however, provide PhLi. The equilibrium can be driven to the product side by removing the product hydrocarbon or precipitating the RLi. 

Table 5.24. Carbon isotopic compositions of fatty acids and n-alkanes relative to the Ci6 fatty acid and Ci6 n-alkane, respectively (%o PDB) (Duan et al., 2004) (With permission from Duan Y)...

Our experience to this point has been that C—H bonds are not very acidic. Alkanes, for example, have of approximately 60. Compared with them, however, aldehydes, ketones, and esters have relatively acidic hydrogens on their a-carbon atoms (Table 20.1). 

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 leaving abilities as determined by the decomposition of alkoxides are CF3 > Ph > H > t-Bu > Me > i-Pr > Et. Assuming that the intermediate has structure 19a, this order reflects the relative stabilities of the (R ketone) complexes and provides an estimate of the relative acidities of the alkanes. 

Why are acid-base properties presented for alkanes The answer is because it is important to define relative acidity as it relates to other compounds. Establishing that alkanes are considered to be extremely weak acids or bases gives a benchmark for categorizing other weak protonic acids and bases. In Chapter 15, a reaction will be discussed that generates an alkane as a conjugate acid. Categorizing alkanes as very weak acids rather than nonacids will help explain that reaction (see Sections 15.3 and 15.5.2). 

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. 

Polyesters from alkane diols and alkane dicarboxylic acids are freely degraded by biological systems, but their applications have been limited because of their relatively low molecular weight and poor physical strength. 

Reactions catalyzed by silica and silica-alumina surfaces (with or without added acid) closely parallel the carbonium ion reactions in homogeneous acid. The principal advantage of such acidic surfaces is that they allow alkylations and dealkylations to be conducted at temperatures in the 200 00°C region where dealkylation becomes thermodynamically favorable relative to alkylation. They are thus the catalysts of choice for cracking of petroleum distillates to branched olefins and branched alkanes. The acidic surfaces also possess the advantage of little oxidizing action. 

Mote stable catalysts ate obtained by using fluorinated graphite or fluorinated alumina as backbones, and Lewis acid halides, such as SbF, TaF, and NbF, which have a relatively low vapor pressure. These Lewis acids ate attached to the fluorinated soHd supports through fluorine bridging. They show high reactivity in Friedel-Crafts type reactions including the isomerization of straight-chain alkanes such as / -hexane. 

An additional curious feature of alkylaromatic oxidation is that, under conditions where the initial attack involves electron transfer, the relative rate of attack on different alkyl groups attached to the same aromatic ring is quite different from that observed in alkane oxidation. For example, the oxidation of -cymene can lead to high yields of -isopropylbenzoic acid (2,205,297,298). 

Alkylation of isobutylene and isobutane in the presence of an acidic catalyst yields isooctane. This reaction proceeds through the same mechanism as dimerization except that during the last step, a proton is transferred from a surrounding alkane instead of one being abstracted by a base. The cation thus formed bonds with the base. Alkylation of aromatics with butylenes is another addition reaction and follows the same general rules with regard to relative rates and product stmcture. Thus 1- and 2-butenes yield j -butyl derivatives and isobutylene yields tert-huty derivatives. 

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