Alkanes acidic hydrogen

As usual, R is a hydrocarbon group or, in the simplest case, a hydrogen atom. The acidic hydrogen atom is the one bonded to oxygen. The IUPAC name of a carboxylic add can be obtained by substituting the suffix -oic acid for the final e in the name of the corresponding alkane. In practice, such names are seldom used. For example, the first two members of the series are commonly referred to as formic add and acetic add. 

Although of relatively weak acidity, amines will react with either carbanionic metal alkyls or hydridic metal hydrides to form amides with the elimination of alkane or hydrogen, respectively. The easiest and most exploited method for the synthesis of lithium and magnesium amides is to treat lithium alkyls or Grignard reagents (normally commercially available) with the corresponding amine (equations 25,47 264 and 2749). 

The reaction of branched-chain alkanes with hydrogen peroxide in Magic Acid-S02C1F solution has been carried out with various ratios of alkane and hydrogen peroxide and at different temperatures.602 Some of the results are summarized in Table 5.35. 

In alkylations of metal halides with A1R3 usually only one alkyl group is transferred, since the dialkyl aluminum halides are much less powerful alkylating agents than the trialkyls. The alkylaluminums react with compounds having acidic hydrogen atoms to give the alkane in some cases alkylation may occur, for example,... 

Several of the trialkylaluminum and alkylaluminum halides and hydrides mentioned above are commercially available. Alkynyl, alkenyl, cyclopentadienyl, and aryl derivatives are, in general, not commercially available and must be synthesized for laboratory use. Alkynyl derivatives can be prepared by salt metathesis, as in the reaction of Et2AlCl with NaC=CEt to give Et2AlC=CEt. The acidity of terminal alkynes is sufficient for preparation of alkynyl aluminum compounds by alkane or hydrogen elimination upon reaction with a trialkylaluminum or an aluminum hydride (equation 17), respectively. TriaUcynyl aluminum compounds are typically isolated as Lewis base adducts to stabilize them against otherwise facile polymerization. Alkenyl compounds of aluminnm have similarly been prepared. 

Amines react with organoaluminum compounds to give Lewis acid-base adducts. However, ammoiua and primary and secondary amines may coordinate to organoaluminum compounds with subsequent alkane or hydrogen evolution to form heterocyclic aminoalanes, oligomeric iminoalanes, or in the case of ammonia, aluminum nitride.A classic example is the... 

Shilov chemistry, developed from 1970, employs [Pt(II)CLt] salts to oxidize alkanes RH to ROH or RCl with modest efficiency. Pt(IV) is an efficient (but economically impractical) primary oxidant that makes the process catalytic. This discovery strongly contributed to the continuing activity in CH activation. Periana developed a related and much more efficient system for methane oxidation to methanol using 2,2 -bipyrimidine ligands and sulfuric acid as solvent. In this case, the sulfuric acid is the primary oxidant and the methanol formed is protected by being converted in situ to MeOSOsH, an ester that strongly resists further oxidation. This area is more fully described under the entry Alkane Carbon-Hydrogen Bond Activation. 

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 ... 

Because of the greater proportion of s character in the sp o orbitals of the carbon atoms than that in sp hybrids (alkenes) or sp hybrids (alkanes), any bonding pair of electrons can come closer to carbon nuclei in acetylene than in alkenes or alkanes. Hence hydrogen is released as a proton more easily in the acetylenic carbon atoms. The increased acidity of hydrogen attached to a carbon atom with a triple bond results in the formation of a class of carbides with certain metals, called acetylides. These acetylides are unstable and highly sensitive to shock and heat, exploding violently (see Chapter 30). 

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). 

The overall order of alkane acidities deduced from the decomposition of silicon anions is the same as that obtained from the decomposition of alkoxides " " . The ethyl anion has the highest proton affinity, i.e., ethane is the weakest acid. The acidities of propane (secondary hydrogen) and of cyclobutane are both lower than that of methane, but all other alkanes are more acidic. The acidity of methane is particularly enhanced by a cyclopropyl substituent and by a t-butyl group. 

Alkylation of Aromatics. Aromatic hydrocarbons containing a replaceable hydrogen can be alkylated unless steric effects prevent introduction of the alkyl group (61,78-82). The reaction is called the Friedel-Crafts alkylation, first realized in the presence of aluminum chloride, which is the catalyst still the most frequently used and studied in Friedel-Crafts reactions. In addition, many other acid catalysts are effective (80,82-84). These include other Lewis acids (other aluminum halides, gallium chloride, boron trifluoride, ferric chloride, zinc chloride, stannous and stannic chloride, antimony chloride) and protic acids (hydrogen fluoride, concentrated sulfuric acid, phosphoric acid, polyphosphoric acid, trifluo-romethanesulfonic acid, and alkane- and arenesulfonic acids). 

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