Bonding in alkanes

The most frequently used organocuprates are those in which the alkyl group is primary. Steric hindrance makes secondary and tertiary dialkylcuprates less reactive, and they tend to decompose before they react with the alkyl halide. The reaction of cuprate reagents with alkyl halides follows the usual Sn2 order CH3 > primary > secondary > tertiary, and I > Br > Cl > F. p-Toluenesulfonates are somewhat more reactive than halides. Because the alkyl halide and dialkylcuprate reagent should both be primary in order to produce satisfactory yields of coupled products, the reaction is limited to the fonnation of RCH2—CH2R and RCH2—CH3 bonds in alkanes. 

Although many of the aromatic compounds based on benzene have pleasant odors, they are usually toxic, and some are carcinogenic. Volatile aromatic hydrocarbons are highly flammable and burn with a luminous, sooty flame. The effects of molecular size (in simple arenes as well as in substituted aromatics) and of molecular symmetry (e.g., xylene isomers) are noticeable in physical properties [48, p. 212 49, p. 375 50, p. 41]. Since the hybrid bonds of benzene rings are as stable as the single bonds in alkanes, aromatic compounds can participate in chemical reactions without disrupting the ring structure. 

Carbon-carbon single bonds in alkanes are formed by a overlap of carbon sjy hybrid orbitals. Rotation is possible around a bonds because of their cylindrical... 

The importance of palladium acetate lies in its ability to catalyse a wide range of organic syntheses functionalizing C-H bonds in alkanes and in aromatics, and in oxidizing alkenes. It has been used industrially in the... 

Spectroscopy of the PES for reactions of transition metal (M ) and metal oxide cations (MO ) is particularly interesting due to their rich and complex chemistry. Transition metal M+ can activate C—H bonds in hydrocarbons, including methane, and activate C—C bonds in alkanes [18-20] MO are excellent (and often selective) oxidants, capable of converting methane to methanol [21] and benzene to phenol [22-24]. Transition metal cations tend to be more reactive than the neutrals for two general reasons. First, most neutral transition metal atoms have a ground electronic state, and this... 

Ans. (a) The extra pair of electrons between two carbon atoms leaves two fewer electrons to bond to hydrogen atoms, (ft) The extra bond between the two end carbon atoms leaves two fewer electrons to bond to hydrogen atoms, (c) There are no multiple bonds in alkanes and cycloalkanes. 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

We have already explained. In terms of hybridisation, how a carbon atom can form four sp hybrid orbitals (see p. 47). We can apply this concept to explain the bonding in alkanes. Ethane is taken as an example of a typical alkane. The four sp hybrid orbitals on each carbon atom will overlap end-on with four other orbitals three hydrogen Is orbitals and one sp hybrid orbital on the other carbon atom. Four cr bonds will be formed and they will adopt a tetrahedral arrangement. This is illustrated for ethane in the diagram. 

Abstract This chapter covers oxidation of C-H and C-C bonds in alkanes. Section 4.1 concerns oxidation of C-H bonds aldehydes and other CH species (4.1.1), methylene (-CH groups) (4.1.2) and methyl (-CH ) groups (4.1.3). This is followed by the oxidation of cyclic alkanes (4.1.4) and large-scale alkane oxidations (4.1.5). Alkane oxidations not considered here but covered in Chapter 1 are hsted in Section 4.1.6. The final section (4.2) concerns oxidative cleavage of C-C bonds. 

Alkanes have similar chemical properties, hut their physical properties vary with molecular weight and the shape of the molecule. The low polarity of all the bonds in alkanes means that the only intermolecular forces between molecules of alkanes are the weak dipole-dipole forces (see 2.5.1), which are easily overcome. As a result, compared with other functional groups, alkanes have low melting and boihng points, and low solubility in polar solvents, e.g. water, but high solubility in nonpolar solvents, e.g. hexane and dichloromethane. Most cycloalkanes also have low polarity. 

Much of the work on the catalytic activation of C—H bonds in alkanes follows from the important observation of Garnett and Hodges (21) that, in the presence of the PtCl42- ion, aromatic compounds exchange hydrogen with deuterium in the D20—CH3C02D solvent. Temperatures in the range 80°-100°C are used and the solvent also contains a mineral acid to stabilize the platinum(II) from disproportionation ... 

The contents of Sections II, III, and IV show that the activation of C—H bonds in alkanes by transition metal compounds has much in common with the activation of C—H bonds in aromatic compounds. It appears, therefore, to be more profitable at the present time to draw mechanistic parallels between alkanes and aromatic systems, as has been done here, than, say, between alkanes and molecular hydrogen, although, of course, much work has been done on hydrogen activation (59). 

An obvious use of an electronegativity scale is to predict the direction of electrical polarity of a covalent bond with ionic character. Table 2.2 tells us that the C—H bond in alkanes (CnH2n+2) is polar in the same sense as the 0—H bonds in water, although to a much lesser degree ... 

The first step of the activation of butane and cyclohexane has been assumed to be the cleavage of a secondary C—H bond, with minor contributions from primary C — H bonds in the case of butane. This picture is supported only by indirect evidence. When the relative rates of reaction of various alkanes were compared on a V-Mg oxide and Mg2V207 catalyst (Table VIII), it was found that alkanes with only primary carbons (ethane) reacted most slowly. Those with secondary carbons (propane, butane, and cyclohexane) reacted faster, with the rate being faster for those with more secondary carbon atoms. Finally, the alkane with one tertiary carbon (2-methylpropane) reacted faster than the ones with either a single or no secondary carbon (26). From these data, it was estimated that the relative rates of reaction of a primary, secondary, and tertiary C—H bond in alkanes on the V-Mg oxide catalyst were 1, 6, and 32, respectively (26). 

The (r-donor ability of the C—C and C—H bonds in alkanes was demonstrated from a variety of examples. The order of reactivity of single bonds was found to be tertiary C—H > C—C > secondary C H primary C H, although various specific factors such as steric hindrance can influence the relative reactivities. 

The hydrogen peroxonium ion may be considered as an incipient OH+ ion capable of electrophilic hydroxylation of single (effect reactions similar to such previously described electrophilic reactions as protolysis, alkylation, chlorination (chlorolysis), and nitration (nitrolysis). 

Since ozone is a strong 1,3-dipole,635 or at least has a strong polarizability (even if a singlet biradical structure is also feasible), it is expected to be readily protonated in superacids, in manner analogous to its alkylation by alkylcarbenium ions. Protonated ozone HC>3+, once formed, should have a much higher affinity (i.e., be a more powerful electrophile) for cr-donor single bonds in alkanes than neutral ozone. 

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

Cis/Trans Isomerism There is normally free rotation around the carbon-carbon single bonds in alkanes. The alkene functional group has two carbon-carbon bonds. The introduction of the second bond freezes rotation around the... 

Aromatic compounds have special characteristics of aromaticity, which include a low hydro-gen carbon atomic ratio, C-C bonds that are quite strong and of intermediate length between such bonds in alkanes and those in alkenes, tendency to undergo substitution reactions rather than the addition reactions characteristic of alkenes, and delocalization of n electrons over several carbon atoms. The last phenomenon adds substantial stability to aromatic compounds and is known as resonance stabilization. 

The activation and transformation of C-H bonds in alkanes by homogeneous transition metal catalysts should to be a topic of further research. No processes are close to commercialization on a technical scale. So far the organometallic approach by Shilov is one of the most promising ways of producing a practical system [55]. 

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