Reactivity of alkanes

An older name for alkanes is paraffin hydrocarbons Paraffin is derived from the Latin words parum affims ( with little affinity ) and testifies to the low level of reactivity of alkanes... 

Although the activation and functionalization of C-H bonds of alkanes are the important, promising routes for synthesis of functionalized materials, it is difficult to achieve the functionalization of alkanes because they are unreactive due to the low reactivity of alkane C-H bonds. Carboxylation of alkanes to carboxylic acids is one of the interesting and important functionalization processes. 

Arguments similar to those stated above can be used to explain the relative chemical inertness of fluoropolymers. Consider the reactivity of alkanes vs. perfluoroalkanes as shown in Table 4.2 (abstracted from Sheppard and Sharts Statistically, FA based materials will have many more types of bonds, in addition to C—F, than fluoropolymers. These bonds will be subject to the same chemical fate during assault by aggressive reagents as bonds in their hydrocarbon counterparts. Similar reasoning can be used to explain the relative thermal stability of FAs compared to fluoropolymers. Thus, incorporation of perfluoroalkyl groups will not make the modified material less stable than the native one. 

Ozone reacts slowly with saturated hydrocarbons usually to give alcohols.92 93 The reactivity of alkanes toward ozone is several orders of magnitude less than that of alkenes. Oxidation of saturated hydrocarbons takes place preferentially at the tertiary carbon. In liquid-phase ozonation94 the order of reactivity of the primary, secondary and tertiary C—H bonds is 1 13 110. The formation of tertiary alcohols occurs with high degree (60-94%) of stereoselectivity.94-96... 

Discuss the chemical reactivity of alkanes. Why are they chemically reactive or unreactive ... 

Equation (1) depicts an early example of an intermolecular addition of an alkane C-H bond to a low valent transition metal complex [12], Mechanistic investigations provided strong evidence that these reactions occur via concerted oxidative addition wherein the metal activates the C-H bond directly by formation of the dative bond, followed by formation of an alkylmetal hydride as the product (Boxl). Considering the overall low reactivity of alkanes, transition metals were able to make the C-H bonds more reactive or activate them via a new process. Many in the modern organometallic community equated C-H bond activation with the concerted oxidative addition mechanism [10b,c]. 

As discussed in Section 6.6, numerical analyses can be used with some success to simulate the chemical features of low-temperature hydrocarbon oxidation. However, there is progress yet to be made on a number of fronts by both experimentalists and numerical modellers. The kinetics and mechanism of certain classes of compounds, such as the alkenes and aromatics, have not been investigated fully in the low-temperature region, the information on aromatic compounds being particularly sparse (Chapter 1). By contrast, there is much better understanding of the low-temperature oxidation of alkanes. Following Section 1.4.1, the reactivity of alkanes arises within the mechanistic structure... 

It was shown in Fig. 53 that the hydrocarbon conversion which is reached over the catalyst at fixed reaction conditions depends on the type of the hydrocarbon. Alkenic and aromatic hydrocarbons are more reactive than alkanic hydrocarbons. The reactivity of alkanic hydrocarbons increases with the number of carbon atoms in the molecule. 

Compare the reactivities of alkanes and alkenes. Explain the difference. 

An older name for alkanes is paraffin hydrocarbons. Paraffin is derived from the Latin words parum affinis ( with little affinity ) and testifies to the low level of reactivity of alkanes. Like most other organic compounds, however, alkanes bum readily in air. This combination with oxygen is known as combustion and is quite exothermic. All hydrocarbons yield carbon dioxide and water as the products of their combustion. 

Table II. 10. Relative reactivities of alkanes and arylalkanes toward oiione and some radicals.

Chemical properties of alkanes The main chemical property of alkanes is their low reactivity. Recall that many chemical reactions occur when a reactant with a full electric charge, such as an ion, or with a partial charge, such as a polar molecule, is attracted to another reactant with the opposite charge. Molecules such as alkanes, in which atoms are connected by nonpolar bonds, have no charge. As a result, they have little attraction for ions or polar molecules. The low reactivity of alkanes can also be attributed to the relatively strong C-C and C-H bonds. 

The reactivity of alkanes and cycloalkanes are very low. Alkenes and alkynes containing double and triple bonds are reactive. The addition reactions follow Markovnikov rule. Conjugated dienes undergo Diels-Alder reactions. The substances that may react violently with unsaturated hydrocarbons are halogens, strong oxidizers, and nitrogen dioxide. Alkynes may form acetylides with certain metals. 

Besides the unusual regioselectivity of alkane oxidation (tertiary C-H bonds are less reactive than secondary C-H bonds ) another interesting feature of the reactive species in Gif-reactions is the unusual relative reactivity of alkane C-H bonds compared with some other readily oxidizable compounds such as alcohols, thiols and triphenylphosphine The explanation of such behavior is not clear so far. 

Due to the low reactivity of alkanes, their analysis by derivatization is very difficult. Moreover, most of the literature on analysis of functional groups does not include analytical methods for separation and identification of alkanes. 

The properties and reactivity of alkane and silane - - complexes are closely related to those of dihydrogen complexes. However, the thermodynamic stabilities of the complexes are much different. The silane complexes have been studied in less detail than dihydrogen complexes, but, broadly speaking, they are similar in stability to the dihydrogen complexes or only slightly less stable. Alkane complexes have been studied intensively as reactive intermediates, " but detailed structural and reactivity studies have not been conducted because alkane complexes are unstable in solution. The most detailed data have been gained by NMR spectroscopic studies of CpRe(CO)2(RH) complexes. These data imply that the metal binds in an Tj -mode to a single C-H bond of the alkane. 

The high reactivity of alkanes possessing tertiary carbon now may be explained by their ability to readily participate in the hydride transfer step. Since carbocations do not readily abstract hydride ion from secondary (or primary) carbon atoms, n-alkanes usually do not enter the alkylation reaction unless they first isomerize to isoalkanes containing tertiary carbon atoms. 

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