Primary C-H bonds

Only 20—40% of the HNO is converted ia the reactor to nitroparaffins. The remaining HNO produces mainly nitrogen oxides (and mainly NO) and acts primarily as an oxidising agent. Conversions of HNO to nitroparaffins are up to about 20% when methane is nitrated. Conversions are, however, often ia the 36—40% range for nitrations of propane and / -butane. These differences ia HNO conversions are explained by the types of C—H bonds ia the paraffins. Only primary C—H bonds exist ia methane and ethane. In propane and / -butane, both primary and secondary C—H bonds exist. Secondary C—H bonds are considerably weaker than primary C—H bonds. The kinetics of reaction 6 (a desired reaction for production of nitroparaffins) are hence considerably higher for both propane and / -butane as compared to methane and ethane. Experimental results also iadicate for propane nitration that more 2-nitropropane [79-46-9] is produced than 1-nitropropane [108-03-2]. Obviously the hydroxyl radical attacks the secondary bonds preferentially even though there are more primary bonds than secondary bonds. 

The radical stabilization provided by various functional groups results in reduced bond dissociation energies for bonds to the stabilized radical center. Some bond dissociation energy values are given in Table 12.6. As an example of the effect of substituents on bond dissociation energies, it can be seen that the primary C—H bonds in acetonitrile (86 kcal/mol) and acetone (92kcal/mol) are significantly weaker than a primaiy C—H... 

The degree to which allylic radicals ar e stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals. Compare, for exanple, the bond dissociation energies of the primary C—H bonds of propane and propene ... 

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 on page 156 hints at the answer. The data in Table 5.3 indicate that a tertiary C—H bond (390 kj/mol 93 kcal/mol) is weaker than a secondary C-H bond (401 kj/mol 96 kcal/mol), which is in turn weaker than a primary C H bond (420 kj/mol 100 kcal/mol). Since less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

The air-stable complex Cp (PMe3)IrCl2 efficiently catalyzes the exchange of deuterium from D20 into both activated and nonactivated C-H bonds of organic molecules without added acid or stabilizers. Selectivity is observed in many cases, with activation of primary C-H bonds occurring preferentially (Eq. 2.8).29... 

Alkanes are oxidized first to alcohols and then to ketones6,34,45,51 52 as shown in Fig. 6.6. The order of reactivity is tertiary C—H > secondary C—H > primary C-H. In many cases oxidation of primary C—H bonds is below detection.52... 

Another remarkable property of iodorhodium(III) porphyrins is their ability to decompose excess diazo compound, thereby initiating carbene transfer reactions 398). This observation led to the use of iodorhodium(III) me.vo-tetraarylporphyrins as cyclopropanation catalysts with enhanced syn anti selectivity (see Sect. 2.2.3) s7, i°o) as wep as catalysts for carbenoid insertion into aliphatic C—H bonds, whereby an unusually high affinity for primary C—H bonds was achieved (see Sect. 6.1)287). These selectivities, unapproached by any other transition metal catalyst,... 

The primary C-H bond is often activated in these and related systems, whereas secondary C-H bonds are often intact, although in general chemistry the reactivity of a C-H bond is often in the order tertiary > secondary > primary.24... 

Replacing one or several of the hydrogen atoms in methane by one or several other atoms than hydrogen automatically creates secondary or tertiary C-H bonds. Secondary and tertiary C-H bonds are more reactive than a primary C-H bond. During oxidation reactions, this leads to an easier oxidation of the reaction products than methane, and consequently to a low(er) reaction selectivity. Such reactions therefore produce complicated reactant mixmres that require costly... 

The reason for this low heat of reaction is that while a primary C-H bond (98 kcal/mol) is being broken, a C-C pi-bond (60 kcal/mol) is simultaneously being formed. 

A similar conclusion applies to a Mg-V-O catalyst in which Mg3(V04)2 is the active component. The relative rates of reaction for different alkanes on this catalyst follow the order ethane < propane < butane 2-methylpropane < cyclohexane (Table I) [12-14]. This order parallels the order of the strength of C-H bonds present in the molecule, which is primary C-H > secondary C-H > tertiary C-H. Ethane, which contains only primary C-H bonds, reacts the slowest, whereas propane, butane, and cyclohexane react faster with rates related to the number of secondary carbon atoms in the molecule, and 2-methylpropane, with only one tertiary carbon and the rest primary carbons, reacts faster than propane which contains only one secondary carbon. Similar to a Mg-V-O catalyst, the relative rates of oxidation of light alkanes on a Mg2V207 catalyst follow the same order (Table I). 

The C-H bond strength is largest for primary C-H bonds at —101 kcal mol-1, decreasing to 98 kcal mol-1 for secondary and 96 kcal mol-1 for tertiary C-H bonds (Lide, 1998-1999). Hence one expects that, all else being equal, a tertiary C-H will react faster than a secondary C-H, which in turn will react faster than a primary C-H. Greiner (1970), whose measurements of the absolute rate constants for OH reactions in the mid-1960s provided the first clue of the potential importance of OH in the troposphere, suggested that... 

The magnitude of the rate constants, their observed pressure dependence, and the products of the reactions are consistent with the mechanism involving the initial addition of OH to the triple bond. For example, the OH-l-butyne reaction at 298 K is about a factor of three faster than the reaction with n-butane (see Table 6.2), despite the fact that it has fewer abstractable hydrogens and the = C — H bond is much stronger than a primary -C-H bond ( 125 vs 100 kcal mol -1). In addition, a pressure dependence is not consistent with a simple hydrogen atom abstraction (see Chapter 5.A.2). 

The first preparative use of intramolecular C—H insertion in organic synthesis was based on the observation that on flash vacuum pyrolysis, a conjugated alkynyl ketone such as 1-(1-methyl-cyclopentyl)-2-propynonc is smoothly converted to a mixture of the cyclizcd enones 1 and 223. This elegant reaction apparently proceeds via isomerization of the alkyne to the corresponding alkylidene carbcne, followed by subsequent intramolecular C-H insertion. It should be noted that despite a 3 2 statistical predominance of primary C-H bonds over secondary C—H bonds, a marked preference for insertion into the latter (methylene) is observed. 

This reaction apparently proceeds by way of the normal phosphonate condensation product, the diazoalkylidene, which then spontaneously loses nitrogen to form the transient alkylidene car-bene. Careful work showed that, after statistical corrections were applied, the reactivity of a C-H bond toward insertion was approximately 0.003 for primary C-H bonds (methyl), 1.0 for secondary C-H bonds (methylene), 7.5 for benzylic (methylene) C-H bonds and 18.6 for tertiary C-H bonds. These relative reactivities are very similar to those previously observed for intramolecular C-H insertion by an alkylidene carbenoid generated from a vinyl bromide27. It was shown subsequently that the alkylidene carbene insertion reaction proceeds with retention of absolute configuration28. Using this approach, (l )-3-dimethyl-3-phenyl-l-cyclopentene and (i )-4-methyl-4-phenyl-2-cyclohexcnonc were prepared in high enantiomeric purity. 

Alkanes appear to react with platinum(IV) in an identical manner to benzene (34, 84) chloromethane and chloroethane can be detected as the reaction products from methane and ethane, respectively. When propane, butane, or hexane is the reactant, the terminal chloro isomers predominate over the internal isomers. This was interpreted to mean that primary C—H bonds are the most reactive (34), but a more detailed study has shown that this conclusion does not necessarily follow from the experimental results (84). When cyclohexane is the reactant, dehydrogenation (or chlorination and then dehydrohalogenation) occurs to give benzene as one of the reaction products (29, 34, 84). 

Unlike higher alkanes, ethane contains only primary C—H bonds, and the dehydrogenation product ethene contains only vinylic C—H bonds. As shown in Table I, these are strong bonds. Thus one would expect that, compared to other alkanes, the activation of ethane would require the highest temperature, but the reaction might be the most selective in terms of the formation of alkene. Indeed, this appears to be the case. 

Propane contains one secondary carbon, and the secondary C—H bonds are weaker than the primary C—H bonds (Table I). Therefore, propane is... 

The selectivity is high at low conversions and decreases as the conversion increases. Thus the sequential reaction of Eq. (3) is the primary reaction pathway for these catalysts, and the activation of propane is mostly by breaking of a secondary C—H bond to form a propyl species, with breaking of a primary C—H bond contributing to a minor extent. 

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). 

Simplest Mechanism. The kinetics and products of the liquid-phase oxidation of neat isobutane (tert-BuH) are largely explained by the following steps. (We have found no significant reactions of primary C—H bonds.) Similar steps also apply to the liquid-phase oxidation of cumene (6,12). 

It appears, then, that alkylperoxy radical isomerization is capable of producing hydroperoxyalkyl radicals during the oxidation of all alkanes and that alkene-hydroperoxy radical addition will serve a similar function during the oxidation of those alkanes which contain a high proportion of primary C—H bonds. It remains to determine the proportion of hydroperoxy alkyl radicals arriving by each route as equilibrium is approached. 

It is of interest to review ideas as to the point of hydrogen abstraction. Through 1939 most investigators believed that attack of paraffins was at the primary C—H bonds at the end of a chain (75, 109, 167, 168, 217, 220, 224), Attack at the a-carbon atom of substituted benzenes (217) and at the end methyl of olefins (109) was proposed. Preferential attack at 1° C—H bonds fitted in with the comparative ease of oxidation of n-paraffins and their low knock ratings. 

These results are in agreement with the alkane behavior in superacid media and indicate the ease of oxidation of tertiary alkanes. However, high acidity levels are necessary for the oxidation of alkanes possessing only primary C—H bonds. 

Propane as a degradation product of polyethylene (a byproduct in the reaction) was ruled out because ethylene alone under the same conditions does not give any propane. Under similar conditions but under hydrogen pressure, polyethylene reacts quantitatively to form C3 to C6 alkanes, 85% of which are isobutane and isopentane. These results further substantiate the direct alkane alkylation reaction and the intermediacy of the pentacoordinate carbonium ion. Siskin also found that when ethylene was allowed to react with ethane in a flow system, n-butane was obtained as the sole product, indicating that the ethyl cation is alkylating the primary C—H bond through a five-coordinate carbonium ion [Eq. (5.66)]. 

Oxidation of alkanes to alcohols anil or ketones.1 This dioxirane oxidizes hydrocarbons in CH2Cl2/l,l,l-trifluoro-2-propanone (TFP) at -22 to 0° to alcohols or further oxidation products in high yield. Tertiary C-H bonds are attacked more rapidly than secondary ones, and primary C-H bonds are scarcely affected. The oxidation apparently involves insertion of O-atom. Oxidations can be stereospecific, as in the case of cis- and trans- 1,2-dimethyIcyclohexane. 

Specific dehydrogenation at the terminal positions of alkanes is a reaction that would be of high utility. The 1-alkenes obtained by such a reaction are the basis of a variety of additional products. Felkin and co-workers discovered that metal complexes are able to mediate the transfer of hydrogen from alkanes 13 to olefins 14 (Scheme 4) [17]. The specific advantages of a transition metal catalyst can be applied to the benefit of the chemoselectivity of this reaction. In a kinetically controlled process, it is predominantly primary C-H bonds that add to the metal complex. A subsequent /Miydride elimination affords the terminal alkenes... 

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