Radicals with alkanes

If the hydrocarbon radical cation has a definitive structure, proton loss occurs from one particular, well-defined position and these transformations are more selective than the alternative C-H abstractions from alkanes with radical reagents (Eq. 2). For example, C-H substitutions of the adamantane cage with radical reagents always give mixtures of 1 and 2-substituted adamantanes [2], As the adamantane radical cation (4) has one single structure, proton transfer from the radical cation to the solvent occurs highly selectively. Scheme 2 shows the geometry of 4 and the structure of the complex of the adamantane radical cation with acetonitrile (S) where the tertiary C-H bond is already half-broken. 

The names of alkanes with radical branches use the name from Table 18-2 for the longest continuous chain of carbon atoms. The radicals are named using the -yl ending presented in Table 18-3, and their positions along the carbon chain are denoted by a number. For example,... 

The elementary steps (1) through (3) describe a free radical chain mech anism for the reaction of an alkane with a halogen... 

Structurally simple alJkyl halides can sometimes be prepared by reaction of an alkane with Cl2 or Br2 through a radical chain-reaction pathway (Section 5.3). Although inert to most reagents, alkanes react readily with Cl2 or Br2 in the presence of light to give alkyl halide substitution products. The reaction occurs by the radical mechanism shown in Figure 10.1 for chlorination. 

Even with the limitation on yield implied by the statistical process, cross-dimerization is still useful when one of the reactants is an alkane, because the products are easy to separate, and because of the few other ways to functionalize an alkane. The cross-coupling of an alkane with trioxane is especially valuable, because hydrolysis of the product (10-6) gives an aldehyde, thus achieving the conversion RH RCHO. The mechanism probably involves abstraction of H by the excited Hg atom, and coupling of the resulting radicals. 

Alkane free radical addition to aklenes and alkynes generates products with more carbon atoms than the reactants—that is, the result is alkylation. Generally,... 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

Deprotonation is a typical direction of cation-radical reactivity. Cation-radicals are usually strong H acids (e.g., the alkane cation-radicals pass its protons to the alcohol molecules Sviridenko et al. 2001). Bases that conjugate with these H acids are radicals RH+ —> H+ + R. Scheme 1.19 displays two real cases of this deprotonation (Neugebauer et al. 1972). 

Dinitrogen pentoxide reacts with alkanes in carbon tetrachloride at 0 °C via a radical mechanism to give nitration products which can include nitrate esters.Reactions of alkanes with dinitrogen pentoxide in nitric acid are complex and of little synthetic value. 1-Adamantyl nitrate is one of the products obtained from the photochemical irradiation of a solution of adamantane and dinitrogen pentoxide in methylene chloride. ... 

When two free radicals combine, they form a molecule with aU covalent bonds that is a stable and unreactive molecule we call P . It consists of n monomer units (M) linked together with an initiator unit (A) at each end, AM, A. From ethylene, this polymer is the molecule A CH2)2nA. Note that the product molecule is simply a linear alkane with no double bonds remaining. This molecule is totally unreactive, and it is called the dead polymer, ff n is large enough, we neglect the end groups and caU it simply (C2iin)n-... 

In 1992, Hari Prasad Rao and Ramaswamy reported on the oxyfunctionalization of alkanes with H2O2 using a vanadium silicate molecular sieve s . With this catalyst acyclic and cyclic alkanes were oxidized to a mixture of the corresponding alcohols (primary and secondary ones), aldehydes and ketones. Unfortunately, most of the early attempts were of rather limited success due to low turnover frequencies and radical producing side reactions as observed, for example, by Mansuy and coworkers in 1988. ... 

Oxygenation of alkanes with hydrogen peroxide in homogeneous reactions can be obtained also with Keggin-type iron-substituted polyoxometalates. Evidence against radical reactions has been offered. 

Ketones. Reactions of ketones are similar to those of alkanes, with abstraction by OH, N03, and Cl occurring from the alkyl chains. In the case of acetone, for example, these reactions generate the radical CH3C0CH202 in air. As for typical R02, this reacts rapidly with NO (k m = 8 X 10 12 cm3 molecule-1 s"1) and with N02 ( 29x = 6.4 X 10-12 cm3 molecule -1 s-1) (Sehested et al., 1998b), giving calculated lifetimes with respect to reaction with 100 ppt each of NO and N02 of about 1 min. In remote areas where the NOx concentrations can be much smaller (see later and Chapter 11), reactions of CH3C0CH202 with H02 and other R02 may also become important. 

The free-radical-induced reaction of alkanes with sulfuryl chloride characteristically results in the chlorination of hydrocarbons. However, when pyridine is added to the irradiated reactants, sulfochlorination occurs in quite satisfactory yield. For example, the irradiated reaction of cyclohexane and sulfuryl chloride in the presence of pyridine resulted in a 54.8% yield of cyclohexanesulfonyl chloride and only 9.4% of chlorocyclohexane.161... 

An unusual radical halogenation of hydrocarbons by phase-transfer catalysis may be performed by reacting alkanes with tetrahalomethanes. The reaction is initiated by single-electron oxidation of OH- by CHlg4. The tetrahalomethane radical anion formed decomposes to the 12 trihalomethyl radical [Eq. (10.56)], which is then involved in C—H activation and propagation steps 286... 

Thermal reactions of light alkanes with oxygen in the combustion process have been studied extensively (6, 7). These studies were typically conducted at high temperatures—flame temperatures. The elementary reactions of the hydrocarbon species often involve reactions with atomic (H, O) or free radical species (OH, alkyl, etc.). The initiation step is the homolytic cleavage of C—C single bonds to form alkyl radicals. The C—C bonds are the weakest bonds in an alkane molecule (Table I). The chain-propagation step... 

A free radical chain reaction proceeds through a succession of free radicals. In the photochemical chlorination of an alkane, the initiating step is the homolytic lission of chlorine molecules to produce chloroalkanc molecules and chlorine free radicals. These two reactions constitute the propagating step. However, the chlorine free radicals may also combine to form chlorine molecules or react with the alkane free radicals to form chloroalkane molecules. Both of these reactions constitute terminating steps of the chain reaction. Il should be noted, however, that the foregoing sequence cannot take place in the dark. Exposure to light allows the series of reactions then to proceed rather violently. 

The mechanism of oxidation of alkanes with dimethyldioxirane has been examined by measurement of the primary kinetic isotope effect for the oxidation of cyclohexane and methylcyclohexane in solution and in the gas phase. These experiments indicated that the major products (cyclohexanol and methylcyclohexanol) are probably formed via an electrophilic oxygen-insertion reaction while minor by-products may arise from radical reactions.90... 

Ingold KU, LusztykJ, Raner KD (1990) The unusual and the unexpected in an old reaction. The pho-tochloronation of alkanes with molecular chlorine in solution. Acc Chem Res 23 219-225 Jeevarajan AS, Carmichael I, Fessenden RW (1990) ESR Measurement of the piC of caboxyl radical and ab calculation of the carbon-13 hyperfine constant. J Phys Chem 94 1372-1376 Kemsley KG, Moore JS, Phillips GO, Sosnowski A (1974) Reaction of radical probes with substituted phenols as models for the investigation of tyrosine in aldolase and chemically modified aldolase. Acta Vitaminol Enzymol 28 263-267... 

These unusual selectivities, which are analogous to those observed in the absence of strong acid activators (see the foregoing), is not easily explained by a mechanism simply involving hydrogen abstraction by a free radical. It was concluded298 that these reactions involve reversible formation of alkyl radicals by direct reaction of the alkane with Co(III) ... 

Traditionally, this radical addition reaction has been used for the preparation of new poly halo organic compounds by reaction of simple polyhalo alkanes with alkenes689 698. The products from these reactions have often been used as key intermediates for the synthesis of heterocyclic compounds690. In some cases it has been found that metal complexes significantly speed up reactions and/or increase yields. 

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