Oxygen ions with alkanes

Alkanes—Continued reactions—Continued with ozonide ions, 135 with superoxide ions, 134-35 role of oxygen ions in oxidation. 138-41 Alkenes, reactions with oxygen ions, 134 with ozonide ions, 135 with superoxide ions, 134-35 Aluminosilicate gels, alkali cations, 241... 

The paramagnetic oxygen ions 0 , 01> and 0J have been formed on magnesium oxide and studied by EPR spectroscopy. The reactivity of these ions with hydrocarbons follows the sequence 0 >>03> >02. Both with alkanes and alkenes the initial reaction is thought to be hydrogen atom abstraction. 

The proposed surface intermediates are summarized in Table II. The intermediates in each reaction of alkanes includes alkox-ide ions, regardless of the type of active oxygen species. Although carboxylate ions are believed to be the intermediate in the reactions of C2 and C3 alkenes, the type of carboxylate formed with 0 as a reactant is different from the type of carboxylate ion formed when 07 or 07 was a reactant. With 0 ions the carbon number of the carboxylate ions is the same as that of the hydrocarbon reactant, but with 07 or 07 the carboxylate ions have carbon numbers smaller than the parent hydrocarbon. The reaction schemes of Ci alkenes are somewhat complicated, yet it appears that they react in a manner more similar to C2-C1 alkanes than to C2 or C3 alkenes. 

Efficient oxidation of alkanes with molecular oxygen can be attained using N-hydroxyphthalimide combined with Co(acac) (n = 2,3).1121 According to a new concept, photocatalyzed selective oxidation of small hydrocarbons at room temperature is carried out over alkali or alkaline-earth ion-exchanged zeolites.1122... 

A carbonyl group dramatically increases the acidity of the protons on the a carbon atom because deprotonation gives a resonance-stabilized enolate ion. Most of the enolate ion s negative charge resides on the electronegative oxygen atom. The pKa for removal of an a proton from a typical ketone or aldehyde is about 20, showing that a typical ketone or aldehyde is much more acidic than an alkane or an alkene (pKa > 40), or even an alkyne (pKa = 25). Still, a ketone or aldehyde is less acidic than water (pKa = 15.7) or an alcohol (pA a = 16 to 18). When a simple ketone or aldehyde is treated with hydroxide ion or an alkoxide ion, the equilibrium mixture contains only a small fraction of the deprotonated, enolate form. 

Oxidation of n-butane. In the presence of oxygen, Co(l 1) is converted into Co(lll), the actual catalyst for oxidation of alkanes by oxygen thus oxidation of n-butane by Co(lll) ion at 100° at a pressure of 17-24 atm. gives acetic acid (83.5% yield) together with traces of n-butyric acid, propionic acid, and methyl ethyl ketone. Oxidation of n-pentane under similar conditions gives acetic acid (48% yield) and propionic acid (27% yield). Isobutane is relatively inactive. The reaction involves electron transfer in which cobalt ions function as chain carriers. 

The characteristic transformation of branched alkanes with ozone in superacidic media is C-C bond cleavage. When a stream of oxygen containing 1-5% ozone was passed through a solution of isobutane in FSO3F-SbF5-SO2ClF solution held at -78°C, the H and C NMR spectra of the resultant solution were consistent with the formation of dimethylmethylcarboxonium ion 67 [Eq. (6.52)] in 45% yield. Similar treatment of isopentane, 2,3-dimethylbutanc, and 2,2,3-trimethylbutane resulted in the formation of related carboxonium ions as the major products. 

Since during the Purex process TBP, alkane, and aqueous nitric acid solution are in mixture or contact condition, the radiation chemical transformations depend on the composition, concentration of nitric acid, contaminant metal ions, irradiation conditions, and oxygen concentration (Triphathi and Ramanujam 2003 Katsumura 2004). Under aerated condition, the organic radicals react with oxygen forming peroxy radicals. After successive reactions a variety of alcohols, ketones, peroxides, and carbonyl compounds form. The ratio of nitration products to oxidation products is 0.8, and the ratio increases if there is no sufficient supply of O2. 

Two reviews address the reactions of bare transition metal atoms and ions with hydrocarbons in the gas phase and the reactions of monosubstituted alkanes with bare transition metal ions. 2 A study of the reactivity of ground-state, neutral transition metal atoms from the left hand side of the 4d series (Y through Mo) shows that they are unreactive towards linear alkanes but that they will react with cyclopropane and alkenes.3 Atomic metal cations form a 1 1 adduct with tribenzocyclotriyne in a Fourier-transform ion cyclotron resonance spectrometer. Reaction of molecular oxygen with M(C2H4) results in ligand exchange to M02 for the early first row transition metals. Activation of the O—O bond and product formation is observed for ccnnplexes of Sc+,Ti+andV+.5... 

The reactions of halide and azide ions (see above), cyanide ion (see Chap. 7) and thiocyanate ions (see Sect. 13.7) have all been discussed. In the general context of halides and pseudohalides, it should be noted that nitrite ion has been successfully phase-transferred [31, 32]. The yields for the formation of nitroalkanes in either crown [31 ] or quaternary ion catalyzed [32] processes are fair to good for primary alkanes and poor for secondary systems. Bromocyclohexane, for example, afforded only traces of nitrocyclohexane when treated with nitrite ion, the by-products resulting presumably from oxygen attack (nitrite ester formation) or elimination. The conversion of n-octy bromide into the corresponding nitro-compound is formulated in equation 9.14 and several examples of the transformation are recorded in Table 9.6. 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

The Mo+ ion was generally less reactive to alkenes than the other ions. Additions of the alkene to the Mo+ and [MoO]+ with loss of H2 or 2H2 were the major reaction products similar to the alkanes. The reactions of [Mo02]+ with alkenes gave a variety of products, the major reactions being reduction by loss of an oxygen atom. 

The 02 ion on MgO does not react with CO or alkanes at 77 K but the EPR signal disappears slowly at room temperature (361). Similarly, on ZnO (390) it reacts only slowly with propylene at room temperature and not with CO, H2, or ethylene. A slow reaction with propylene is also observed for 02 on V2Os/MgO at room temperature (391). Yoshida et al. (392) have studied the reactivity of adsorbed oxygen with olefins on the V20j/Si02 system. Adsorption of propylene destroyed the signal from 02 slowly at room temperature and the reaction products, aldehydes with some acrolein, were desorbed as the temperature was raised to 150°C. More quantitative... 

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