Reactivities of hydrocarbon ions

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. 

All the absolute values of k reported in the literature are collected in Table 1. In all cases the figures should refer to the reaction of the ion-pair with monomer. The results are too fragmentary and in some cases of uncertain accuracy for a detailed discussion of the effect of environment on reactivity. A few points are clear. The reactivity of the relatively unsolvated ion-pair in hydrocarbon solvents is relatively large and may even be comparable with that of the solvated ion-pair in tetrahydrofuran despite the large difference in dielectric constant. The reactivity of the ether-solvated ion-pair in solvents of lower dielectric constant is lower than either. The first effect of etherate formation is to decrease the reactivity of the ion-pair which can be increased again by an increase of the dielectric constant of the solvent. 

This structural variation notwithstanding, only a few cationic transition-metal ions react efficiently with molecular oxygen under gas-phase conditions (see below). In contrast, many anionic metal complexes and clusters are readily oxidized by O2 to afford various metal-oxide anions [19]. From a conceptual point of view, however, anionic species appear to be inadequate reagents for the activation of hydrocarbons, because they generally require electrophilic attack. At present, only a few oxidations by transition-metal oxide anions have been reported to occur in the gas phase, and they are mostly limited to relatively polar substrates, such as the CH3OH CH2O conversion [20]. Because of the lower reactivity of hydrocarbons, their C-H bond activation by metal-oxide anions is likely to be limited to radical pathways driven purely thermodynamically, i.e., when Z)(0-H) exceeds Z)(C-H) of the substrate [21]. As radical-type pathways are prone to create selectivity problems, and over-oxidation is particularly difficult to control, the anionic route appears less attractive as far as partial oxidation of alkanes is concerned. 

A series of experiments on the reactivity of metal ions with nitriles, RCN, led to the discovery of the remote functionalization mechanism. The initial interaction of the metal ion involves coordination at the nitrile group. The insertion of the metal atom into a C—or C—C bond occurs only after the alkyl chain becomes long enough (at least three or four methylene groups) to interact with a remote bond. The dissociation of the metal-hydride or metal-alkyl intermediate results in a loss of H2 alkene or alkane molecules, depending on the structure of the hydrocarbon group R. 

In this chapter, we will first describe and evaluate the various experimental methods which have been used to obtain information about the structures of hydrocarbon ions. We will then review the information currently available on the structures of these ions formed in various systems. Finally, we will examine the rates of some of the reactions of various isomeric ions for information about the effect of structure on ionic reactivity. 

TMED, (CH3)2NCH2CH2N(CH3)2. B.p. 122 C a hygroscopic base which forms a hydrocarbon-soluble stable chelate with lithium ions and promotes enhanced reactivity of compounds of lithium, e.g. LiAlH4, UC4H9, due to enhanced kinetic basicity of the chelate. Used in polymerization catalysts, tetramethyl lead, TML 5 lead tetramethyl. 

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

Protonation of formic acid similarly leads, after the formation at low temperature of the parent carboxonium ion, to the formyl cation. The persistent formyl cation was observed by high-pressure NMR only recently (Horvath and Gladysz). An equilibrium with diprotonated carbon monoxide causing rapid exchange can be involved, which also explains the observed high reactivity of carbon monoxide in supera-cidic media. Not only aromatic but also saturated hydrocarbons (such as isoalkanes and adamantanes) can be readily formylated. 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

Unsaturated fluorocarbons are much more reactive toward nucleophiles than then hydrocarbon counterparts owing to fluorme s ability to both stabihze carban ions and mductively increase the electrophihcity of multiple bonds and aromatic nngs Nucleophihc attack dominates the chemistry of unsaturated fluorocarbons, and the role of fluonde ion in fluorocarbon chemistry is analogous to that of the proton in hydrocarbon chemistry [129] Like the related electrophilic reactions for hydrocarbons, there are fluonde-promoted isomenzations and dimenzations (equation 9), oligomenzations (equation 10), additions (equation 11), and amomc Fnedel-Crafts alkylations (equation 12) that all proceed via carbamomc intermediates [729 7 7]... 

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