Stability, of alkyls

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

In discussing the elFect of structure on the stabilization of alkyl cations on the basis of the carbonylation-decarbonylation equilibrium constants, it is assumed that—to a first approximation—the stabilization of the alkyloxocarbonium ions does not depend on the structure of the alkyl group. The stabilization of the positive charge in the alkyloxocarbonium ion is mainly due to the resonance RC = 0 <-> RC = 0+, and the elFect of R on this stabilization is only of minor importance. It has been shown by Brouwer (1968a) that even in the case of (tertiary) alkylcarbonium ions, which would be much more sensitive to variation of R attached to the electron-deficient centre, the stabilization is practically independent of the structure of the alkyl groups. Another argument is found in the fact that the equilibrium concentrations of isomeric alkyloxocarbonium ions differ by at most a factor of 2-3 from each other (Section III). Therefore, the value of K provides a quantitative measure of the stabilization of an alkyl cation. In the case of R = t-adamantyl this equilibrium constant is 30 times larger than when R = t-butyl or t-pentyl, which means that the non-planar t-adamantyl ion is RT In 30= 2-1 kcal... 

The order of stability of alkyl radicals is the same as for carbocations. 

The conformational anomeric effects design the contrasteric effects observed in acetals which render the more sterically encumbered gauche/ gauche conformers more stable than their anti/gauche and anti/anti conformers. Such effects were first evidenced by Jungins in 1905 and rediscovered by Edward in 1955 and by Lemieux and Chiu in 1958. They observed the higher stability of alkyl a-D-glucopyranosides in comparison with their (3-anomers (Fig. 5).8... 

The energies are usually expressed as electron volts. The IRE for the bond in ethane is zero and for CHgNa it is 2.56 ev. The stability of alkyl carbon-metal bonds for a variety of metals has been evaluated by Jaffe and Doak (5). They point out that not only is the (the measure of covalent energy) for the C—M bonds of transition metals appreciably smaller (perhaps one-half) than the corresponding values for other elements, but the ionic resonance energy of the alkyl-transition metal bonds is also appreciably smaller (perhaps one-third) than that of alkyl-alkali or alkyl-alkaline earth metal bonds. 

SCHEME 7. Test of the configurational stability of alkyl hthium carbenoids (a) fast equilibrium between enantiomeric lithium carbenoids (b) configurational stable hthium carbenoid 13... 

SCHEME 10. Decreasing stability of alkyl- and alkoxyalkyUithium compounds... 

It seems beyond debate that when an exchange reaction of a hydrocarbon (HC) with D2 is observed and the initial product distributions are binomial (random distribution of D atoms), single c-metal-carbon bonds are being formed. Nevertheless, this conclusion was puzzling in the period when virtually no homogeneous alkyl-metal complexes were known and the stability of alkyl-metal complexes was doubted for principal reasons (see, e.g., 169). However, it appeared that these complexes can be rather stable when one blocks a very fast and easy elimination of one of the H atoms in the jS-position, which step decomposes the alkyl-metal bond into an olefin and a bound hydrogen atom (170,171). On the other hand, this means that the transition... 

The same reaction scheme can be written for (Z) -2-phenyl-2-butene, except that paths B and E would lead to erythro and threo aldehydes. In cobalt catalysis this isomerization could explain both the lack of stereospecificity and the lack of influence of the sterochemistry of the starting olefin on the distribution of aldehydes 26 and 27. This hypothesis agrees well with results with a-ethylstyrene. On the other hand, when rhodium is used, extensive isomerization occurs less readily probably because of a better stability of alkyl- and acylrhodium carbonyls, and one can thus achieve a high degree of stereospecificity. 

Having introduced the correlation of Fig. 1, we may return to the stabilities of alkyl cations. Rate constants for the hydration of secondary and primary alkenes have been measured in concentrated solutions of aqueous sulfuric acid by Lucchini and Modena97 and by Tidwell and Kresge42 using proton nuclear magnetic resonance (NMR) or UV to monitor progress of the reactions. It is conceivable that the reactions involve a concerted addition of a proton and water molecule to the alkenyl double bonds. However, the very weak basicity of water under the conditions of reaction makes this unlikely, and the steep acidity dependences of the reactions (e.g., m =-1.65) is... 

X-ray diffractometry is the most powerful method to determine atomic coordinates of molecules in the solid state. X-ray crystal structure analysis was, however, rarely applied in the early years of development of persistent, long-lived alkyl carbocations and studies were only performed to investigate structures of carbocations of aryl derivatives and aromatic systems.65 This is due to the low thermal stability of alkyl carbocations and to the difficulties in obtaining single crystals of carbocations suitable for analysis. Since then, however, methods and instrumentation have improved significantly and X-ray crystal structure analysis has become a powerful tool to solve structural problems of carbocations.65,66... 

The influence of fluorine substituents on the stability of alkyl radicals derives from the same complex interplay of inductive and resonance effects that affects their structure. Simple orbital interaction theory predicts that substituents of the -X type (that is, electronegative substituents bearing lone pairs) should destabilize inductively by virtue of their group electronegativities, and stabilize by resonance to the extent of their ability to delocalize the odd electron. 

Measures have been taken to improve the thermal stability of the organoclays. Extraction of excess modifier can improve the thermal stability of alkyl ammoniums, sometimes with improvements up... 

The early observations consisted of a review of enthalpies of formation of hydrocarbons and their stabilities from F. D. Rossini, Chem. Rev., 27, 1 (1940). The exception we know of is the relative stabilities of alkyl lithiums where methyl lithium is stabilized relative to the -alkyl family (and exhibits a methyl deviation opposite to the methyl halides) and the secondary and tertiary lithium alkyls are destabilized. See J. F. Liebman, J. A. Martinho Simoes and S. W. Slayden, in Lithium Chemistry A Theoretical and Experimental Overview (Eds. P. v. R. Schleyer and A.- M. Sapse), Wiley, New York, in press. 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

Even in the transition state, the angles are increasing towards 120° and all interactions with the leaving group are diminishing as it moves away. There is steric acceleration in the S l reaction rather than steric hindrance. This, as well as the stability of /-alkyl cations, is why f-alkyl compounds react by the SnI mechanism. 

Alkene insertions into M—CR3 bonds proceed via a similar transition state, with the CR3 moiety tilting towards the terminal alkene-C, with retention of configuration of the CR3 group. Note that in either case in the M(R)(alkene) complex two coordination sites are involved, whereas in the product only one is occupied. For this reason the stability of alkyls depends on blocking this second coordination site to prevent the reverse reaction. 

Kolhnar, H. The stability of alkyl anions. A molecular orbital theoretical study, J. Am. Chem. Soc. 1978,100,2665-2669. 

The stabilities of the metal-carbon bond formed from oxidative additions are as varied as their mechanistic pathways. Metal-carbon bond strengths increase going down a triad in an isostructural series of complexes. Alkyl migration to CO ligands on the metal to form acyl derivatives is more facile in first-row transition metals because of their lower metal-carbon bond energies. The thermal stability of alkyls vs. acyls does not follow any pattern, except that the availability of a sixth coordination site in ML (acyl) complexes favors the alkyl carbonyl isomer. The corresponding acyl, which can be made by running the reaction of the alkyl or aryl halide in CO (at 1-3 atm), is more stable by... 

Figure 5.9. Relative stabilities of alkyl cations. (Plots aligned with each other for easy comparison.)...

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