Carbon-hydrogen bonds secondary

Fluorine is well known for its ability to fluonnate seleeti vely terttary carbon-h drogen bonds that are slightly activated by polar substituents Hypofluorite reagents also react with carbon-hydrogen bonds, both secondary and tertiary but tertiary bonds react better [6]... 

Only limited success was achieved in determining the relative reactivity of primary, secondary, and tertiary carbon-hydrogen bonds to sulphonyl nitrenes 8>. Insertion of p-toluenesulphonyl nitrene into 2-methylbutane gave a mixture of products which could not be completely resolved. The ratio of (primary) (secondary + tertiary) = [38 + 39 40 + 41] was 1.53, compared to a ratio of 5.6 for carbethoxynitrene58>, indicating the lowered selectivity of the sulphonyl nitrene relative to the carbethoxynitrene, as might be expected from the possible resonance stabilization of the latter species. 

Rate measurements on 2-methyl-5-deuteriothiophene have shown the existence of a small kinetic isotope effect ( H/fcD = 1.18 + 0.02). There appears to be no way of deciding if such a small effect is a primary (i.e., indicating a partial weakening of the carbon-hydrogen bond in the transition state) or a secondary one. 

Photolysis of diazirine in the presence of a large excess of propane yielded n- and isobutane and in the presence of n-butane yielded n- and isopentane. From the relative rates of attack on the primaiy and secondary carbon-hydrogen bonds in these compounds, it was concluded that methylene derived from diazirine showed approximately the same discrimination as methylene formed by the photolysis of ketene. The results obtained, using methylene derived from the photolysis of diazomethane, gave a product ratio closer to the simple statistical ratio of the number of carbon-hydrogen bonds without correction factors for the type involved and indicated almost no differentiation between the types. 

Advantage can be taken of the enhanced polarity of a carbon-silicon bond over that of a carbon-hydrogen bond in the displacement of 2-tri-methylsilyl groups by reaction with a number of carbonyl reagents to give imidazoles substituted at C-2 by secondary alcohol, acyl, aroyl, ester, and amide functions. ... 

The carbon is electrically neutral, but it does have a greater electronegativity than hydrogen, and so it attracts slightly the electrons within the carbon/ hydrogen bonds. Now suggest the relative order of stability of the primary, secondary and tertiary carbon radicals. tertiary>secondary>primary... 

Ramalingam and coworkers have performed detailed computational studies at various levels of theory with respect to the insertion of HCIC and CbC singlet carbenes into the carbon-hydrogen bond of small alkanes and identified two insertion modes (Fig. 6.8)." Of these, the a approach has been found to be preferred over the n mode at all levels of theory because the k attack leads to the eclipsed conformation (compare transition state 143 to 144, which is a secondary saddle point). In the initial phase of the insertion process there is a net charge flow from the alkane to the carbine. Barriers for the insertion... 

The reactions of F-atoms with the secondary C—H bonds in cyclohydrocarbons (c-CaHg, c-CjHio, c-CsHij) [572, 576] produce HF with a somewhat lower fraction of the available energy in HF vibration ((Fy) 45—55%), but the vibrational distributions are still inverted with a peak in v = 2. A possible explanation of the difference between the energy disposal involving abstraction from secondary compared with primary carbon—hydrogen bonds may be that the radical stabilisation energy for the cyclane products is not available for excitation of the HF. 

In the oxidation of tertiary carbon—hydrogen bonds, where termination cannot involve this mechanism, the rate of light emission cannot be directly proportional to the rate of termination [34,56], However, it is known that in oxidation of such hydrocarbons, termination can involve at least some primary or secondary peroxy radicals which are formed indirectly from fragmentation of the alkoxy radical corresponding to the parent hydrocarbon (see Sect. 3.4). 

Kossiakov and Bice 2) have shown that the energy of activation for the fission of a carbon-hydrogen bond is least at a tertiary carbon atom, the energy for this case being 2 kcaJ. less than for a secondary carbon atom and 4 kcal. less than for a primary carbon atom. It would therefore seem that, for work of this type, hydrocarbons containing one or more tertiary carbon atoms would be most suitable for study. Davies and Elton (S) showed that optically active 2-phenylbutane imdergoes racemization when adsorbed on charcoal, homolysis occurring at the tertiary carbon atom. 

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