Carbon free bond enthalpie

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

An interesting example of the influence of the number and positions of substituents on the thermodynamic polymerizability was provided by studies of cationic polymerization of 3,4-dialkoxy-THF. When two substituents were in cis-position, polymers with DP up to 35 could be obtained while trans-monomer essentially did not polymerize. As shown in Scheme 34, if substituents in cyclic monomer are in ds-position, there is a considerable additional strain due to their steric repulsion this strain is partly released when a rigid cyclic monomer is converted to a polymer chain in which, due to free rotation around carbon-carbon and carbon-oxygen bonds, the strain can be minimized. In trans-monomers, steric repulsion is considerably lower thus the gain in enthalpy is less significant. 

The mechanistic basis of free-radical reactions was considered in Chapter 12 of Part A. Several mechanistic points are crucial in development of free-radical reactions for synthetic applications. Successful free-radical reactions are usually chain processes. The lifetimes of the intermediate radicals are very short. To meet the requirement of synthesis for high selectivity and efficiency, all steps in a desired process must be fast in comparison with competing reactions. Because of the requirement that all steps be quite fast, only steps that are exothermic or very slightly endothermic can participate in chain processes. Comparison of two sets of radical processes can illustrate this point. Let us compare the enthalpy of addition of a radical to a carbon-carbon double bond with addition to a carbonyl group ... 

In Fig. 2a the free enthalpies of activation AG (300 °C) of the thermolysis reactions of symmetrical hexaalkylethanes 11 (Cq—Cq series) — the weakest bond connects two quaternary carbons — are plotted against their ground state strain Hs as obtained from MM2 calculations 14 32 37>. The large range of stability differences encompassed by this series is easily judged from the scale on the right side of Fig. 2 in which is given for each compound the temperature at which the half-life is 1 h. 

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