C-X bonds strengths

The rate constants for (3-elimination processes are correlated with the C-X bond strength (91,140). Thus the rate determining step in these processes involves the breaking of the C-X bond. (3-Elimination... 

According to the spectroscopic and other qualitative proposals, the first radical anion formed in the case of aryl halides is of % nature. This radical anion gives, by an intramolecular ET to the C—X bond, a a radical anion, which dissociates into the aryl radical and the anion of the leaving group. The C—X bond strength is decreased once the radical anion is formed, as has been shown by thermodynamic cycles40. 

Recent thermochemical determinations suggest that A = 14.21 kcal/mol and B 17.21 kcal/mol (llg). To obtain, for instance, benzylic C-X bond strengths, the stabilization energy calculated from equation 9 is subtracted from C-X bond strengths for corresponding saturated species. Recent studies of alkyl-benzene pyrolysis (117) imply that within current uncertainties in relative bond strengths this formula applies to secondary and... 

The rates of dehalogenation strongly depend on the nature of the halogen, increasing with decreasing C-X bond strength (I > Br > Cl). 

The radical chain dehalogenation with Bu3Sn-H is a very useful reaction. The ease of dehalogenation follows the C-X bond strength I > Br > Cl > F the weakest C-X bond is preferred. The initiator for this reaction is commonly AIBN. An example mechanism involving this reaction is shown in Section 11.7, Approaches to Radical Mechanisms. 

The increasing C-X bond strength (apart from resonance) is attributed to the electrostatic attraction associated with the difference in electronegativity. This is accentuated by the already existing C=0 charge separation and leads to a favorable juxtaposition of positive and negative charge. ... 

The role of the halogen atom in the precursors was also explored. Although bromine was mostly used, iodine and chlorine were also employed. While the former gave very efficient cydisation (but with a far more complex mixture of products), the latter gave no reaction at all, reflecting the order of C-X bond strength, Cl>Br>I. 

Several authors have noted, although sometimes on a purely qualitative basis, that the rates of the reactions of a particular nucleophile with an alkyl or aryl halide vary inversely with the C—X bond strength (i.e. I > Br > Cl), as would be expected for a nucleophilic displacement reaction (Ochiai et al., 1969 Hart-Davis and Graham, 1970, 1971 Schrauzer and Deutsch, 1969 ... 

If one looks at the average bond dissociation energies for X2, C-X, H-X, and C-H bonds (Table 2.2), an average heat of reaction for the halogenation of alkanes can be calculated. The results in kcal/mol are as follows F = -101, Cl = -22, Br = -4, and I = 16. The variation in these numbers comes from a continual decrease in H-X and C-X bond strengths in the series F, Cl, Br, and I. These heats of reaction reflect a dramatic change in reactivity. Free radical fluorination is so exothermic that it occurs spontaneously and very explosively. Chlorination and bromination can be controlled and are useful reactions. Free radical iodination rarely occurs. 

The reactivity order for metals approximates the electropositive character of the metal with the more active metals displacing the less active metals— that is trans-metallation occurs. For alkyl and aryl halides, the reaction order is RI—>RBr—> RCl —> RF, and alkyl —> aryl. These trends reflect the decrease in C-X bond strengths. 

In Summary The halogen orbitals become increasingly diffuse along the series F, Cl, Br, 1. Hence, (1) the C-X bond strength decreases (2) the C-X bond becomes longer (3) for the same R, the boiling points increase (4) the polarizability of X becomes greater and (5) London interactions increase. We shall see next that these interrelated effects also play an important role in the reactions of haloalkanes. 

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