Bromine free energy change

Table 12-2 shows the contribution made by various substituents to the free-energy change from the axial to the equatorial orientations of the substituent. Thus, for bromine, the free-energy change, AG°, is —0.5 kcal mole, which means that at 25°, the equilibrium constant, K, for the axial <= equatorial equilibrium is about 2.3 (from —2.303 RT log K = AG° see Section 4-4A). 

Either by using this value in the equation above, or using the graph above, we see that the Free energy change is zero if the liquid is saturated with bromine. 

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. 

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