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188 ChemActivity 20 Radical Halogenation of Aanes Model 8: Selectivity of the Photo-Halogenation Reaction • Radical halogenation with F, is unselective, violent and dangerous limiting the usefulness of this reaction in organic synthesis. • Radical halogenation with le is so slow as to be useless in organic synthesis • Radical halogenation with Br, is right It is very useful in organic synthesis. The rate is manageable and reactions with Br, are very selective. (A selective reaction gives close to 100% of a single product. An unselective reaction gives a mixture of products.) Table 8: Relative Preference of Halogens for 19,2° 3° Ally & Benzyl Sites Halogen (X) X sub, for X sub for sub for a X sub, fora X sub, for a H in a l' H in a 2" Hina 3 H in allyl in benzy Position Position Position Position Position Fluorine (F) 1 1 .2 1.4 1.5 1.6 Chlorine (CI) 1 4 Bromine (Br) 1 100 2000 10,000 100.000 For Example: Сн Rxn VI HC CH₂ H₃C CH₂ H₃C cs CH La CH₂ bv. H₃ cha Rxn v HC CH₂ H₃C c L3 Products v Br CH Br Rxn VIII HC CH CH BE: hv –BI H₂ C CH3 H₃C 2000 de Critical Thinking Questions 14. Fluorination and Chlorination are less selective than Bromination. Do the product ratios for Rxn VI-VII support this statement? Explain. 15. Explain how the ratio 1.4 to 9 was arrived at in Rxn VI in Model 8. 16. Is your explanation consistent with the 2000:9 ratio in Rxn VIIT?

Answer 1

Solution:

14)  From the given reactions, VI to VIII, it can be seen that the product ratio in bromination is 2000 : 9, while for chlorination and fluorination are 5 : 9 and 1.4 : 9 respectively. Thus, the higher ratio in bromination suggest its selectivity towards alkanes.

15) Fluorination is less selective than both bromination and chlorination, because the transition state is highly exothermic in nature. Thus, according to Hammond's postulate, in an exothermic reaction, the transition state is closer to the reactants than to the products in energy. Thus, reaction is highly less selective.

16) The explanation for selective bromination is as follows,

In bromination, the abstraction of a hydrogen atom by Br⋅ is endothermic while abstraction of a hydrogen atom by Cl⋅ and F- are exothermic.Thus, transition states for the endothermic bromination have a larger energy difference than those for exothermic chlorination and fluorination. Therefore, according to Hammond's postulate, in an exothermic reaction, the transition state is closer to the reactants than to the products in energy. On the other hand endothermic reactions have product like transition state.