Endothermic reaction bromination

According to the Hammond postulate, the transition state for abstraction by chlorine resembles the reactant because this is an exothermic reaction. In contrast, the transition state for abstraction by bromine resembles the product because it is an endothermic reaction (see Figure 21.2). In the case of abstraction by chlorine the carbon-hydrogen bond is only slightly broken in the transition state, and the stability... 

According to the Hammond postulate, the transition state of an endothermic reaction resembles the produets, so the energy of aetivation to form the more stable 2° radical is lower and it is formed faster, as shown in the energy diagram in Figure 15.5. Because the 2° radical [(CH3)2CH-] is converted to 2-bromopropane [(CH3)2CHBr] in the seeond propagation step, this 2° alkyl halide is the major product of bromination. 

Conversely, the structure of the transition state of an endothermic reaction step occurs later and looks more like the products of that step than like the reactants, and changes in products have a large effect on the rate. Hammond s postulate accounts for the fact that bromination of an alkane is more regioselective than chlorination. 

Activation energies of endothermic reactions that involve both bond formation and bond rupture will be greater than the heat of reaction, AH°. Two examples illustrate this principle, namely, the first chain-propagating step in the chlorination of methane and the corresponding step in the bromination of methane ... 

Comparisons can be made for less extreme cases as well. Comparing two endothermal reactions, we can predict that the more endothermal one will be the more selective. For example, hydrogen atom abstraction from propane by bromine atoms is more endothermal and more selective than by chlorine atoms. 

Chlorination of alkanes is less exothermic than fluonnation and bromination less exothermic than chlorination Iodine is unique among the halogens m that its reaction with alkanes is endothermic and alkyl iodides are never prepared by lodmation of alkanes... 

Both reactions are endothermic, but the interaction of bromine atoms with HBr is much more so than the interaction with molecular hydrogen. Consequently, the former reaction will occur much less frequently than the latter. 

Rice, Fryling, and Weselowski (J. Amer. Chem. Soc., 1924, 46, 2405) make all reaction rates proportional to the concentration of what they call residual molecules, which have to be formed endothermically from one of the reactants. The proportion of these increases with temperature and accounts for the increase in reaction rate. Something of this kind may be true in special cases, for example, in the formation of HBr the residual molecule would be the bromine atom. But this resolution into atoms is only the limiting case of ordinary activation, and it is difficult indeed to see what the residual molecule could be, or what tautomeric change could occur in the simple decomposition of hydrogen iodide or nitrous oxide. 

The transition states forming the 1° and 2° radicals for the endothermic bromination have a larger energy difference than those for the exothermic chlorination, even though the energy difference of the products is the same (13 kJ, or 3 kcal) in both reactions. 

The systems involving bromine atoms also show the fast drop-off as AH deviates from 0, especially marked in the toluene to cumene series, which implies a small value of the intrinsic barrier. There is a further problem that there is a substantial equilibrium isotope effect of about a factor of 26b accompanied in the aliphatic cases by considerable opportunity for reversibility, thus A H/fcD values in the endothermic attack of Br- on aliphatic RH are unlikely to be less than 2. Correspondingly, isotope effects in the reverse directions can be inverse. This has not been observed, but the trend shown in the last several reactions, in which isotope effects as low as 1.0 are observed suggests this possibility. We attach no meaning to (fcH/ D)max for abstraction from HBr because of the failure of the one-dimensional model. Both chlorine and bromine then fit the scheme of highly variable isotope effects associated with low intrinsic barriers. 

A bromine radical can abstract either a 1° or a 2° hydrogen from propane, generating either a 1 ° radical or a 2° radical. Calculating AH° using bond dissociation energies reveals that both reactions are endothermic, but it takes less energy to form the more stable 2° radical. 

The first propagation step of the bromination radical chain is significantly endothermic. A small change in product radical stability is reflected in the barrier for reaction and consequently in the rate of reaction. Thus the first propagation step (and the chain reaction) that goes the fastest forms the most stable product radical. Bromination will select for allylic and benzylic > tertiary > secondary > primary > vinyl and phenyl. 

The reaction for fluorine (Rl) is highly exothermic, while the reactions for chlorine (R2), bromine (R3), and iodine (R4) are endothermic. The heats of these reactions are 30.8, — 1.2, — 16.7, and — 32.7 kcal/mol for reactions (Rl), (R2), (R3), and (R4), respectively. According to Hammond s postulate, reaction (Rl) should have an early TS, and reactions (R2) and (R3) should have late TSs. What the electronic states are during these reactions, and how the CASVB method describes the electronic structure, are our interests in this section. 

Figure 9.6 The reaction between sodium and bromine. A, Despite the endothermic electron-transfer process, all the Group 1A(1) metals react exothermically with any of the Group 7A(17) non-metals to form solid alkali-metal halides. The reactants in the example shown are sodium (in beaker under mineral oil) and bromine. B, The reaction is usually rapid and vigorous.

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