Halogen reactivity order

It should be pointed out that the existence of stable structures of the intermediate-complex type (also known as a-complexes or Wheland complexes) is not of itself evidence for their being obligate intermediates in aromatic nucleophilic substitution. The lack of an element effect is suggested, but not established as in benzene derivatives (see Sections I,D,2 and II, D). The activated order of halogen reactivity F > Cl Br I has been observed in quantita-tivei36a,i37 Tables II, VII-XIII) and in many qualitative studies (see Section II, D). The reverse sequence applies to some less-activated compounds such as 3-halopyridines, but not in general.Bimolecular kinetics has been established by Chapman and others (Sections III, A and IV, A) for various reactions. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

The reaction of an a-halo sulfone with a base to give an alkene is called the Ramberg-Bdcklund reaction. The reaction is quite general for a-halo sulfones with an (x hydrogen, despite the unreactive nature of a-halo sulfones in normal 8 2 reactions (p. 437). Halogen reactivity is in the order I>Br>Cl. Phase-transfer catalysis has been used. In general, mixtures of cis and trans isomers are obtained, but usually the less stable cis isomer predominates. The mechanism involves formation of an episulfone, and then elimination of SO9. There is much evidence for... 

As usual, the reactivity order with respect to halogen in C=C-X is I > Br > Q. The organic halide in the couplings with alkynylzinc halides is usnaily an iodide, though also activated vinylic bromides (e.g. BrCH=CHCOOC2H5) have been found to react smoothly. We... 

In each case D for CH3—H is 105 kcal/mol (438 kJ/mol), while D values for the other bonds involved are given in Tablel4.4.109 F2 is so reactive110 that neither uv light nor any other initiation is needed (total AH = -101 kcal/mol -425 kJ/mol) 111 while Bn and I2 essentially do not react with methane. The second step is exothermic in all four cases, but it cannot take place before the first, and it is this step that is very unfavorable for Br2 and 12. It is apparent that the most important single factor causing the order of halogen reactivity... 

The addition of HX to alkenes proceeds according to the Markownikoff rule, i.e. the halogen is attached to the more substituted carbon atom [321—323], The reactivity order of butenes was found to be dependent on the nature of the catalyst. Over MgS04, the order was isobutene > trans-2-butene > 1-butene > cis-2-butene but with CaCl2, the reactivity decreased in the order isobutene > m-2-butene > 1-butene > trans-2-butene [321], Propene is more reactive than ethylene [318], Earlier reports that tert-butylchloride is formed from 1-butene and hydrogen... 

The reactivity order also appears to correlate with the C-X bond energy, inasmuch as the tertiary alkyl halides both are more reactive and have weaker carbon-halogen bonds than either primary or secondary halides (see Table 4-6). In fact, elimination of HX from haloalkenes or haloarenes with relatively strong C-X bonds, such as chloroethene or chlorobenzene, is much less facile than for haloalkanes. Nonetheless, elimination does occur under the right conditions and constitutes one of the most useful general methods for the synthesis of alkynes. For example,... 

The reactivity of an alkali metal increases as its ionization energy decreases, giving a reactivity order Cs > Rb > K > Na > Li. Cesium is the most reactive, combining almost explosively with the halogens. 

Unlike the metallic elements, halogens become less reactive going down the periodic table because of their generally decreasing electron affinity. Thus, their reactivity order is F2 > CI2 > Br2 > I2. Fluorine often reacts violently, chlorine and bromine somewhat less so, and iodine often sluggishly. 

Under neutral conditions, the positional reactivity order for the halogenation of quinoline appears to be 3 > 6 > 8, whereas isoquinoline gives mainly 4-substitution. For isoquinoline, the fact that reaction occurs on the free base is adequate explanation for the change in orientation, since, contrary to common belief, the 4-position is shown by calculations and gas-phase studies of reactivity to be the most reactive in the neutral isoquinolines (see Section U.G.) indeed, more reactive than benzene. 

Halogen compounds are readily reduced, converting RX into alkanes in good yields. The reactivity order is I > Br > Cl. The process is very solvent-dependent in THF only primary halides are reduced, but in HMPA primary, secondary, tertiary, and aryl halides are all reduced. 

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