Halogen atom abstraction reactions

If one takes out of this list the polyhalomethanes and tries to understand what the chemistry is about, certainly a simple-minded view is that this is simply a halogen atom abstraction reaction such as one finds in a gas phase reaction, like sodium atom with organic halides. There is a linear free energy correlation in that the logarithms of the rate constants bear a linear relation to one another. 

The issue of the role of bridged radicals in the stereochemistry of halogenation has recently been examined computationally and a new interpretation offered. The structure, rotational barriers, and for halogen atom abstraction for P-haloethyl radicals were studied. For the reactions where X=C1 or Br, the halogen atom abstraction reaction shows a preference for a trans TS. 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

One can set up to do this using the competition between dimerization and halogen atom abstraction from RX to form the rhodium(III) halide complex. As a function of [RX], the product ratio is quite easily evaluated. From that, one can get the rate constant ratio but, knowing independently the rate constant for dimerization, it is possible to extract from those data rate constants for reactions of the rhodium(II) complex with these organic halides. The rate constants obtained are listed in Table I. 

Cp2Fe2(CO)4] promotes photosubstitution of CO by RNC in [CpFe (CO)2I].120 The propagation step in these reactions is halogen atom abstraction from reactant by the 17-electron radicals [CpMo(CO)2PR3] and [CpFe (CO)2(CNR)], respectively. A radical chain pathway has also been proposed for the [Cp2Fe2(CO)4]-promoted photosubstitution in Eq. (27).121 In... 

Radicals can undergo other reactions as well as monomer addition. Atom abstraction reactions usually involve transferof a hydrogen or halogen atom. An example from micromolecular chemistry involves the chlorination of hydrocarbons at about 200°C or during irradiation with light of wavelength less than 4875 x 10 m ... 

Laser flash photolysis of [CpM(CO>3]2 (M = W, Mb, and Cr) provides a convenient source of CpM(CO)3, an organometallic free radical with 17 valence electrons. It is a transient and highly reactive species. Depending on the circumstances and the other reagents present, the radical will dimerize, undergo halogen and hydrogen atom abstraction reactions, and electron transfer reactions. With tetramethyl-phenylenediamine, there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical. 

Halogen atom abstraction. The metal radicals react with alkyl and aralkyl halides (RX, X = Cl, Br, I) to yield CpM(CO)3X and R. The metal halide product is a species known independently, and its formation in these reactions was verified by IR. The organic free radicals were not detected directly, but were inferred from the final organic products determined by GC these products were consistent with those known to be formed when R undergoes dimerization and disproportionation reactions. 

Halogen atom abstraction occurs also with metal halides, both (NH3)5Co -X and X-Rh (dmgH)2PPh3 having been studied. For the latter, 1 1 = x 0 Br mol s for the reactions of... 

In an atom abstraction reaction, a radical X- attacks a Y-Z cr bond to give a new closed-shell species X—Y and a new radical -Z. One of the electrons in the old cr bond goes to form a bond to X, and the other one ends up on Z. The transferred atom, Y, is usually H or halogen, but not always. 

Probably the best-known example of a free-radical reaction is the halogenation of alkanes with Br2 or NBS. This chain reaction is initiated by homolytic cleavage of Br2 induced by light. The propagation part consists of two atom abstraction reactions. 

In this reaction, a metal radical is produced by homolysis of the metal-metal bond (Equation (34)). A typical behavior of metal radicals is halogen atom abstraction from an alkyl halide, such as CGI4 (Equation (35)). The CCI3 radical thus produced can then react with a vinyl monomer to initiate a polymerization reaction. 

These reaction coordinate diagrams for the formation of primary and tertiary alkyl radicals by halogen atom abstraction from 2-methylpropane illustrate a larger difference in the activation energies for the reaction with a bromine atom (b) than with a chlorine atom (a). This difference is consistent with the higher selectivity of bromination. 

Radicals for addition reactions can be generated by halogen-atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism, shown in Fig. 10,3. The rates of each of these steps must exceed those of competing chain termination reactions in order for good yields to be obtained. The most important competitions are between the addition step and reaction of the intermediate R- with Bu3SnH and between the H-abstraction step ki and addition to another molecule of the alkene. If the addition step is not fast enough, the radical R- will... 

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