Alkyl halide radical propagation

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

The two propagation steps shown above for X2 are those that lead directly to the principal products (RX and HX), but many other propagation steps are possible and many occur. Similarly, the only termination step shown is the one that leads to RX, but any two radicals may combine. Thus, products like H2, higher alkanes, and higher alkyl halides can be accounted for by steps like these (these are for... 

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 several guidelines that should be followed in order to increase the chemoselectivity of the monoadduct. Firstly, radical concentration must be low in order to suppress radical termination reactions (rate constant of activation [fcal and fca2] < < rate constant of deactivation kd t andfcd2]). Secondly, further activation of the monoadduct should be avoided ( al> >kd2). Lastly, formation of oligomers should be suppressed, indicating that the rate of deactivation (kd 2[Cu"LmX]) should be much larger than the rate of propagation ( [alkene]). Alkyl halides for copper-catalyzed ATRA are typically chosen such that if addition occurs, then the newly... 

Atom Transfer Radical Polymerisation (ATRP) was discovered independently by Wang and Matyjaszewski, and Sawamoto s group in 1995. Since then, this field has become a hot topic in synthetic polymer chemistry, with over 1000 papers published worldwide and more than 100 patent applications filed to date. ATRP is based on Kharasch chemistry overall it involves the insertion of vinyl monomers between the R-X bond of an alkyl halide-based initiator. At any given time in the reaction, most of the polymer chains are capped with halogen atoms (Cl or Br), and are therefore dormant and do not propagate see Figure 1. 

The tributyltin radical abstracts a halogen atom from the alkyl halide and the chain is propagated as follows—... 

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. 

How fast an alkyl halide is formed depends upon how fast the alkyl radical is formed. Here also, as was the case with methane (Sec. 2.20), of the two chain-propagating steps, step (2) is more difficult than step (3), and hence controls the rate of overall reaction. Formation of the alkyl radical is difficult, but once formed the radical is readily converted into the alkyi halide (see Fig. 3.5). 

Like RAFT, ITP is a degenerative transfer polymerization using alkyl halides [10,11]. ITP was developed in the late 1970s by Tatemoto et al. [226-229]. In ITP, a transfer agent RI reacts with a propagating radical to form the dormant polymer chain P -1. The new radical R can then reinitiate the polymerization. In ITP, the concentration of the polymer chains is indeed equal to the sum of the concentrations of the transfer agent and of the initiator consumed. The newly formed polymer chain P- can then propagate or react with the dormant polymer chain P -1 or R -1 [230]. The mechanism of ITP with alkyl iodide is shown in Scheme 41. 

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