Propene allylic bromination

Propene undergoes allylic bromination when treated with A-bromosuccinimide (NBS) in CCI4 in the presence of peroxides or light. 

If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the free-radical reaction and unfavorable for the ionic reaction. Chemists of the Shell Development Company found that, at a temperature of 500-600°, a mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH—CH2— = allyl). Bromine behaves similarly. 

To predict which of the various C—H bonds in propene is most likely to break when a mixture of propene and bromine or chlorine is heated, we need to look at bond dissociation enthalpies. We find that the bond dissociation enthalpy of an allylic C—H bond in propene (Table 8.7) is approximately 92 kj (22 kcal)/mol less than that of a vinylic C—H bond and 50 kJ (12 kcal)/mol less than a C—H bond of ethane. The allyl radical is even more stable than a 3° radical this unusual stability also applies to carbocations. The reason the allylic C—H bond is so weak is discussed in Section 8.6B. Note from Table 8.7 that the benzyl radical CgH5CH2- is stabilized in exactly the same way as the allyl radical and for the same reason benzylic compounds undergo many of the same reactions as allylic compounds (Section 21.5). 

Following is a balanced equation for the allylic bromination of propene. 

Interestingly, bromination under radical conditions appears to occur with about equal facility to bromoethene (ethylene bromide [H2C=CFIBr]) and 3-bromo-l-propene (allyl bromide [H2C=CH-CH2Br]). 

Scheme 7.21. A representation of the addition of radiolabeled bromine (indicated as Br2> to 3-bromo-l-propene (allyl bromide, H2C=CHCH2Br). It is important to note that the initially formed bromonium ion is capable of being opened by internal (backside) attack from the bromine atom on the neighboring carbon. This is an example of the process called neighboring group participation (this chapter, Rearrangement). In this particular case, the newly formed bromonium ion is, except for the position of the label ( Br), the mirror image of the bromonium ion from which it was generated. Each of these bromonium ions is capable of being attacked by exogenous labeled bromide anion. Thus, depending on the specific bromonium ion, the final relationship of the two isotopically unique bromine atoms in the product (1,2,3-tribromopropane) will be either 1,2 or 1,3.

In a subsequent report, Ziegler and co-workers systematically studied the halogenation of several allylic substrates using a variety of reagents including A -bromo and A -chloro pthalimide, chloramine-T, N-bromo and A -chlorosaccharin, A -chloro-A -benzoyl-p-toluenesulfonamide, N-chloro-di-/ -toluylsulfonamide, and most notably, iV-bromo and N-chlorosuccinimide. Successful allylic bromination was reported for several substrates, among which were the conversion of cyclohexene (7) into bromocyclohexene (8) and the conversion of phenyl propene 9 into cinnamyl bromide (10). 

Microwave spectroscopic studies of l-fluoro-2-propene show that the cis conformation is more stable than the gauche conformation by approximately 306 cal/mol293 However, when fluorine is replaced by chlorine, bromine, or iodine, the gauche conformation becomes more stable294. These results confirm our expectations that the conformational preferences of allyl halides may depend on a balance of nonbonded attractive effects and a—it interaction effects. 

The first significant attempt to study the kinetics of the thermal decomposition of chlorinated and brominated hydrocarbons and related compounds was by Lessig in 1932. These studies were complicated by the catalytic activity of the pyrex or quartz glass walls of the vessels in which the gas phase reactions were carried out. It was later found that the heterogeneous reactions could be suppressed by using seasoned reaction vessels in which a fine coating of carbonised material had formed on the walls from previous decompositions of similar compounds. Substances such as allyl bromide and 3-chloro-2-methyl-propene have been found particularly effective in producing carbonised films. 

What if, instead of taking just a proton, we had also taken away two electrons from propene In reality we can get such a structure quite straightforwardly from allyl bromide (prop-2-enyl bromide or l-bromoprop-2-ene). Carbon 1 in this compound has four atoms attached to it (a carbon, two hydrogens, and a bromine atom) so it is tetrahedral (or sp hybridized). 

Allylic hydrogens are especially reactive in radical substitution reactions. We can synthesize allylic halides by substitution of allylic hydrogens. For example, when propene reacts with bromine or chlorine at high temperatures or under radical conditions where the concentration of the halogen is small, the result is allylic substitution. 

Chain Propagation Chain propagation involves the formation of products. Reaction of a radical and a nonradical gives a new radical. (Both radicals formed in the initiation can abstract hydrogen atoms. We show only the Br reaction.) In the first propagation step, a bromine atom abstracts an allylic hydrogen (the weakest C—H bond in propene) to produce an allyl radical. The allyl radical, in turn, reacts with a bromine molecule to form allyl bromide and a new bromine atom. 

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