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  Alkenes Radical Additions Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Alkenes Radical Additions

1. Addition of Radicals to Alkenes

Protons and other electrophiles are not the single reactive species that start addition reactions to carbon-carbon double bonds. Unusually, this first became manifest as a result of incompatible reports concerning the regioselectivity of HBr additions. As described before, the acid-induced addition of HBr to 1-butene gave predominantly 2-bromobutane, the Markovnikov Rule product. Though, in some early experiments wherein peroxide contaminated reactants were used, 1-bromobutane was the major product. Additional study displayed that a substitute radical chain-reaction, started by peroxides, was responsible for the anti-Markovnikov product. This is displayed by the following equations.

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The weak O-O bond of a peroxide initiator is divided homolytically by hight or thermal energy. The resultant alkoxy radical then abstracts a hydrogen atom from HBr in a powerfully exothermic reaction. In the first step of a chain reaction once a bromine atom is formed it adds to the π-bond of the alkene. Further than this, is regioselective, giving the more stable carbon radical like an intermediate. The second step is carbon radical abstraction of other hydrogen from HBr, generating the a new bromine atom and anti-Markovnikov alkyl bromide. Each of the steps in this chain reaction is exothermic, so once started the method continues until radicals are lost to termination events. 
This free radical chain addition competes very favorably with the slower ionic addition of HBr depicted earlier, especially in non-polar solvents. It is significant to remind, though, that HBr is special in this respect. Since one of the chain steps becomes endothermic (the first for HI &the second for HCl). &). The radical addition process is unfavorable for HCl and HI. 

Other radical addition reactions to alkenes have been observed, one case being the peroxide induced addition of carbon tetrachloride displayed in the equation given below:

RCH=CH2   +   CCl4 (peroxide initiator)   ->   RCHClCH2CCl3

The best known and most significant use of free radical addition to alkenes is probably polymerization. Because the addition of carbon radicals to double bonds is energetically favorable, concentrated solutions of alkenes are prone to radical-initiated polymerization, as demonstrated for propene by the equation. The blue colored R-group depicts an initiating radical species or a growing polymer chain; the propene monomers are colored maroon. The addition all the time occurs so that the more stable radical intermediate is formed.

RCH2(CH3)CH· + CH3CH=CH2 -> RCH2(CH3)CH-CH2(CH3)CH· + CH3CH=CH2 -> RCH2(CH3)CHCH2(CH3)CH-CH2(CH3)CH· -> etc.

2. Allylic Substitution

We noted earlier that in free radical halogenation reactions allylic and benzylic sites are exceptionally reactive. Because carbon-carbon double bonds add bromine and chlorine in liquid phase solutions, radical substitution reactions by these halogens are frequently carried out at elevated temperature in the gas phase (first equation below). Creation of the ionic π-complexes which are intermediates in halogen addition is unfavorable in the absence of polar solvents, and entropy generally favors substitution over addition.
The brominating reagent, N-bromosuccinimide (NBS), has proven helpful for achieving benzylic or allylic substitution in CCl4 solution at temperatures below its boiling point (77 ºC). One this type of  application is displayed in the second equation.

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In the NBS reaction the predominance of allylic substitution over addition is interesting. The N-Br bond is without any doubt weak (probably less than 50 kcal/mol) so bromine atom abstraction by radicals should be very favorable. By removing an allylic hydrogen from the alkene the resultant succinimyl radical might then establish a chain reaction. One drawback with this technique is that NBS is very insoluble in CCl4, about 0.006 mole / liter at reflux. Even though it is possible that the allylic bromination occurs at a solid-liquid interface, proof for another pathway has been obtained. Very low concentrations of bromine may be generated from NBS in the non-polar solvent employed for these reactions. This would serve like a source of bromine atoms that would abstract allylic hydrogens irreversibly (an exothermic reaction) in competition with reversible addition to the double bond. In this way The HBr produced is known to react with NBS, giving a new succinimide and bromine molecule, as displayed here. Ionic addition of bromine to the double bond would be slowest in these circumstances (CCl4 is a nonpolar solvent).

HBr   +   (CH2CO)2NBr   ->   Br2   +   (CH2CO)2NH

This technique is must be identical as that for the free radical halogenation of alkanes, with NBS serving as a source of very low concentrations of bromine. Unsymmetrical allylic radicals will react to give two regioisomers. So, 1-octene on bromination with NBS yields a mixture of 3-bromo-1-octene (ca. 18%) and 1-bromo-2-octene (82%) - both cis and trans isomers.

RCH2CH=CH2   +   (CH2CO)2NBr   ->   RCHBrCH=CH2   +   RCH=CHCH2Br   +   (CH2CO)2NH

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