Free
Radical Addition to Alkenes
~ Organic Chemistry 342 Website Tutorial ~
Purpose
Organic Chemistry is the study of carbon-containing compounds and the reactions they undergo. One such reaction is the free radical addition to alkenes. The purpose of this web page is to supply one with a better understanding of this reaction.
A Brief
Overview
Since s bonds are stronger
than p
bonds, double bonds tend to react to convert the double bond into s
bonds in an ADDITION REACTION.
Protons and other electrophiles are not the only reactive species that initiate addition reactions to carbon-carbon double bonds. Curiously, this first became evident as a result of conflicting reports concerning the regioselectivity of HBr additions. The acid-induced addition of HBr to 1-butene gives predominantly 2-bromobutane, the Markovnikov’s rule product.
However, in some early experiments in which peroxide contaminated reactants were used, 1-bromobutane was the chief product. Further study showed that an alternative radical chain-reaction, initiated by peroxides, was responsible for the anti-Markovnikov product. This is shown by the following equations.
Each of the steps in this chain reaction is exothermic, so once started the process continues until radicals are lost to termination events. Note also that the bromine atom adds to the double bond in a selective manner that generates the more stable carbon radical as an intermediate. However, this mode of addition is unfavorable for HCl and HI because one of the chain steps becomes endothermic (the second for HCl & the first for HI).
Other radical addition reactions to alkenes have been observed, the best known of these being polymerizations. Since the addition of carbon radicals to double bonds is energetically favorable, concentrated solutions of alkenes are prone to radical-initiated polymerization, as illustrated for propene by the following equation. The blue colored R-group represents an initiating radical species or a growing polymer chain; the propene monomers are colored maroon. The addition always 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. |
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Other websites that may be of interest:
Free Radical Reactions - WSU chemistry
Free Radical Addition to Alkenes – Indiana/Purdue University
Amanda Anderson
Last Modified: 30 April 2002