Free Radicals Add to Double Bonds

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

One possible interpretation is a change to a free radical chain mechanism. Bromine radical is first produced which then adds to the alkene. The resulting free radical reacts with hydrogen bromide to yield the final alkyl bromide and regenerate bromine radical. 

Examine spin density surfaces for l-bromo-2-propyl radical and 2-bromo-l-propyl radical (resulting from bromine atom addition to propene). Eor which is the unpaired electron more delocalized Compare energies for the two radicals. Is the more delocalized radical also the lower-energy radical Could this result have been anticipated using resonance arguments  

Does the lower energy radical intermediate lead to the lower energy product Compare energies for 1-propyl bromide and 2-propyl bromide to tell. 

Spin density surface for l-bromo-2-propyl radical shows location of the unpaired electron. 

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The second way in which fats deteriorate is oxidative lipolysis. This is an entirely different process in which oxygen free radicals add across double bonds. Oxidative rancidity can be prevented or reduced by several different routes. One way is to ensure that no double bonds are present. Another is to use anti-oxidants that act as free radical traps. Exposure to oxygen and ultraviolet light should be avoided. Reducing the temperature has no effect since free radical processes have a zero activation energy. 

In oxidative rancidity, oxygen, normally in the form of a free radical, adds across double bonds. This is a zero activation energy process so it is not inhibited by reducing the temperature. The end products of this process can be unpleasant in both taste and smell. Oxidative rancidity tends to appear suddenly and then progress rapidly, and may be prevented by using saturated fats. 

Aldehydes, formates, primary, and secondary alcohols, amines, ethers, alkyl halides, compounds of the type Z—CH2—Z, and a few other compounds add to double bonds in the presence of free-radical initiators/ This is formally the addition of RH to a double bond, but the R is not just any carbon but one connected to an oxygen or a nitrogen, a halogen, or to two Z groups (defined as on p. 548). The addition of aldehydes is illustrated above. Formates and formamides " add similarly ... 

Sulfonyl halides add to double bonds, to give P-halo sulfones, in the presence of free-radical initiators or UV light. A particularly good catalyst is cuprous chlo-ride. Triple bonds behave similarly, to give P-halo-a,(3-unsaturated sulfones. 

This new free radical adds to the double bond of another monomer molecule, growing the polymer chain. The polymerization process ends as the unpaired electrons of two free radicals combine to form a single bond ... 

The mechanism of free radical polymerization of ethylene is outlined m Figure 6 17 Dissociation of a peroxide initiates the process m step 1 The resulting per oxy radical adds to the carbon-carbon double bond m step 2 giving a new radical which then adds to a second molecule of ethylene m step 3 The carbon-carbon bond forming process m step 3 can be repeated thousands of times to give long carbon chains... 

Stannanes also add across double bonds offiuonnated olefins in a free radical reaction Trimethylstannane undergoes stereospecific addition to hexafluorocyclo-butene to afford trans 1,2,3,3,4,4 hexafluoro 1 (trimethylstannyl)cyclobutane [5] (equation 1)... 

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

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