Homolytic Bond Making and Bond Breaking

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Chapter: Organic Chemistry : Curved-Arrow Notation

Heterolytic processes make up a large proportion of organic transformations because most bonds are somewhat polarized.


HOMOLYTIC BOND MAKING AND BOND BREAKING

Heterolytic processes make up a large proportion of organic transformations because most bonds are somewhat polarized. Heterolytic cleavage is merely an increase of this polarity to the limit at which there is no bond remaining; that is, electron movement follows in the direction established by the bond polarity to give a cation – anion pair.

If a bond is particularly weak and/or nonpolar, bond cleavage can occur by a nonpolar or homolytic process. One electron of the shared pair goes with each of the two bonded atoms. Bond breaking then is the movement of single electrons rather than electron pairs and is indicated in curved-arrow notation as “half-headed” arrows. Homolytic cleavage of a bond does not result in the formation of charge but does result in the formation of unpaired electron intermediates called free radicals. Free radicals normally have seven electrons in the valence shell and as a consequence are very reactive intermediates. Common examples of compounds which undergo homolytic bond cleavages include halogens (Br2, Cl2, F2), peroxides (R–O–O–R), and azocompounds (R–N=N–R).


All of these free-radical precursors are characterized by relatively weak, nonpolar bonds which, upon heating, break to give free-radical intermediates. Free radicals are very reactive and proceed to products by a variety of one-electron, or homolytic, reactions.

Homolytic bond formation can occur when two free-radical species contact each other. Each has an available unpaired electron, and if these two electrons are shared, a new bond will result.


This is simply the reverse of the homolytic cleavage. It is a very exothermic process (by the amount equal to the energy of the bond being formed), and it occurs at a very fast rate.

Homolytic bond formation can also occur by the reaction of a free radical with a bonded pair of electrons. Two common examples of this behavior are hydrogen (or other atom) abstraction reactions and free-radical addition to double bonds. Atom abstraction reactions take place by the interaction of a free radical with a σ-bonded atom. One electron of the σ bond pairs with the unpaired electron of the free radical to produce a new bond. The remaining electron of the σ bond remains on the fragment from which the atom has been abstracted and produces a new free-radical species. This process is energetically driven by bond strengths; that is, atom abstraction only occurs if the bond that is formed is stronger than the one that is broken. In the example of hydrogen abstraction shown below, a phenyl radical readily abstracts a benzylic hydrogen from toluene to give benzene plus the benzyl free radical because the aromatic C–H bond (103 kcal/mol) that is formed is appreciably stronger than the benzylic C–H bond (85 kcal/mol) that is broken.


Addition to π bonds is a second very common reaction of free radicals. Inter-action of the free radical with the π-electron pair causes one of the π electrons to pair up with the unpaired electron of the free radical to produce a new bond to one of the π-bonded atoms. The remaining π electron is now unpaired and thus forms a new free-radical species. The process is often very favorable since the new σ bond (70 – 90 kcal/mol) formed in the addition process is normally much stronger than the π bond (60 kcal/mol) which is broken in the reaction. In the above example a new carbon – carbon σ bond is formed by free-radical addition to produce a new carbon-centered free radical; however, a wide variety of other free-radical species add readily to olefins.

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