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V. Fragmentation Reactions

  • Page ID
    23829
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    A. β-Fragmentation

    Homolytic β-fragmentation of a radical is an elementary reaction that cleaves a bond to one of the atoms adjacent to a radical center. (Other names for β-fragmentation are β-cleavage and β-scission.) This type of reaction sometimes produces an unsaturated carbohydrate by expelling a noncarbo­hydrate radical (eq 27), and other times it gives a carbo­hy­drate radical and an unsaturated noncarbohydrate (eq 28). Equation 29 illustrates the first of these possibilities with a reaction in which the radical 4 fragments to give C6H5SO2· and the unsaturated carbohydrate 5.21 Being able to form a stabil­ized radical such as C6H5SO2· is an essential factor in this reaction.

    (27).png

    (28).png

    (29).png

    A β-fragmentation reaction producing a carbohydrate radical and an unsaturated noncar­bo­hydrate is one driven by formation of a compound with a thermo­dy­nam­ically stabilized multiple bond (usually a carbon–oxygen double bond) and a radical that also is stabilized (usually by an oxy­gen atom attached to the radical center). The reaction shown in eq 30 fits this pattern because it produces formaldehyde and the oxygen stabilized radical 6.22 Forming an aromatic ring is another way for providing a substantial driving force for β‑fragmentation (eq 31).23 A further option for β-fragmentation is ring opening, a possibility that presents itself when a radical is centered on an atom attached to the ring (eq 32).24

    (30).png

    (31).png

    (32).png

    B. Heterolytic β-Fragmentation

    When a radical is centered on a carbon atom that has an effective nucleo­fuge attached to a neighboring carbon atom, the possibility exists for formation of a radical cation (eq 33). The bond from the neighboring carbon atom to the leaving group needs to be one that does not cleave homo­lytically with ease; otherwise, β-frag­men­ta­tion producing ionic intermediates could be preempted by homolytic fragmentation. Heterolytic β-fragmentation occurs in the reac­tion shown in eq 34.25

    (33).png

    (34).png

    C. α-Fragmentation

    α-Fragmentation is an elementary reaction in which a bond attached to a radical center cleaves homolytically. This reaction is rare because it requires the energy-demanding step of bond breaking without the energetic compen­sation of bond formation. One situation in which α-frag­men­tation takes place is in the formation of the isonitrile and stabilized, sulfur-centered radical shown in eq 35.26 A second occurs in the fragmentation of the hypervalent radical shown in eq 36.19

    (35).png

    (36).png

    D. Bond Homolysis

    Bond homolysis either produces a pair of radicals (eq 37), or if the bond being broken is part of a ring system, a diradical. Thermal reaction cleaves the weakest bond in a molecule; thus, when the cobal­oxime 7 is heated, the carbon–cobalt bond, one of the weakest covalent bonds known, breaks homo­lytically at temperatures well below those necessary for cleavage of other bonds in the molecule (eq 38).27 This bond homolysis involves electron transfer with cobalt acting as the electron acceptor.

    (37).png

    (38).png

    Photochemical reaction offers a range of possibilities for bond homol­ysis (eq 39). Success depends both upon a compound being able to absorb the incident light and on this light supplying sufficient energy for bond breaking. Absorp­tion of visible light provides the energy needed to cleave weaker covalent bonds, such as the iodine–oxygen bond in the reaction shown in eq 40.28 UV radiation is energetic enough to break stronger bonds, such as the carbon–carbon bonds in the reactions pictured in eq 41.29

    (39).png

    (40).png

    (41).png

    Unlike thermal reaction, bond breaking during a photochemical process does not necessarily cleave the weakest bond in a molecule. Selectivity in bond breaking during photolysis results from a combination of factors that control the reactivity of elec­tron­ic­ally excited molecules. In the reaction shown in eq 41, for instance, exci­tation energy is quickly localized in the keto group in the substrate. This localization leads to one of the characteristic reactions of an excited aldehyde or ketone, namely, breaking ­the bond between the carbonyl carbon atom and one of its attached car­bon atoms.30


    This page titled V. Fragmentation Reactions is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.

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