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10.4: Preparing Alkyl Halides from Alkenes - Allylic Bromination

  • Page ID
    31496
  • Objectives

    After completing this section, you should be able to

    1. write the equation for the bromination of a symmetrical alkene using N-bromosuccinimide.
    2. predict the product formed when a given symmetrical alkene is treated with N-bromosuccinimide.
    3. identify the reagent, the symmetrical alkene, or both, needed to produce a given allyl halide by allylic bromination.
    4. list the following radicals in order of increasing or decreasing stability: allyl, vinyl, primary alkyl, secondary alkyl, tertiary alkyl, methyl.
    5. explain the ease of forming an allyl radical, and the difficulty of forming a vinyl radical, in terms of relative $\ce{\sf{C–H}}$ bond dissociation energies.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    Study Notes

    We have discussed the electrophilic addition of X2 and HX to alkenes as a route to forming alkyl halides (Sections 7.8 and 8.2). In this section we introduce bromination at the allyic position with N-bromosuccinimide (NBS). Notice that at the moment we are restricting our studies to the allylic bromination of symmetrical alkenes, such as cyclohexene. When we introduce an element of asymmetry, we find that more than one allyl radical can be formed; therefore, we must assess the relative stability of each radical when trying to predict which product will predominate. The method of doing this assessment is described in the next section.

    R (in R–H) methyl ethyl i-propyl

    t-butyl

    phenyl benzyl allyl vinyl
    Bond Dissociation Energy
    (kcal/mole)
    103 98 95 93 110 85 88 112

    The covalent bond homolyses that define the bond dissociation energies listed above are described by the general equation:

    R3C-H + energy ——> R3C· + H·

    Since the hydrogen atom is common to all the cases cited here, we can attribute the differences in bond dissociation energies to differences in the stability of the alkyl radicals (R3C·) as the carbon substitution changes. This leads us to the conclusion that:

    alkyl radical stability increases in the order: phenyl < primary (1º) < secondary (2º) < tertiary (3º) < allyl ≈ benzyl.

    Because alkyl radicals are important intermediates in many reactions, this stability relationship will prove to be very useful in future discussions. The enhanced stability of allyl and benzyl radicals may be attributed to resonance stabilization (Chapter 2). Formulas for the allyl and benzyl radicals are shown below. Draw structural formulas for the chief canonical forms contributing to the resonance hybrid in each case.

     

    The poor stability of phenyl radicals, C6H5·, may in turn be attributed to the different hybridization state of the carbon bearing the unpaired electron (sp2 vs. sp3).

    The hydrogen halide (HCl, HBr, and HI) react with alkenes in an electrophlic addtion reaction discussed in chapter to yield halogenated products. Bromine and chlorine can also react with alkenes to for dihalogenated products as discussed in chapter 8. Another method for preparing alkyl halides from alkenes is with N-bromosuccinimide or NBS in the presence of light. We noted that benzylic and allylic sites are exceptionally reactive in free radical halogenation reactions. Since carbon-carbon double bonds add chlorine and bromine in liquid phase solutions, radical substitution reactions by these halogens are often carried out at elevated temperature in the gas phase (first equation below).

    The brominating reagent, N-bromosuccinimide (NBS), has proven useful for achieving allylic or benzylic substitution in CCl4 solution at temperatures below its boiling point (77 ºC). One such application is shown in the second equation. the allylic bromination with NBS is analagous to the alkane halogenation reaction (Section 10.3) since it also occurs as a radical chain reaction. The NBS serves as the source for the bromine, which is used in the initiation step to create a bromine radical that then abstracts a proton from the allylic position in the propagation step. The radical created then reacts with the NBS to to become bromiated and the cycle continues until it is terminated.

    CH2CHCH3 reacts with chlorine to produce CH2CHCH2cl and hydrochloric acid.

    The predominance of allylic substitution over other positions comes down to bond dissociation energies. The relative bond dissociation energies are shown in the table at the top of this section. The C-H bond that we are focusing on as the point of difference for each of the energies shows that the allylic C-H bond has a strength of about 88 kcal/mol. This means that the allylic radical created is more stable than a typical alkyl radical with the same substitution by about 9 kcal/mol. Therefore, this radical is the most likely one to form and thus react.

    Unsymmetrical allylic radicals will react to give two regioisomers. Thus, 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)2N

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