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10.2: Reactions with Phosphorus Halides

  • Page ID
    182976
  • Carbocation containment

    Synthetic organic chemists use phosphorus tribromide and thionyl chloride to convert an alcohol into a better leaving group without carbocation rearrangement.

    image084.png

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    Despite their general usefulness, phosphorous tribromide and thionyl chloride have shortcomings. Hindered 1º- and 2º-alcohols react sluggishly with the former, and may form rearrangement products, as noted in the following equation.

    pbr3oh1.gif

    Below, an abbreviated mechanism for the reaction is displayed. The initially formed trialkylphosphite ester may be isolated if the HBr byproduct is scavenged by base. In the presence of HBr a series of acid-base and SN2 reactions take place, along with the transient formation of carbocation intermediates. Rearrangement (pink arrows) of the carbocations leads to isomeric products.

    pbr3oh2.gif


    Reaction of thionyl chloride with chiral 2º-alcohols has been observed to proceed with either inversion or retention. In the presence of a base such as pyridine, the intermediate chlorosulfite ester reacts to form an "pyridinium" salt, which undergoes a relatively clean SN2 reaction to the inverted chloride. In ether and similar solvents the chlorosulfite reacts with retention of configuration, presumably by way of a tight or intimate ion pair. This is classified as an SNi reaction (nucleophilic substitution internal). The carbocation partner in the ion pair may also rearrange. These reactions are illustrated by the following equations. An alternative explanation for the retention of configuration, involving an initial solvent molecule displacement of the chlorosulfite group (as SO2 and chloride anion), followed by chloride ion displacement of the solvent moiety, has been suggested. In this case, two inversions lead to retention.

    Example: Conversion of Alcohols to Alkyl Chlorides

    There’s one important thing to note here: see the stereochemistry? It’s been inverted.*(white lie alert – see below) That’s an important difference between SOCl2 and tosyl chloirde, TsCl, which leaves the stereochemistry alone. The TsCl reaction is studied in section 12.3.

    Mechanisms

    Since the reaction proceeds through a backside SN2 reaction, there is inversion of configuration at the carbon

    The PBr3 reaction is thought to involve two successive SN2-like steps:

    image088.png

    Notice that these reactions result in inversion of stereochemistry in the resulting alkyl halide.

     

    ExerciseS

    2.  Draw the mechanism of the reaction of thinoylchloride with cyclohexanol, given below.

    3.  Draw the expected product of the reaction of cylohexanol with the following reagents.

    (a) SOCl2 (b) PBr3

    Answer

    2.

    3.

    (a)  (b)