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4.13: Competition between substitution and elimination

  • Page ID
    227567
  • Having discussed the many factors that influence nucleophilic substitution and elimination reactions of alkyl halides, we must now consider the practical problem of predicting the most likely outcome when a given alkyl halide is reacted with a given nucleophile. As we noted earlier, several variables must be considered, the most important being the structure of the alkyl group and the nature of the nucleophilic reactant. In general, in order for an SN1 or E1 reaction to occur, the relevant carbocation intermediate must be relatively stable. Strong nucleophiles favor substitution, and strong bases, especially strong hindered bases (such as tert-butoxide) favor elimination.

    The nature of the halogen substituent on the alkyl halide is usually not very significant if it is Cl, Br or I. In cases where both SN2 and E2 reactions compete, chlorides generally give more elimination than do iodides, since the greater electronegativity of chlorine increases the acidity of beta-hydrogens. Indeed, although alkyl fluorides are relatively unreactive, when reactions with basic nucleophiles are forced, elimination occurs (note the high electronegativity of fluorine).

    The following table summarizes the expected outcome of alkyl halide reactions with nucleophiles. It is assumed that the alkyl halides have one or more beta-hydrogens, making elimination possible; and that low dielectric solvents (e.g. acetone, ethanol, tetrahydrofuran & ethyl acetate) are used. When a very polar solvent would significantly influence the reaction this is noted in red. Remember that halogens bonded to sp2 or sp hybridized carbon atoms do not normally undergo substitution or elimination reactions with nucleophilic reagents.

    The most important aspect to remember is that substitution and elimination reactions compete with each other, and in most cases, a mixture of different products is obtained. By changing the experimental condition (mainly the nucleophile and solvent), we can favor the formation of substitution or elimination products

    Nucleophile

    Anionic Nucleophiles
    ( Weak Bases: I, Br, SCN, N3,
    CH3CO2 , RS, CN etc. )

    pKa's from -9 to 10 (left to right)

    Anionic Nucleophiles
    ( Strong Bases: HO, RO )

    pKa's > 15

    Neutral Nucleophiles
    ( H2O, ROH, RSH, R3N )

    pKa's ranging from -2 to 11

    Alkyl Group


    Primary
    RCH2
    Rapid SN2 substitution. The rate may be reduced by substitution of β-carbons. Rapid SN2 substitution. E2 elimination may also occur. e.g. SN2 substitution. 

    Secondary
    R2CH–
    SN2 substitution and / or E2 elimination (depending on the basicity of the nucleophile). Bases weaker than acetate (pKa = 4.8) give less elimination. The rate of substitution may be reduced by branching at the β-carbons, and this will increase elimination. E2 elimination will dominate. SN2 substitution. (N ≈ S >>O)
    In very polar solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be formed slowly.

    Tertiary
    R3C–
    E2 elimination will dominate with most nucleophiles (even if they are weak bases). No SN2 substitution due to steric hindrance. In very polar solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be expected. E2 elimination will dominate. No SN2 substitution will occur. In very polar solvents SN1 and E1 products may be formed. E2 elimination with nitrogen nucleophiles (they are bases). No SN2 substitution. In very polar solvents, SN1 and E1 products may be formed.

    Contributors

    William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry

    Competition between substitution and elimination. (2020, September 13). Retrieved May 23, 2021, from https://chem.libretexts.org/@go/page/14800

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