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11.12: A Summary of Reactivity - SN1, SN2, E1, E1cB, and E2

  • Page ID
    67273
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    Objectives

    After completing this section, you should be able to 

    1. determine whether a specified substrate is most likely to undergo an E1, E2, SN1 or SN2 reaction under a given set of conditions.
    2. describe the conditions under which a given substrate is most likely to react by a specified mechanism (E1, E2, SN1 or SN2). 
    Study Notes

    This section summarizes much of what has been discussed in the chapter. It focuses on how a given substrate will behave under certain conditions, but does not deal with the stereochemistry of the products. 

    Content

    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 nucleophile 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 high dielectric solvent would significantly influence the reaction this is noted in red. Note that halogens bonded to sp2 or sp hybridized carbon atoms do not normally undergo substitution or elimination reactions with nucleophilic reagents.

    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, as in the case of neopentyl. Rapid SN2 substitution. E2 elimination may also occur. e.g.
    ClCH2CH2Cl + KOH ——> CH2=CHCl
    SN2 substitution. (N ≈ S >>O)

    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 high dielectric ionizing 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 high dielectric ionizing solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be expected. E2 elimination will dominate. No SN2 substitution will occur. In high dielectric ionizing solvents SN1 and E1 products may be formed. E2 elimination with nitrogen nucleophiles (they are bases). No SN2 substitution. In high dielectric ionizing solvents SN1 and E1 products may be formed.

    Allyl
    H2C=CHCH2
    Rapid SN2 substitution for 1º and 2º-halides. For 3º-halides a very slow SN2 substitution or, if the nucleophile is moderately basic, E2 elimination. In high dielectric ionizing solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be observed. Rapid SN2 substitution for 1º halides. E2 elimination will compete with substitution in 2º-halides, and dominate in the case of 3º-halides. In high dielectric ionizing solvents SN1 and E1 products may be formed. Nitrogen and sulfur nucleophiles will give SN2 substitution in the case of 1º and 2º-halides. 3º-halides will probably give E2 elimination with nitrogen nucleophiles (they are bases). In high dielectric ionizing solvents SN1 and E1 products may be formed. Water hydrolysis will be favorable for 2º & 3º-halides.

    Benzyl
    C6H5CH2
    Rapid SN2 substitution for 1º and 2º-halides. For 3º-halides a very slow SN2 substitution or, if the nucleophile is moderately basic, E2 elimination. In high dielectric ionizing solvents, such as water, dimethyl sulfoxide & acetonitrile, SN1 and E1 products may be observed. Rapid SN2 substitution for 1º halides (note there are no β hydrogens). E2 elimination will compete with substitution in 2º-halides, and dominate in the case of 3º-halides. In high dielectric ionizing solvents SN1 and E1 products may be formed. Nitrogen and sulfur nucleophiles will give SN2 substitution in the case of 1º and 2º-halides. 3º-halides will probably give E2 elimination with nitrogen nucleophiles (they are bases). In high dielectric ionizing solvents SN1 and E1 products may be formed. Water hydrolysis will be favorable for 2º & 3º-halides.
    Worked Example\(\PageIndex{1}\)

    1) For the following, please determine what kind of reaction is occurring and predict the product(s).

    a)

    Exampl 1 Question .svg

    b)

    Example 2 Question .svg

    c)

    Example 3 Question .svg

    Answer

    a) The substrate is a secondary halide so the product is determined by the nature of the nucleophile used in the reaction. Cyanide (-CN) is a good nucleophile which is a weak base. The fact that the nucleophile is a weak base means that an E2 reaction is not favored. Also, the fact that cyanide is a good nucleophile means that SN2 substitutions are favored over SN1. When a secondary halide is reacted with a good nucleophile which is a weak base, the preferred reaction is SN2. After SN2 substitution the product is a nitrile.

    Exampl 1 Answer .svg

    b) The substrate is a secondary halide so the product is determined by the nature of the nucleophile used in the reaction. Methoxide (-OCH3) is a strong base so it could prefer to remove a hydrogen from the substrate. This makes an E2 elimination the preferred reaction and an alkene the product.

    Example 2 Answer .svg

    c) The substrate is a secondary halide so the product is determined by the nature of the nucleophile used in the reaction. Methanol (HOCH3) is a weak nucleophile that is a weak base. Being a weak base means that methanol is not capable of actively removing a hydrogen to cause an E2 reaction. Also, because methanol is a weak nucleophile is not capable of attacking the substrate and causing an SN2 reaction. For the methanol nucleophile to react the substrate must first eject it leaving group to form the highly reactive carbocation intermediate. Formation of the carbocation is the rate determine step for both the SN1 and E1 reactions so they each form a separate product.

    Example 3 Answer .svg

    Exercises

    Exercise \(\PageIndex{1}\)

    Identify the dominant reaction mechanism (SN1, SN2, E1, or E2) for the following reactions.

    11.12.png

    Answer

    A) SN2

    B) E1

    C) SN1

    D) E2

    Exercises \(\PageIndex{2}\)
    4) Identify all the products of the following reactions and specify the major product.

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    b)KlfOvoOna7_5A6I6kuMNiGaJ-o2on_Z7bX5sF0rbrpfQyvhtP4FqUfPZ-YI62ASiRuLEyDPRsKRuST0CjJ0_IDhafBD-eyfld340-CmIXZPq48Tb9eTo9ArPSnkvarI4GFVC0ifq

    c)LOmc6XfFjGNgy9RrUcID6LMp53yNhKNn4mAMsosn-I4kEI8kRnYw-VipxXQYuxdCQmCW812hR5Yc2igS2MaoCFV0jqi9g5_D3tl1dVtLxWdg91MdzUx1eiKHUO5Lp2IbN_NKfHTe

    d)EOgI5PPDAmQt4ieBkeCPHA_kqbi4r-VGSSBEN5S1sGyHkO3FVXMHECnq-mPSourY-TvP3S0N8xb-CdfS5fq6cCtr_QrX-rXafHHFSQoEQPJW7n4HDBWDa1N9L08bDoKNt6zCKmdo

    e)

    5) The following

    all the products of the following reactions and specify the major product.

    a)3XmYuZpxO8gU8lU_0-ji68K3-v_v5cURUT1UUa3jFtTAFSBu6deCqJWrpEdq6bQuUT54nc7fvk1-BU7zH6PdUDMydnXLKB0kv56z7J_1EZWrIJ45UyWyk_-SiXMAzl1NMiTqeabj

    b)KlfOvoOna7_5A6I6kuMNiGaJ-o2on_Z7bX5sF0rbrpfQyvhtP4FqUfPZ-YI62ASiRuLEyDPRsKRuST0CjJ0_IDhafBD-eyfld340-CmIXZPq48Tb9eTo9ArPSnkvarI4GFVC0ifq

    c)LOmc6XfFjGNgy9RrUcID6LMp53yNhKNn4mAMsosn-I4kEI8kRnYw-VipxXQYuxdCQmCW812hR5Yc2igS2MaoCFV0jqi9g5_D3tl1dVtLxWdg91MdzUx1eiKHUO5Lp2IbN_NKfHTe

    d)EOgI5PPDAmQt4ieBkeCPHA_kqbi4r-VGSSBEN5S1sGyHkO3FVXMHECnq-mPSourY-TvP3S0N8xb-CdfS5fq6cCtr_QrX-rXafHHFSQoEQPJW7n4HDBWDa1N9L08bDoKNt6zCKmdo

    e)

     
    Answer
     

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    b)2QKalqyamcJMJglDMQ5BLXgTcnfA4OEx540WBUqJCZKl2s3vi8Vhbw3xCYBfewisvCutqm-KuwYlptitSWUPkVEqD45X_QzfChfL6xXCgmxKPr6iGZ_IYFqjfhmtGdqgS28yenyu

    c)8goeD5NH4HG95180mnLWhm6B_mxtG6q6CW_CovgQS3wc9am2tHSop3DjXY0-OWhJvItBJLyb8UkSFBb7BJhmbQA8sueL66Qe5ApYhiodvJrIHwPex5ze-onx5CqhNPBxjJ2Irf_y

    d)BrJ5jZ_x0cTuSw3x7GkNoGdHORKqg1xvSe3iXIsiVpLU4iI-5fYQTGZe8qgtEJwaldHaIkQFUjVzcna4HB5Hhpayz-iloF4AEw_e6jDYElRkruyIoQvQW4W962eb-VZL0s-BJrY7

    e)

    Contributors and Attributions


    11.12: A Summary of Reactivity - SN1, SN2, E1, E1cB, and E2 is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.