7.2 SN1 Reactions

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    SN1 reactions – Substitution Nucleophilic Unimolecular

    sn1 mechanism

    Unimolecular – Only one reactant molecule is involved in the slow step and the rate law is 1st order.

    Rate = k[substrate]

    -The substrate is often al alkyl halide (Cl, Br, and I are good leaving groups).

    -The intermediate is a carbocation.

    -Carbocation rearrangements are possible.

    -Addition of AgNO3 can facilitate carbocation formation.

    carbocation agno3

    Predict the major substitution product for the following reaction.

    sn1 example 1

    Show the mechanism for how the product below was formed.

    sn1 example 2 mechanism

    -SN1 reactions result in racemization at a chiral center (slight excess of the inverted product).

    sn1 racemization

    Substrate Effects

    -All about carbocation stability

    sn1 substrate effects

    Nucleophile Effects

    -Weak nucleophiles (or low concentrations of strong nucleophiles) are ok as it isn’t involved in the slow step.
    -Water and alcohols are common nucleophiles in SN1 reactions.

    Solvent Effects

    Polar protic solvents are necessary to stabilize the carbocation (ion-diple forces).
    -Water and alcohols are common solvents.
    -Solvolysis – Many SN1 reactions involve the solvent as a reactant and are called solvolysis reactions.

    carbocation stabilized by ion dipole

    Leaving Group Effects

    -Generally the weaker the base the better the leaving group; the better the leaving group the faster the reaction.

    sn2 leaving group