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Narrative
For this animation of an S_N1 reaction, the tetrahedral alkyl halide ionizes to form a planar, trigonal carbocation which may be attacked from either side by a nucleophile leading to racemization of a chiral halide.

Discussion
S_N1 (substitution nucleophilic first order) reactions involve ionization of the alkyl halide to form a carbocation. This step is the slow, rate determining step, of the reaction. The carbocation is then attacked by a nucleophile in a fast second step to form the product. The stereochemistry of this process is simulated with a model. The tetrahedral alkyl halide forms a planar, trigonal carbocation which may be attacked from either side by a nucleophile leading to racemization of chiral halides. The order of reactivity or tertiary > secondary > primary halides parallels the stability of the corresponding carbocations.