The S_N1 reaction

Whereas the S_N2 reaction proceeds concerted and in a stereospecific way, the S_N1 reaction is a two-step reaction which is not stereospecific. Let's look at the S_N1 reaction of tert-butyl bromide with OH^-:

What you see on the left is the first step of an S_N1 reaction. After Br^- has moved away from the central carbon of the tert-butyl group, the so-called carbocation becomes a flat structure (as can be seen on the right); consecutive attack by a nucleophile, like water, can occur from both sides of the structure. Both attacks are equally likely to occur.

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Although the example above doesn't start from an optically active molecule, you can probably imagine what the result would be if the starting material is, for example, optically pure 1-bromo-1-phenylethane.

The first of two steps depicted above, breaking the C-Br bond, is the rate determining step. Attack of the carbocation by the nucleophile (e.g. OH^-, or H₂O) is fast, compared to the dissociation of the C-Br bond.

In the next part, you will see that certain reactions can take place during the lifetime of the carbocation, between the bond dissociation and nucleophilic attack. These rearrangements of the carbocation occur due to the relative stability of the different carbocations.

The SN1 reaction

The S_N1 reaction