Stereochemistry of SN1 Reactions

Two products are formed when a chiral substrate that possesses an asymmetric, electrophilic carbon is applied in an SN1 reaction. One of them has the same absolute configuration as the starting product (if, according to the CIP rules, the leaving group and the nucleophile have the same position in the priority order of the substituents), which is called retention. In contrast, the other product possesses the opposite absolute configuration, known as inversion. In SN1 reactions, the nucleofuge exits the substrate before the nucleophilic attack can ever occur. Thus, an intermediate carbocation is then formed. Due to the carbocation's trigonal planar shape, its two enantiotopic sides are susceptible to attack by the nucleophile with the same probability.

Fig.1Influence of the leaving group on the stereochemistry of the product.

However, practically speaking, only an incomplete racemization occurs frequently enough. Then, a small excess of the inversion product is detected. This result is particularly often observed when substrates with a relatively poor leaving group (e.g. chloride) are applied. A back-side attack of the nucleophile on the substrate can then take place before the C-X bond is completely cleaved. That is, the SN1 reaction is inclined to display a partial SN2 characteristic.

In contrast to the chloride anion, the bromide anion is a very good leaving group. Therefore, complete racemization is more easily obtainable when bromine is exchanged in a nucleophilic substitution. This result may be experimentally illustrated, as R- and S-1-phenylethanol have distinguishable scents. Therefore, it can easily be detected through the mere human sense of smell whether only one enantiomer has been formed or not.

An example of a nucleophilic substitution that results in an incomplete racemization is the hydrolysis of 1-chloroethylbenzene. In the hydrolysis of 1-bromoethylbenzene, however, complete racemization is obtained.

Fig.2Different degrees of racemization in the hydrolysis of 1-halogenoethylbenzenes.