Recap
In the previous article, we explained two different concepts relating to nucleophilic substitution reactions. The first of these is the intimate ion pair concept, which can be viewed as an additional concept or addition to the SN1-type mechanism, because it cannot be treated as a mechanism of its own, given that it only splits one step into more substeps. The intimate ion pair concept suggests that the dissociation of a substrate into cation (the electrophilic portion that is subsequently attacked by a nucleophile) and anion (the leaving group) is split into a series of sub-steps. One of the ‘intermediates’ in the sub-step is the intimate ion pair, which refers to ions that remain close together but yet a covalent bond is not formed. The intimate ion pair may proceed further to separate themselves into two separation ions, but since the dissociation is reversible, the intimate ion pair may also reform the covalent bond and become the starting material again, and this process is known as internal return.
Fig. 1: Demonstration of the formation of an intimate ion pair.
The next part of the previous article was discussion of tetrahedral mechanisms, which are sometimes also termed addition-elimination reactions. These refer to nucleophilic substitution on vinylic carbons, which are able to form a tetrahedral intermediate, giving the mechanism its name. A similar reaction, the addition-elimination reaction, first involves an addition transformation, which is followed by elimination. Although this is not technically a substitution reaction, it does result in a net substitution, and as such it can be treated as a substitution reaction. For general substitution reactions, however, it is very difficult for vinylic carbons to undergo them. The stereochemistry of substitution products formed by vinylic substrates is typically random. The addition-elimination reaction mechanism is shown below in Fig. 2.
Fig. 2: Addition-elimination reaction mechanism.
SNi Mechanism
The first mechanism we will be discussing in this article is the SNi mechanism, which is rather similar to a so-called ‘combination’ of the SN1 and SN2 reactions, as it involves both a dissociation of a molecule and the attack by the nucleophile on the substrate. What the SNi mechanism stands for is debatable; it can either mean substitution nucleophilic internal (with internal return) or substitution nucleophilic intramolecular. It is also somewhat linked with the concept of the intimate ion pair, as this is formed during the SNi mechanism. There is only one reactant in the SNi mechanism. This reactant will undergo dissociation to form the cation and anion (the leaving group). There is then further dissociation of the anion into a stable compound as well as the negatively-charged nucleophile. This nucleophile attacks the cation and forms a bond to it, which results in the formation of the SNi product. The product would be highly similar to that of the SN2 product, with the notable exception that instead of the expected Walden inversion of stereochemistry, there is retention of stereochemistry. This is demonstrated by the reaction in Fig. 3, which shows chlorination by the chlorinating agent thionyl chloride (SOCl2). First, the complex dissociates into the alkyl cation and the SO2Cl- anion. This is an intimate ion pair. In the next part of the reaction, the chloride from this anion attacks the alkyl cation, while the rest of the anion forms sulfur dioxide, SO2. This forms the final product.
Fig. 3: Example of the SNi mechanism.
Part 3 of this article is here.
P.S. Sorry for this short article, but I will be going overseas until the end of this month, so there will probably be nothing new until then.