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SN1 Reaction Overview
Unimolecularnucleophilic substitution reactions (SN1 for short) are two-step reactions in which a nucleophile replaces a leaving group. The ‘unimolecular’ part of the name stems from the first order rate law for SN1 reactions, meaning the rate of these reactions is dependent exclusively on the concentration of the electrophilic substrate.
SN1 Reaction Mechanism
With generic alkyl halide, an SN1 reaction proceeds through two distinct steps. In the first step the bond to the leaving group is broken and a carbocation intermediate is formed, while in the second step a new nucleophile attacks the electrophilic center of our carbocation intermediate. Rather surprisingly, even a simple solvent like water can act as a nucleophile in the reaction and end up donating the group to substitute for the leaving group.
Notice that after carbocation formation, only 3 pairs of electrons remain on the central carbon. As a result, this carbocation intermediate displays trigonal planar electronic geometry, sp2 hybridization and a positive net charge. The geometry of this intermediate is the cause of racemization of products, meaning we will see both stereoisomers being produced.
Due to this planar geometry of the electrophilic carbon, the nucleophile can attack from either side during the following step - and in many cases, is equally likely to attack from either side. This is why SN1 reactions, when they produce chiral products, produce both possible configurations.
Properties of SN1 reactions
Of the two steps of an SN1 reaction, the formation of the carbocation in the first step is the least favorable and therefore rate limiting step of the reaction. Since only the substrate with the leaving group is involved in this step, this is the only reactant whose concentration will at all determine the rate of an SN1 reaction. This makes SN1 reactions first order, as seen in equation 1.
rate = k[Electrophile]
Equation 1. Rate of an SN1 reaction
Notice also that SN1 reactions form an intermediate. While intermediates tend to not be entirely stable (‘metastable’), they do represent a local minimum of Gibbs free energy within the reaction coordinate. In other words, they are a temporary stable state, and far more easily observable than a transition state.
SN1 reactions are also notable for their tendency to produce racemic products. As we observed during the reaction pathway, proceeding through a trigonal planar intermediate virtually guarantees, in the absence of strong steric hinderance from large groups, that the nucleophile may attack from either direction and produce either of two possible configurations of products. Bear in mind however that for this to occur, a chiral product must be formed - if the final product lacks 4 different substituents, we cannot expect racemization.
Conditions for SN1 reactions
The SN1 pathway is largely favored in highly substituted molecules, such as tertiary and secondary alkyl halides. Neither primary nor methyl alkyl halides undergo SN1 reactions at all. The primary determinant of favorability for an SN1 reaction is stability of the carbocation intermediate - and such an intermediate is most substituted when tertiary, as neighboring groups can donate electron density to contribute to the stability of this structure.
Weak nucleophiles are preferred in SN1 reactions, because strong nucleophiles will preferentially cause SN2 reactions to happen. An SN2 reaction proceeds primarily through nucleophilic attack, and therefore strong nucleophiles virtually guarantee that the SN2 pathway would be preferred. Polar protic solvents are conducive to the SN1 pathway, as they stabilize the carbocation in solution. Table 1 provides a summary of the key difference between SN1 and SN2 reactions.
The MCAT is likely to ask you to predict similar mechanisms to these illustrated here, or to reason from experimental results about which pathway may be at play. So, in your studies, try to pay close attention to the driving forces behind each pathway and learn how to predict which one might occur under which conditions.
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