Predicting SN1 vs. SN2 mechanisms; competition between nucleophilic substitution and elimination reactions
When considering whether a nucleophilic substitution is likely to occur via an SN1 or SN2 mechanism, we really need to consider three factors:
1) The electrophile: when the leaving group is attached to a methyl group or a primary carbon, an SN2 mechanism is favored (here the electrophile is unhindered by surrounded groups, and any carbocation intermediate would be high-energy and thus unlikely). When the leaving group is attached to a tertiary, allylic, or benzylic carbon, a carbocation intermediate will be relatively stable and thus an SN1 mechanism is favored.
2) The nucleophile: powerful nucleophiles, especially those with negative charges, favor the SN2 mechanism. Weaker nucleophiles such as water or alcohols favor the SN1 mechanism.
3) The solvent: Polar aprotic solvents favor the SN2 mechanism by enhancing the reactivity of the nucleophile. Polar protic solvents favor the SN1 mechanism by stabilizing the carbocation intermediate. SN1 reactions are frequently solvolysis reactions.
For example, the reaction below has a tertiary alkyl bromide as the electrophile, a weak nucleophile, and a polar protic solvent (we’ll assume that methanol is the solvent). Thus we’d confidently predict an SN1 reaction mechanism. Because substitution occurs at a chiral carbon, we can also predict that the reaction will proceed with racemization.
|Write out a verbal explanation of the SN1 mechanism above.|
Solution (there is not one exact way to explain a reaction, but this is an example for a well written response): (1R, 3S)-1-bromo-1,3-dimethylcyclohexane is reacted with methanol. The bromine to C1 bond breaks forming a carbocation intermediate and bromide ion. One of the lone pairs on the methanol oxygen attacks the carbocation, forming a new carbon-oxygen bond, yielding a protonated ether intermediate. One of the electron pairs from the bromide attack the hydrogen from the protonated ether, causing the oxygen-hydrogen bond to break. This yields the racemic mixture of ether products (1R,3S)-1-methoxy-1,3-dimethylcyclohexane and (1S,3S)-1-methoxy-1,3-dimethylcyclohexane as well as hydrogen bromide.
In the reaction below, on the other hand, the electrophile is a secondary alkyl bromide – with these, both SN1 and SN2 mechanisms are possible, depending on the nucleophile and the solvent. In this example, the nucleophile (a thiolate anion) is strong, and a polar protic solvent is used – so the SN2 mechanism is heavily favored. The reaction is expected to proceed with inversion of configuration.
Determine whether each substitution reaction shown below is likely to proceed by an SN1 or SN2 mechanism.