In this tutorial, you will learn about the second type of nucleophilic substitution reaction – sn2 – and its mechanism.
Be sure to check out our first tutorial about sn1 reactions first to fully understand the topic!
Topics Covered in Other Articles:
- Sn1 reactions
- What is a nucleophile
- What is an electrophile
- Periodic trends
- Kinetic molecular theory
- Steric hindrance
- Carbocation: an ion with a positively charged carbon.
- Leaving group: the atom or group of atoms that detach from the molecule during the course of the reaction. The leaving group accepts electrons, taking them from the broken bond and using them to fill its valence shell to leave as a neutral species.
- Nucleophilic substitution reaction: a reaction involving a nucleophile replacing a leaving group on a molecule. This happens in either one or two steps, depending on the type of reaction.
- Sn1 reaction: nucleophilic substitution reaction that happens in two separate steps.
- Sn2 reaction: nucleophilic substitution reaction that happens in one singular step.
- Stereocenter: an atom (typically carbon) with four unique attached groups.
- Steric hindrance: non-bonding interactions between molecules, resulting from their physical shape, that affect the ways in which they interact.
Sn2 Nuclephilic Substitution Reaction
Sn2 reactions are quite similar to sn1 reactions: both are types of nucleophilic substitution reactions, resulting in a version of the original reactant with the leaving group substituted for the nucleophile. However, it is important to understand the distinctions between the two, as sn2 reactions have a very different mechanism.
Notably, they happen all in one step, as opposed to the two-step mechanism of sn1. The nucleophile attacks the electrophile while the leaving group is still attached, which forces the leaving group to break away from the molecule. This is a stark contrast to the mechanism of sn1, where the leaving group leaves on its own before the nucleophile enters the reactions.
In the intermediate structure, both the nucleophile and leaving group are attached to the molecule. The leaving group then detaches itself to create the final product. Similar to sn1, the leaving group takes the electrons from its bond as it goes, thus leaving as a neutral species.
Note the sterics of this mechanism: the nucleophile attacks from an angle of 180 degrees from the leaving group. This is known as a backside attack. In the intermediate, the nucleophile and leaving group are on the same plane, both semi-attached to the carbon. In the final product, the leaving group has already left, and the nucleophile is bonded to the carbon, but still at an angle of 180 degrees from the original position of the leaving group. This is called an inversion of configuration, which happens at the stereocenter, the carbon in this case.
Example of an sn2 reaction
The diagram above shows an sn2 reaction mechanism between methanethiol and methyl iodide. The methanethiol, though not charged, is a strong nucleophile due to its lone pairs, and attacks the methyl iodide. As a result, there is a brief intermediate structure where both the CH3SH and I are attached to the molecule; shortly after that, the iodine leaves as a negatively charged species, as it takes the electrons from its bond with it.