Core Concepts
In this tutorial, you will learn about the first type of nucleophilic substitution reaction – sn1 – by looking at its mechanism and going through an example.
Topics Covered in Other Articles
Vocabulary
- 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.
- Sn1 reaction: nucleophilic substitution reaction that happens in two separate steps.
- 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 react.
What is a nucleophilic substitution reaction?
A nucleophilic substitution reaction involves a nucleophilic molecule replacing another atom or group of atoms, called the leaving group, on a molecule. The nucleophilic molecule is rich in electrons, which attack the substrate molecule. The leaving group on the substrate molecule departs with a newly-gained electron pair. The nucleophilic molecule bonds with the substrate molecule. Nucleophilic substitution reactions are common in organic chemistry, and take the general form of: Nuc + LG-R → Nuc-R + LG.
This reaction can happen in either one or two steps, depending on the type of reaction. In this tutorial, we will be discussing the two-step version of this reaction: the sn1 reaction.
Sn1 Nucleophilic Substitution Reaction
An sn1 reaction is a type of nucleophilic substitution reaction that happens in two separate, distinct steps. In the first step of the mechanism, the leaving group detaches from the molecule, taking the electrons from its bond with it. Thus, the leaving group exits as a neutral species, having a full octet. The carbon loses one of its electrons in this process, and thus gains a positive charge.
At this stage, the electrophile molecule is left with a carbocation, and this positive charge is the site where the nucleophile attaches and donates its electrons, quenching that positive charge to leave the final product uncharged.
The first step is the rate-determining step, because it happens slower than the nucleophilic attack, meaning that the entire reaction can only proceed at the speed of the first step. The rate of the sn1 reaction only depends on the concentration of the electrophile molecule and not that of the nucleophile.
Sn1 Mechanism
Note the sterics of this mechanism: the nucleophile can approach from either side of the molecule because the leaving group has already fully left. The nucleophile “attacks” the carbocation, which is equally blocked from all sides, resulting in two equally likely products. However, if the other two substituents (Y and Z) were different or unequal in size, this would affect the sterics, making the least hindered approach more likely.
Example of an sn1 reaction
The diagram above shows an sn1 reaction mechanism between methyl tert-butyl ether and hydrogen bromide. Firstly, the oxygen atom in the electrophile bonds with a hydrogen that has dissociated from the HBr. This is because CH3O is a poor leaving group, but CH3OH is a good leaving group. Thus, as the oxygen bonds with this hydrogen, the leaving group improves and can carry out the first step of the sn1 reaction, detaching from the molecule. The electrophile is now left with a carbocation, and the remaining Br–, a strong nucleophile, can “attack” and create a bond to that carbon.