In this tutorial, you will learn how to explicitly distinguish between the different aspects of sn1 vs. sn2 reactions, and to identify the factors that make each more likely to occur.
- Sn1 reactions
- Sn2 reactions
- What is a nucleophile
- What is an electrophile
- Steric Hindrance
- Cations and anions
- Aprotic (solvent): a solvent that does not contain hydrogen atoms bonded to oxygen, nitrogen, or fluorine, and thus cannot hydrogen bond. It may contain hydrogen atoms elsewhere, such as bonded to carbon.
- 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.
- Protic (solvent): a solvent that contains a hydrogen bonded to an oxygen, nitrogen, or fluorine atom, which can serve as a source of H+ atoms. This is a solvent that has the ability to hydrogen bond.
- Solvation: a process where solvent molecules surround and interact with dissolved solute molecules.
- Steric Hindrance: non-bonding interactions between molecules, resulting from their physical shape, that affect the ways in which they interact.
1. Sn1 vs. Sn2 Rate Equations
The numbers associated with Sn1 and Sn2 reactions can seem counterintuitive at first. If you think about the number of steps involved in these reactions, they seem backwards. However, the numbers refer to the number of reactants involved in the rate-determining step, not to the number of steps. The slowest step in a reaction is the one that limits the rate of the overall reaction, just like the neck of a bottle determines how quickly you can pour out its contents.
In an Sn1 reaction, this slowest step is the dissociation of the electrophile, when the leaving group leaves. This process is not dependent on the concentration of nucleophile, because the nucleophile only takes part in the second step. As a result, we can write the rate equation as R = k[electrophile], that is, the rate of reaction is related by the rate constant k to the concentration of ONE reactant, the electrophile. Another way of saying this is that the reaction is “unimolecular,” and this is why we call it Sn1: Substitution – nucleophilic – unimolecular.
Similarly, because TWO reactants must come together in the rate determining (and only) step of an Sn2 reaction, we call this type of reaction “bimolecular” and write its rate equation as R = k[electrophile][nucleophile]. This leads to the name Sn2: Substitution – nucleophilic – bimolecular.
2. Sn1 vs. Sn2 Electrophiles
The position of the leaving group on the electrophile is perhaps the most significant when it comes to distinguishing between sn1 vs. sn2 reactions.
Sn1: if the leaving group is attached to a tertiary carbon, it is most likely to undergo an sn1 reaction; if attached to a secondary carbon, less likely, and if attached to a primary carbon, very unlikely – essentially impossible. This is because the first step in an sn1 reaction is the carbocation formation, as the leaving group detaches itself. A tertiary carbocation is relatively stable, while a primary carbocation is very unstable. Thus, the more stable the resulting carbocation, the more likely an sn1 reaction is.
To summarize: Tertiary > secondary > primary
Sn2: if the leaving group is attached to a primary carbon, it is most likely to undergo an sn2 reaction; if attached to a secondary carbon, less likely, and if attached to a tertiary carbon, very unlikely – essentially impossible. This is because in an sn2 reaction, the nucleophile “attacks” the electrophile as-is, so there physically has to be space for it to do so. A primary carbon is the only connected to one other carbon, thus it has the least steric hindrance; a tertiary carbon, however, is connected to three other carbons, and thus there will be multiple other groups getting in the way of the nucleophile. Thus, the more steric hindrance, the less like sn2 is to occur.
To summarize, the trend is directly opposite to that of sn1: Primary > secondary > tertiary
3. Sn1 vs. Sn2 Nucleophiles
Sn1: In sn1 reactions, the nucleophile tends to be uncharged and weaker, as it is “attacking” a carbocation. This means that it will not take very much strength for the second step, the nucleophilic attack, to occur – the charge of the electrophile encourages it already. Often, in an sn1 reaction, the nucleophile is the solvent that the reaction is occurring in.
Some examples of nucleophiles common to sn1 reactions are: CH3OH, H2O
Sn2: In sn2 reactions, the nucleophile displaces the leaving group, meaning it must be strong enough to do so. Often, this means that the nucleophile is charged – if not, then it must be a strong neutral nucleophile. That being said, pay attention to sterics as well, as a very bulky nucleophile will be unable to do an sn2 reaction.
Some examples of nucleophiles common to sn2 reactions are: KOEt, NaCN
Note that these are actually charged nucleophiles, since they contain ionic bonds. NaCN, for example, in a reaction acts as Na+ and CN–, making CN– the charged nucleophile.
4. Sn1 vs. Sn2 Solvents
Sn1: Sn1 reactions tend to happen in polar, protic solvents, because they can stabilize the carbocation charge better through their strong solvating power. This essentially means that the protic solvent can surround the charge and interact with it, which stabilizes the charge. In the case of protic solvents, they have the ability to hydrogen bond, but in sn1 reactions they stabilize the carbocation through dipole interactions. Additionally, the polar, protic solvent can hydrogen bond with the leaving group, thus stabilizing it as well.
Some examples of solvents common to sn1 reactions are: water, alcohols, carboxylic acids
Sn2: Sn2 reactions tend to happen in polar, aprotic solvents. This is because they are polar enough to dissolve the nucleophile and allow the reaction to proceed, but do not have the ability to hydrogen bond or as strong solvating power as the solvents for sn1 reactions. This makes sense, as they do not have to stabilize a carbocation in sn2 reactions. In fact, too strong a solvating power, such as the polar, protic solvents, will hinder sn2 reactions because it will solvate the nucleophile, and prevent it from “attacking” the electrophile.
Some examples of solvents common to sn2 reactions are: acetone, DMSO (dimethylsulfoxide), acetonitrile
5. Sn1 vs. Sn2 Leaving Groups
Sn1 and Sn2: Both sn1 and sn2 reactions require good leaving groups, so the nature of the leaving group does not impact the type of reaction very much. However, a very poor leaving group may prevent either reaction from occurring at all.
A good leaving group is one that is highly electronegative, because a leaving group needs to be able to take the electrons from its bond to leave. The more electronegative a species is, the greater its ability to attract electrons, especially those of a bonded pair.
Some examples of good leaving groups common to both sn1 and sn2 reactions are: Cl–, Br–, I–, H2O
Thus, the structure of the electrophile is the easiest way to determine in a reaction will proceed via sn1 vs. sn2. If the leaving group is attached to a primary or tertiary carbon, in most cases you can automatically assume an sn1 or sn2 reaction, respectively. If it is attached to a secondary carbon, the case is a little more ambiguous. You may have to rely on other clues to determine which reaction it will be. In these cases, look at the nucleophile (whether it is charge/uncharged, or strong/weak), and at the solvent (whether it is protic or aprotic).