Explain why one compound was particularly reactive under both sn1 and sn2 reaction conditions.

In this explanation, I shall cite the nucleophilic substitution of the molecule with molecular formula #"C"_3"H"_7"Br"#, undergoing nucleophilic attack by the #"OH"^-# (hydroxide) complex ion.

Also remember that nucleophilic substitution reactions are in competition with elimination reactions, but the sets of conditions to favour each of these two reaction types are different to those that cause one type of nucleophilic substitution to be favoured.

1) The Resulting Carbocation

The most important thing to point out is that primary alkyl halides (also known as haloalkanes or halogenoalkanes , since that is the example being used) always undergo #"S"_N2# reactions, whilst tertiary alkyl halides always undergo #"S"_N1# reactions.

Then first, let us consider the different types of alkyl halides we have at our disposal:

Where R represents an alkyl group (#"CH"_3#, #"CH"_3"CH"_2#, etc.), primary alkyl halides are those with #"C"# atoms sharing one bond with the #"X"# (halogen) group and exactly one bond with another #"C"# atom (the other two bonds being to hydrogen atoms). Secondary - one #"C - X"# bond, exactly two #"C - C"# bonds; tertiary - one #"C - X"# bond, exactly three #"C - C"# bonds.

When an alkyl halide undergoes an #S"_N1# reaction, a carbocation forms: it is of the same type as the alkyl halide was itself (i.e. primary, secondary, tertiary). However, primary carbocations are not stable, and so will not form if there is another possible route for the reaction to take (#"S"_N2#).

There is then increasing stability of carbocations, with secondary carbocations being more stable than primary ones (and so they are able to form) and tertiary carbocations being the most stable of all.

Therefore secondary and tertiary alkyl halides are capable of undergoing #"S"_N1# reactions, but primary ones are not.

2) The Extent of Steric Hindrance

This isn't too complicated: steric hindrance is basically the amount of space the entire molecule takes up, and is related to accessibility by complexity of the molecule.

Tertiary alkyl halides have a lot of steric hindrance, since they have three alkyl groups and so lots of hydrogen atoms - this makes it very difficult for a nucleophile to attack as the halogen group leaves (#"S"_N2# reaction).

Steric hindrance decreases in 'simpler' alkyl halides: both primary and secondary alkyl halides are capable of undergoing an #"S"_N2# reactions as a result, but tertiary alkyl halides are not.

3) Nucleophilic Strength

These final two factors apply only to secondary alkyl halides: the other two types are restricted to only one mechanism, but these may undergo either and will do so in a reaction mixture. These factors only affect which mechanism will occur more frequently.

Strong nucleophiles will attack an electrophilic carbon atom very aggressively; therefore, a strong nucleophile will favour an #"S"_N2# reactionm which forms a transition state (but no detectable intermediate), rather than an #"S"_N1# reaction that would involve them having to 'wait'.

As nucleophilic strength decreases (e.g. through increase in charge), #"S"_N1# reactions begin to become more prominent.

4) Solvent Type

There are two relevant types of solvents: polar protic solvents (EX: water, #"CH"_3"OH"#, etc) have a hydrogen atom bound to an oxygen or nitrogen atom, and polar aprotic solvents (EX: dimethyl sulfoxide/DMSO, acetone, etc) that do not.

Therefore, polar protic solvents exhibit hydrogen bonding: this also allows them to form hydrogen bonds with nucleophiles dissolved in them, which impedes the reactivity of said nucleophiles.

(However polar protics are not all bad, as they really help the original alkyl halide to dissociate, since both are polar.)

Polar protic solvents will favour an #"S"_N1# reaction because, despite the nucleophile being restricted, they greatly assist in the formation of carbocations that are far more readily attacked by nucleophiles.

Polar aprotic solvents are used to favour #"S"_N2# reactions, and a polar protic solvent would only serve to impede rate by deactivating your nucleophile (factor 3).

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.

Related Topics

Vocabulary

• 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

Summary

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).