Now that we’ve looked at the SN2, let’s look at another type of substitution reaction.
This time, we’ll start with the same starting halide as last time (2-bromobutane) and we’ll start with one enantiomer (it’s “optically active”).
A funny thing happens when we dissolve this in water. Look at the bonds formed and bonds broken.
We’re still forming C-O and breaking C-Br, so it’s still a substitution reaction. But there’s some key differences.
- When we measure the rate of this reaction, we find that it only depends on the concentration of the alkyl bromide (even if we correct for the fact that water is present in much greater concentration). In other words, the rate is “unimolecular”
- Unlike the previous case (the SN2 reaction) where we got one enantiomer, with inversion – here we get a mixture of enantiomers (a mixture of retention and inversion).
- When we compare this to other reactions, we find that the rate increases as we go from 1-bromobutane to 2-bromobutane to t-butyl bromide (in other words, the rate increases as we go from primary to secondary to tertiary).
- Finally, unlike the SN2, which proceeds best in polar aprotic solvents (like DMSO) this one proceeds best in polar protic solvents – like water, alcohols, and carboxylic acids.
This is very different than the SN2! So what’s going on here?
Here’s our best theory.
Unlike the SN2, which happens all at once, this reaction happens stepwise. One reaction at a time.
- First, the leaving group leaves. We break the C-Br in this instance, to form a carbocation, and C-Br.
- Then, the nucleophile (water) attacks, which gives us the alcohol as our product.
Let’s see why this makes sense:
- the rate only depends on the alkyl halide because we’re forming an unstable carbocation, which is the slow – rate determining – step.
- The carbocation is flat (sp2 hybridized) – so when the nucleophile attacks a carbon, it can attack from either face of the flat carbocation. This means we can get a mixture of retention and inversion.
- Carbocations are unstable, and their stability increases as we go from primary (least stable) to secondary to tertiary (most stable).
- Carbocations are polar, so they’re going to be stabilized by polar solvents – and polar protic solvents are better at stabilization than polar aprotic solvents are.
We call this the SN1 reaction (Nucleophilic substitution, unimolecular).
It all makes sense if you think of one thing: the big barrier for the SN1 is carbocation stability. Anything that will stabilize the carbocation, will speed up the SN1.
Repeat!: The big barrier to the SN1 is carbocation stability.
Thanks for reading! James
P.S. Further reading: SN1 reaction of alkyl halides with water to form alcohols