SN1 versus SN2 (Edexcel)
Most of the A2 nucleophilic chemistry involved carbonyl groups (see carbonyl chemistry section). However, we are going to revisit the haloalkanes here.
From AS, the basic mechanism is as follows. We are now going to take this one stage further.
SN1 and SN2
SN1 stands for Substitution Nucelophilic Unimolecular and SN2 stands for Substitution Nucleophilic Bimolecular.
Both are simple nucleophilic substitutions but the mechanisms differ slightly depending on the starting haloalkane.
This is also not just to do with organic chemistry but rates are involved too, which is where the numbers 1 and 2 come from.
The ‘1’ tells you that there is one reactant in the rate equation:
Rate = k[A]
The ‘2’ tells you that there are two reactants in the rate equation:
Rate = k[A][B]
SN1
Tertiary haloalkanes will do SN1 reactions and the reaction occurs in 2 steps.
Most people initially assume that the 1 means one step, so please avoid this mistake. As was said above, the 1 means unimolecular and is to do with the rate equation.
The key point here is that it is a tertiary haloalkane.
The CH3 groups are “bulky” and take up a lot of space and act like a shield or barrier. The nucelophile is “blocked” by the electron clouds from the methyl groups.
This means that the nucleophile cannot attack the carbon with the δ+ as normal and results in a two step mechanism.
Therefore, the first step in the mechanism is for the bromide ion to leave.
How does the bromo group just “fall off”? This doesn’t matter, you will never be asked this.
The structure that is formed with the + on the carbon is of course a carbocation intermediate:
The nucleophile can then attack from the front as shown in step 2 above.
Note: the product will be a racemic mixture as the second step of the mechanism is the attack of the nucleophile from above or below the planar C+, both of which have a 50% chance of happening.
SN2
Primary haloalkanes do SN2 reactions that occur in 1 step.
Remember that the 2 refers to bimolecular and is to do with the number of reactants in the rate equation.
The mechanism is one step but it does go through a transition state, which is the middle structure shown below:
A transition state is a theoretical state of high energy (very unstable) where the bonds aren’t completely broken or formed….it’s as the name suggests, in a state of transition.
This is simply looking at things in a bit more detail and again is unnecessary to mention this unless specifically asked about.
Only one product is formed here but the stereochemistry is reversed. If you had one enantiomer to begin with, you end up with the opposite one in the product.
By looking at the mechanism above, they sometimes refer to a “blowing an umbrella inside out” effect.
Note: secondary haloalkanes can do be SN1 or SN2. You can’t tell without some help from the question.