In sophomore organic chemistry, substitution and elimination reactions can make students want to pull their hair out. How do you know what the major product is when there are so many possibilities? The best thing to do, is to examine the "evidence" and look at the big picture. You have to break down each piece, examine it separately and use this to give you an understanding of the overall reaction.
With all of that being said, can we make a determination as to what's going on in this video?
We will come back to this a bit later...
Step 1: Understanding the Substrate Requirements
Each reaction has some different requirements and we will break them down here.
SN2: This reaction requires a strong nucleophile to "attack" an electrophilic center. So, it stands to reason that if anything gets in the way of the nucleophile, this would slow the reaction down. Therefore, substrates that lack steric hinderance are best, or primary carbons. See below:
So, no we know that SN2 reactions prefer primary carbons because the nucleophile is not prohibited from attacking. This also tells us something else about the reaction. Since the reaction involves both the nucleophile and the substrate, the rate must be dependent on both. Remember from the picture above, all we did was change the substrate and our reaction stopped. So, for SN2 reactions, the rate must include both substrate and nucleophile, or rate = k[RX][Nu].
SN1: For these reactions, we can actually see the exact opposite happen. The rate only depends on the substrate and we want there to be a large amount of steric hindrance. Why could this be? Well, clearly the mechanism must be different. For SN1 reactions, the rate only depends on the substrate, not the nucleophile. In fact, the nucleophile is unimportant for the reaction. Therefore, for SN1, the rate = [RX], and that's it! How could this be? It turns out that the mechanism for SN1 is a 2-step process and involves an intermediate.
As you can see, there is a carbocation intermediate. This fact is extremely important! The presence of the intermediate dictates how the reaction proceeds. Anything that stabilizes the intermediate (brings the energy of the intermediate down) helps the intermediate form faster (lower activation energy). So, in other words, anything that stabilizes the carbocation, helps the reaction. Also, the only thing that is important to the reaction is the carbocation/substrate as is indicated in the rate equation. Therefore, we want the exact opposite of what is needed in SN2 reactions, we need bulky substrates or tertiary alkyl halides.
So, we have narrowed down our choices for SN1 and SN2, primary alkyl halide will be SN2 and a tertiary alkyl halide will be SN1. What about secondary alkyl halides? In this case, we need to look at some more evidence. See the chart below for some explanations.
Notice on the chart that there are E1 and E2 reactions. Those will the focus of the next post. However, for this, pay attention to the conditions needed for secondary alkyl halides. If there is a weak nucleophile (something like water or an alcohol) then the reaction will proceed to go via SN1. If there is a strong nucleophile, the reaction will likely proceed via SN2. Also, you can check your solvent. If there is a polar aprotic solvent like dimethylformamide (DMF), hexamethylphosphoramide (HMPA), acetone, dimethyl sulfoxide (DMSO), then the reaction is most likely going to be SN2. If there is not a nucleophile and a solvent listed, then the solvent is acting as the nucleophile and it is typically weak. Check out the video below. Note, both the chart and the video are in the appropriate sections on the site.
Next up, E1 and E2 and putting it all together.