Now, what I want to do is ensure you understand the mechanism we will use every time. It's crucial that you grasp everything about this mechanism. In addition, there are many facts we need to memorize because your professor will expect you to know a significant amount of conceptual information as well. You won't fully understand it unless you know all the concepts behind it. So, let’s start breaking down these different concepts, beginning with the nucleophile. You might already guess what this is, but do you think we need a strong nucleophile or a weak nucleophile to start off this reaction? When something is negatively charged, it is considered strong because this indicates its tendency to actively try to lose electrons. In the description, I mentioned that it's always a negatively charged nucleophile, which signifies it is strong. Thus, a weak nucleophile, one that is neutral or lacks a negative charge, would not be ideal for an SN2 reaction.
Then, the leaving group. Is it better if it's unsubstituted or highly substituted? "Substituted" refers to the presence of R groups. Unsubstituted means the alkyl halide, the leaving group, has fewer R groups, whereas highly substituted means it has many R groups. The mechanism is a backside attack, so think about whether it’s better to have minimal steric hindrance, like just small hydrogens, or bulky groups that take up a lot of space. Clearly, for backside attacks to be favored, you need ample space on the backside; otherwise, it's too cramped for the reaction to occur efficiently.
Next, let’s discuss the reaction coordinate. If I were to draw an energy diagram, what would represent the highest point—the transition state or an intermediate? As I explained earlier, we use a transition state because the reaction happens simultaneously; there's one molecule in between both sides. Is this a concerted mechanism or a two-step mechanism? Concerted means everything happens at once, aligning with the one-step process of SN2, where a bond forms and another breaks concurrently.
Now, let’s look at rate questions. Is the rate unimolecular or bimolecular? SN2 stands for bimolecular, indicating the rate depends on two species. If I increase the concentration of a certain reagent, it affects the rate of product formation. If you double the number of nucleophiles, it increases the chances of hitting the target—the electrophile or leaving group. Molecules engaging in these reactions respond to random motion; if you double the nucleophiles (arrows) and the electrophiles (targets), you quadruple the chance of a successful reaction, emphasizing the bimolecular nature of this rate.
Lastly, let’s tackle stereochemistry questions. If you have a chiral center and perform a backside attack, switching two groups' positions inverts the configuration. If an R configuration starts, it will change to S after the reaction, which is known as inversion of configuration. Retention would mean it stays the R configuration, but that’s not applicable here. Racemic means obtaining an equal mix of R and S enantiomers, but we achieve 100% of the other enantiomer due to the inversion. Always remember, a backside attack means inversion of configuration when discussing chirality.
We will now proceed with a practice question. Rank the following alkyl halides in terms of their reactivity towards an SN2 reaction. All leaving groups are identical in strength, so determine what else could influence their reactivity. Try to solve this, and I will discuss the answer in the next segment.