Substitution vs. Elimination

[saveourpens]

This looks to be real simple but its not for me. This is from a practice test. The answers are all right.

I understand how the first one is Sn2. The substrate is primary and and the solvent is aprotic.

For the second one, its a secondary substrate, with a protic solvent, and a not too basic nucleophile. How in the world do you get Sn2 for that??

The third has a sterically hindered very basic nucleophile, and the substrate is tertiary. Arent you supposed to get E1 for that ?

The fourth has a tertiary substrate with a solvent that acts as a base, ok, sn1 makes sense.

The last one has a secondary substrate with a protic solvent and a sterically unhindered strong base. how is that Sn2? Does the protic solvent weaken the strong base and make it an Sn2 reaction?

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[G1SG2]

For the second one, its a secondary substrate, with a protic solvent, and a not too basic nucleophile. How in the world do you get Sn2 for that?

Secondary substrates are moderately hindered; they can still undergo SN2 reactions, especially with an unhindered nucleophile. You also have a good nucleophile-it has a full minus charge, and its not hindered. Sure, its not as great as methoxide, but it can get the job done. Plus, you have a pretty good leaving group as well. Having a protic solvent doesn't necessarily mean you will get SN1 FOR SURE-it just favors the reaction. You have to take other conditions into consideration as well. So a good nucleophile + a good leaving group + a moderately hindered backside leads to an SN2.

The third has a sterically hindered very basic nucleophile, and the substrate is tertiary. Arent you supposed to get E1 for that ?

Here's what I used to do/still do whenever I seen these type of reactions. First of all, whenever I'd see a tertiary substrate, I'd write NO SN2 right away, because it can't happen. Then, I ask myself if I have a strong base (SB) or good nucleophile (GN). If I have none, I would write no SB/no GN. Under no SB, I would draw a small arrow and write E1, and under no GN I would write SN1. There, you just eliminated E2 and SN2. Here, you have a strong base, but poor nucleophile. So I would write E2 (strong base!) and SN1 (poor nucleophile). We shouldn't be concerned with E1 because we have a strong base. So a strong base with a good leaving group would lead to an E2.

So:
strong base (SB)-->E2
good nucleophile (GN)-->SN2
no SB-->E1
no GN-->SN1

The last one has a secondary substrate with a protic solvent and a sterically unhindered strong base. how is that Sn2? Does the protic solvent weaken the strong base and make it an Sn2 reaction?

Again, we have strong base (E2!) and good nucleophile (SN2!). We have a secondary, moderately hindered substrate but that's okay because we have such a great nucleophile. We also have a good leaving group (halogens are great leaving groups, except for F). Therefore, we would have an SN2. That doesn't mean we won't have competing E2 reactions, especially if we added heat. But the conditions seem to favor SN2.

This method worked for me all throughout orgo, especially with the subs/elim reactions. If you label everything, you'll find yourself eliminating reactions and narrowing it down to the best choice. Hope this helps!

 

[loveoforganic]

Thiolate anion is a better nucleophile than methoxy, just an fyi.

 

[sleepy425]

I just want to point something out. pookiez88, you mention leaving group ability a couple of times, but for this discussion it is irrelevant. For E1, E2, Sn1, and Sn2, the rate equation for each is dependent on the substrate, and the first step in all 4 mechanisms is leaving group dissociation (in Sn2, the nucleophile is also associating simultaneously, and in E2, the beta proton is being pulled off simultaneously, but either way, the leaving group is leaving in the first step).

The point is that enhancing leaving group ability contributes equally to the rates of all 4 mechanisms, so it does not favor one mechanism over the other.

Now, leaving group ability is important to look at, but it doesn't tell you which mechanism the reaction will follow. For example, having F- as a leaving group, or OH- as a leaving group, will tell you that the reaction cannot proceed by any of the 4 mechanisms.

Also, I agree with loveoforganic, thiolates are better nucleophiles than alkoxides for these types of reactions.