0
$\begingroup$

We were studying about nucleophilic substitution reactions. My professor said that in general SN1 reactions are faster than SN2 reactions. I am unable to understand that how can we generalise the rate of two completely different reactions. In this case, what I think is that the rate will depend on our reagent, leaving group, solvent, etc and in some cases SN1 will be faster while in some others SN2. Therefore, I'm a bit confused.

I would be happy if anyone could help me with this.

$\endgroup$
3
  • $\begingroup$ Not always the case: Confusion about the SN1 and SN2 Mechanism; Comparison of the rate of SN1 and SN2 reaction. $\endgroup$
    – andselisk
    Oct 20, 2019 at 12:44
  • $\begingroup$ Yes, but generally, first order kinetics are faster than second order, and the latter slow down much more as the reaction progresses. $\endgroup$
    – Karl
    Oct 20, 2019 at 15:46
  • $\begingroup$ @Karl Is that somewhat related to the fact that in second order reactions, effective collisions are required and this takes time? $\endgroup$ Oct 20, 2019 at 16:42

1 Answer 1

2
$\begingroup$

[OP] In this case, what I think is that the rate will depend on our reagent, leaving group, solvent, etc and in some cases SN1 will be faster while in some others SN2.

SN2 is a second order reaction, so the rate depends on the concentration of the nucleophile. In cases where both SN1 and SN2 mechanisms are plausible (e.g. secondary alkyl halides), you can "push" the mechanism toward SN1 (low concentration of nucleophile, weak nucleophile, protic solvent) or SN2 (high concentration of nucleophile, strong nucleophile, aprotic polar solvent). So even for a given reaction, you can't make the statement that one will be faster than the other. Here is a publication where they studied this.

[OP] My professor said that in general SN1 reactions are faster than SN2 reactions.

I'm not sure your professor would publish that statement. Maybe they said it in passing or for a very specific case. In the comments, there is a discussion about whether SN1 is faster because it does not need a collision. I don't think that that is argument is tight - you need collisions with the solvent to get over the activation barrier.

[OP] I am unable to understand that how can we generalise the rate of two completely different reactions.

You are right, we can't.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.