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My book says, and I understand, that the Sn1 mechanism is much faster for tertiary alkyl halides than primary. And the opposite for Sn2. This all makes sense. What I don't get is the conclusion it comes to next: hence, we predict that the primary alkyl halides will react primarily via Sn2, and the tertiary via Sn1. The reason I get confused here is that nothing was said about the comparative rates for a primary alkyl halide via the Sn1 or Sn2 ( they just compared primary and tertiary for SN1 and SN2). In my mind, which mechanism dominates for primary or tertiary should be dependent on the relative rates for that compound; not rates compared between primary and tertiary.

Is my argument sound? And if so, is there some literature you know of that explains this in such a manner?

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    $\begingroup$ Just measure the absolute rate. Then you know all relative rates. $\endgroup$ – Zhe Mar 31 at 12:40
  • $\begingroup$ If your reactant and product are chiral, you can tell from the stereochemistry of the product which mechanism was at play. SN2 shows inversion of configuration, while SN1 scrambles it (racemic mixture as product). $\endgroup$ – Karsten Theis Mar 31 at 13:35
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Your reasoning is correct. It's the relative rates for a given alkyl halide that matter, and it's difficult to draw conclusions about those, because the relative rates are not fixed, as the $S_N2$ rate is dependent also on the nucleophile concentration.

We can write the rate of the $S_N1$ as $k_1[\text{alkyl halide}]$ and the rate of the $S_N2$ as $k_2[\text{alkyl halide}][\text{nucleophile}]$, so the relative rate at a fixed concentration of the alkyl halide is given by

$$\frac{S_N2}{S_N1}=\frac{k_2[\text{nucleophile}]}{k_1}.$$

The identity of the nucleophile will be a factor as well, since that will affect $k_2$.

So all in all, you can't make a general conclusion that one reaction type will always go faster than the other, although it is likely that there are alkyl halides for which one reaction type is faster in a large majority of cases.

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  • $\begingroup$ the way I see it now is that for primary alkyl halides, the unstable carbocation slows down SN1 much, so generally SN2 wins out. And for tertiary, steric hindrance tends to slow down SN2 much, so generally, the SN1 wins out. That sound right? $\endgroup$ – Sal_99 Apr 2 at 6:52
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    $\begingroup$ Yes, you've got it. $\endgroup$ – Andrew Apr 2 at 13:15

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