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My book says, and I understand, that the $\mathrm{S_N1}$ mechanism is much faster for tertiary alkyl halides than primary. And the opposite for $\mathrm{S_N2}.$ 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 $\mathrm{S_N2},$ and the tertiary via $\mathrm{S_N1}.$

The reason I get confused here is that nothing was said about the comparative rates for a primary alkyl halide via the $\mathrm{S_N1}$ or $\mathrm{S_N2}$ (they just compared primary and tertiary for $\mathrm{S_N1}$ and $\mathrm{S_N2}$). 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
    Commented Mar 31, 2019 at 12:40
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    $\begingroup$ 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 ♦
    Commented Mar 31, 2019 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 $\mathrm{S_N2}$ rate is dependent also on the nucleophile concentration.

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

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

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
    Commented Apr 2, 2019 at 6:52
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    $\begingroup$ Yes, you've got it. $\endgroup$
    – Andrew
    Commented Apr 2, 2019 at 13:15

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