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It is known that $\mathrm{S_N1}$ (Substitution Nucleophilic Unimolecular) follows the order of rate of reaction:

Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$)

For $\mathrm{S_N2}$ (Substitution Nucleophilic Bimolecular) the order of rate:

Tertiary ($3^\circ$) < Secondary ($2^\circ$) < Primary ($1^\circ$)

But what is the effective rate of reaction, if we consider that both $\mathrm{S_N1}$ and $\mathrm{S_N2}$ may occur?

Let us assume that we are using $\ce{NaOH(aq)}$ as the reagent, and using n-butyl chloride ($1^\circ$), sec-butyl chloride ($2^\circ$), and tert-butyl chloride ($3^\circ$). As water is used, substitution will be favoured, and for this discussion, let us neglect any elimination reaction occurring. In this case, what would be the effective rates of reaction for these cases?

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  • $\begingroup$ chemistry.stackexchange.com/questions/35013/… also $\endgroup$
    – Mithoron
    Jan 9, 2018 at 16:11
  • $\begingroup$ For SN1, carbocation stability is the main factor, while for SN2, steric hindrance is the main factor. When we think about carbocation stability, EWGs destabilise the carbocation while ERGs stabilise the carbocation. $\endgroup$ Dec 23, 2023 at 17:53
  • $\begingroup$ So say I had tert-chlorobutane, and tert-methoxybutane, the rate of reaction of methoxybutane would be much higher as -OCH3 group is electron releasing in nature $\endgroup$ Dec 23, 2023 at 17:55
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    $\begingroup$ Here is a paper that studies concurrent SN1 and SN2 reactions in substituted Ar2CHBr compounds, showing how the concentration of the nucleophile, the lifetime of the carbocation and the temperature all have a role in the preference for either SN1 or SN2. $\endgroup$
    – Karsten
    Dec 24, 2023 at 17:01
  • $\begingroup$ @Karsten I am not able to access it. If I could I would have read it and updated my answer accordingly. I am able to see only the supporting info. $\endgroup$ Dec 24, 2023 at 17:03

1 Answer 1

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An interesting interplay: $\mathrm{S_N}$ mechanisms


The effective rate of reaction in a state of affairs wherein both $\mathrm{S_N1}$ and $\mathrm{S_N2}$ reactions can arise is decided via the interaction of the rates of those two mechanisms.

It's crucial to note that the relative rates of $\mathrm{S_N1}$ and $\mathrm{S_N2}$ reactions are influenced with the aid of factors consisting of the substrate, nucleophile, and solvent.

In general, for the given substrates (n-butyl chloride, sec-butyl chloride, and tert-butyl chloride), we can make a few observations:

  1. $1^\circ$ Substrate (n-butyl chloride):
  • $\mathrm{S_N2}$ is favored because of the much less hindered nature of the primary carbon.
  • The effective rate would primarily be undertaken by $\mathrm{S_N2}$.
  1. $2^\circ$ Substrate (sec-butyl chloride):
  • $\mathrm{S_N2}$ is still preferred, however $\mathrm{S_N1}$ may have a greater substantial contribution as compared to the $1^\circ$ substrate.
  • The powerful rate might be a mixture of both mechanisms, with $\mathrm{S_N2}$ being extra distinguished.
  1. $3^\circ$ Substrate (tert-butyl chloride):
  • $\mathrm{S_N1}$ is desired due to the stability of the carbocation intermediate.
  • The effective rate could in general be governed with the aid of $\mathrm{S_N1}$.

In summation, the effective rates for the given substrates might observe this fashion: [$1^\circ > 2^\circ > 3^\circ$]

This implies that for n-butyl chloride ($1^\circ$), the $\mathrm{S_N2}$ mechanism dominates; for sec-butyl chloride ($2^\circ$), it's a combination of both mechanisms however with $\mathrm{S_N2}$ being extra considerable; and for tert-butyl chloride ($3^\circ$), $\mathrm{S_N1}$ is the number one mechanism.


An important point to be noted:

This answer explains how steric hindrance negatively affects the rate of reaction.

$\mathrm{S_N1}$ mechanism has 2 steps, while $\mathrm{S_N2}$ has only 1.

This is not all!

The example will vary from substrate to substrate. It all depends on various other factors such as resonance, hyperconjugation, inductive effects etc.

Thus we will have to carefully consider all the electron displacement effects and then go ahead with making decisions with respect to what comes first.


Sources:

  1. https://typeset.io/questions/what-are-the-different-factors-that-affect-the-rate-of-sn1-2c406c7yp2

  2. https://www.masterorganicchemistry.com/2012/08/08/comparing-the-sn1-and-sn2-reactions/

  3. https://courses.lumenlearning.com/suny-potsdam-organicchemistry/chapter/8-3-factors-affecting-rate-of-nucleophilic-substitution-reactions/

  4. https://www.chemguide.co.uk/mechanisms/nucsub/whatis.html#top


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  • $\begingroup$ In $\mathrm{S_N1}$ the r.d.s. is the carbocation formation and rate is independent of nucleophile concentration, while in $\mathrm{S_N2}$ rate is dependent on nucleophile concentration. $\mathrm{S_N2}$ has only one r.d.s., and the transition state formed in between is transient in nature $\endgroup$ Dec 25, 2023 at 1:08
  • $\begingroup$ Going by what @jimchmst said, it implies that the asker of the question itself has incorrectly put the numbering. I'll edit that as well as update my answer. $\endgroup$ Dec 25, 2023 at 1:10
  • $\begingroup$ Also my inference from @jimchmst's comment is that this exercise is more of an experimental nature rather than that of a theoretical nature. $\endgroup$ Dec 25, 2023 at 1:14
  • $\begingroup$ Thanks for pointing that @jimchmst, actually isobutyl chloride will be a bit of a gray zone because it would rather go rearrange itself into a tertiary carbocation. Thus it's a bit complicated; actual secondary and tertiary halides will give a better insight. $\endgroup$ Dec 25, 2023 at 1:19
  • $\begingroup$ Also in which solvent we carry out sub. of isobutyl chloride also will play a major role in that case. $\endgroup$ Dec 25, 2023 at 1:23

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