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I was going through Peter Sykes organic chemistry where I came across a section where the author is saying:

For halides that do not readily undergo SN1 attack ,this can be promoted by use of silver salt of halogen as $\ce{Ag+}$ promotes carbocation formation by precipitating silver halide, $\ce{AgX}$.

So I am confused what is the role of $\ce{Ag+}$ ions in enhancing and why does precipitation of silver halide occurs? Also can this reaction be utilized to obtain primary carbocation? Lastly we know that primary carbocation is very unstable and thus has a high energy of activation so is this energy provided by the lattice enthalpy of silver halide?

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During $\ce{S_N1}$, there is an equilibrium step where the leaving group leaves, in this case, a halide. This is the mechanism for an alkyl bromide substitution. enter image description here

What $\ce{Ag+}$ allows is the precipitation $\ce{Br-}$ anions produced during the equilibrium step and therefore shift the equilibrium more to the right (the side with the carbocation) according to Le Chatelier's principle. This will promote $\ce{S_N1}$ reactions.

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  • $\begingroup$ So can we obtain a primary carbocation by this procedure? $\endgroup$
    – Arpan
    Jun 23 '21 at 5:46
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    $\begingroup$ In solution, Sn1 and Sn2 reactions occur pretty much simultaneously I believe. In general, for primary carbocations, Sn2 will predominate, about 1000 times the rate of sn1. Adding Ag+ may increase the rate of Sn1 but I would say not too much. Again, there are many factors to this: solvent, ksp of Ag-Hal, any other functional groups, etc. However, I think Sn2 would still be predominant for primary halides even with Ag+ added. I could be wrong though. $\endgroup$
    – M.L
    Jun 23 '21 at 5:51

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