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I understand that a polar solvent will help dissociate the $\ce{HBr}$ into ions. But why a protic solvent?

Why would we want a solvent that would encage the $\ce{Br}$ and $\ce{H}$ ions? Why is the aprotic not favored? My textbook does not discuss this and I searched online, with no result.

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why a protic solvent?

Protic solvents tend to have higher dielectric constants than typical, unreactive, non-protic solvents. Look at the following table.

\begin{array}{|c|c|c|}\hline solvent & dielectric~ constant \\ \hline methanol & 32.6 \\ \hline ethanol & 24.6 \\ \hline methylene~chloride & 9.08 \\ \hline chlorobenzene & 5.69\\ \hline chloroform & 4.81 \\ \hline diethyl ~ether & 4.27 \\ \hline \end{array}

The higher dielectric constant stabilizes ions which means that 1) $\ce{HBr}$ will tend to be more dissociated and 2) (as Dissenter noted) the resultant carbocation intermediate will be more stabilized thereby lowering the energy of the transition state leading to it.

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  • $\begingroup$ Propylene carbonate: 64; Acetonitrile: 39; dimethylformamide: 38; HMPT: 30 $\endgroup$ – Abel Friedman Sep 28 '14 at 20:19
  • $\begingroup$ @AbelFriedman That's why I highlighted unreactive. I don't know the reaction conditions we're discussing, but acid-catalyzed hydration of a nitrile would be a distinct possibility. $\endgroup$ – ron Sep 28 '14 at 20:26
  • $\begingroup$ @AbelFriedman, (and @Dissenter), ron seems to agree with my textbook like Dissenter. I'm confused :O $\endgroup$ – yolo123 Sep 28 '14 at 20:45
  • $\begingroup$ @AbelFriedman Yes, propylene carbonate, DMF and HMPT are also subject to acid hydrolysis and could generate by-products that would contaminate the reaction. $\endgroup$ – ron Sep 28 '14 at 20:45
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High-dielectric solvent assist dissociation because they shield charge. Since the proton's electric field is so high, it always finds a base to bind to, which will in your case be $\ce{Br-}$. So you also have to consider the solvent's basicity. Read this article.

The assertion that you need a protic solvent is wrong, you need a sufficiently basic solvent.

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A polar protic solvent will help stabilize the carbocation that is formed in the process of electrophilic addition. When the nucleophilic $\pi$ bond grabs the hydrogen from the $\ce{HBr}$, one of the carbons that was part of the $\pi$ bond is left deficient of an octet.

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